Oxygen requirements of the earliest animals

Earth's magnetic field and its relationship to the origin of life, evolution and planetary habitability

Earth's magnetic field history can provide insight into why life was able to originate and evolve on our planet, and how habitability has been maintained. The magnetism of minute magnetic inclusions in zircons indicates that the geomagnetic field is at least 4.2 billion years old, corresponding with genetic estimates for the age of the last universal common ancestor. The early establishment of the field would have provided shielding from solar and cosmic radiation, fostering environments for life to develop. The field was also likely important for preserving Earth's water, essential for life as we know it. Between 3.9 and ca. 3.4 billion years ago, zircon magnetism suggests latitudinal stasis of different ancestral terrains, and stagnant lid tectonics. These data also indicate that the solid Earth was stable with respect to the spin axis, consistent with the absence of plate tectonic driving forces. Moreover, these data point to the existence of low-latitude continental nuclei with equable climate locales that could have supported early life. Near the end of the Precambrian (0.591 to 0.565 billion years ago), the dynamo nearly collapsed, but growth of the inner core during earliest Cambrian times renewed the magnetic field and shielding, helping to prevent drying of the planet. Before this renewal, the ultra-weak magnetic shielding may have had an unexpected effect on evolution. The extremely weak field could have allowed enhanced hydrogen escape to space, leading to increased oxygenation of the atmosphere and oceans. In this way, Earth's magnetic field may have assisted the radiation of the macroscopic and mobile animals of the Ediacara fauna. Whether the Ediacara fauna are genetically related to modern life is a matter of debate, but if so, magnetospheric control on atmospheric composition may have led to an acceleration in evolution that ultimately resulted in the emergence of intelligent life.

Resolving the Snowball Earth conundrum: the role of marine dissolved organic carbon pool

The Cryogenian Sturtian (717-660 Ma) and Marinoan glacial deposits (∼650-635 Ma), typically consisting of alternating layers of glaciogenic diamictite and clast-free lithofacies, indicate dynamic glaciers or glacial-interglacial cycles during the global glaciations. This may result from ice sublimation in tropics under a Snowball Earth condition. However, this model fails to explain the deposition patterns observed in mid-latitude continents. We propose the presence of unfrozen oceans while the continents are covered, i.e., the icy-continents. The open-ocean condition requires low atmospheric pCO2 level. We argued that the mantle CO2 degassing could be counterbalanced by a growing pool of dissolved organic carbon (DOC) in the deep ocean, maintaining a small marine dissolved inorganic carbon (DIC) pool and a low atmospheric pCO2 level. The persistent marine productivity in the open ocean would support the expanded DOC pool due to reduced ocean ventilation and limited terrestrial inputs of oxidants. However, the global glaciation with open oceans was climatically unstable. The fluctuation of the DOC pool on a local or regional scale likely contributed to the frequent glacial-interglacial oscillations recorded in the rock records. Additionally, the expansion of the DOC pool removed seawater nutrients, e.g., phosphorus (P), and insufficient nutrient supply prevented the transfer of mantle-degassing carbon as DOC, ultimately leading to the termination of global glaciation. The turnover of the DOC pool, caused by deep ocean ventilation in the deglacial period, significantly increased the atmospheric pCO2 level. This event was followed by intense continental weathering, increased seawater pH, recovery of primary productivity, cap carbonate precipitation, and eventually, the emergence of new life forms and innovations in the biosphere.

A reassessment of the "hard-steps" model for the evolution of intelligent life

According to the "hard-steps" model, the origin of humanity required "successful passage through a number of intermediate steps" (so-called "hard steps") that were intrinsically improbable in the time available for biological evolution on Earth. This model similarly predicts that technological life analogous to human life on Earth is "exceedingly rare" in the Universe. Here, we critically reevaluate core assumptions of the hard-steps model through the lens of historical geobiology. Specifically, we propose an alternative model where there are no hard steps, and evolutionary singularities required for human origins can be explained via mechanisms outside of intrinsic improbability. Furthermore, if Earth's surface environment was initially inhospitable not only to human life, but also to certain key intermediate steps required for human existence, then the timing of human origins was controlled by the sequential opening of new global environmental windows of habitability over Earth history.

Low oxygen but dynamic marine redox conditions permitted the Cambrian Radiation

Whether metazoan diversification during the Cambrian Radiation was driven by increased marine oxygenation remains highly debated. Repeated global oceanic oxygenation events have been inferred during this interval, but the degree of shallow marine oxygenation and its relationship to biodiversification and clade appearance remain uncertain. To resolve this, we interrogate an interval from ~527 to 519 Ma, encompassing multiple proposed global oceanic oxygenation events. We integrate the spatial and temporal distribution of shallow water, in situ reef metazoans, and trilobites, with high-resolution multi-proxy redox data through the highly biodiverse Siberian Platform. We document primarily dysoxic water column conditions, suggesting that early Cambrian metazoans, including motile skeletal benthos, had low oxygen demands. We further document oxygenation events coincident with positive carbon isotope excursions that led to modestly elevated oxygen levels. These events correspond to regional increases in species richness and habitat expansion of mainly endemic species, offering a potentially globally applicable model for biodiversification during the Cambrian Radiation.

Eta-Earth Revisited II: Deriving a Maximum Number of Earth-Like Habitats in the Galactic Disk

In Lammer et al. (2024), we defined Earth-like habitats (EHs) as rocky exoplanets within the habitable zone of complex life (HZCL) on which Earth-like N2-O2-dominated atmospheres with minor amounts of CO2 can exist, and derived a formulation for estimating the maximum number of EHs in the galaxy given realistic probabilistic requirements that have to be met for an EH to evolve. In this study, we apply this formulation to the galactic disk by considering only requirements that are already scientifically quantifiable. By implementing literature models for star formation rate, initial mass function, and the mass distribution of the Milky Way, we calculate the spatial distribution of disk stars as functions of stellar mass and birth age. For the stellar part of our formulation, we apply existing models for the galactic habitable zone and evaluate the thermal stability of nitrogen-dominated atmospheres with different CO2 mixing ratios inside the HZCL by implementing the newest stellar evolution and upper atmosphere models. For the planetary part, we include the frequency of rocky exoplanets, the availability of surface water and subaerial land, and the potential requirement of hosting a large moon by evaluating their importance and implementing these criteria from minima to maxima values as found in the scientific literature. We also discuss further factors that are not yet scientifically quantifiable but may be requirements for EHs to evolve. Based on such an approach, we find that EHs are relatively rare by obtaining plausible maximum numbers of 2.5 - 2.4 + 71.6 × 10 5 and 0.6 - 0.59 + 27.1 × 10 5 planets that can potentially host N2-O2-dominated atmospheres with maximum CO2 mixing ratios of 10% and 1%, respectively, implying that, on average, a minimum of ∼ 10 3 - 10 6 rocky exoplanets in the HZCL are needed for 1 EH to evolve. The actual number of EHs, however, may be substantially lower than our maximum ranges since several requirements with unknown occurrence rates are not included in our model (e.g., the origin of life, working carbon-silicate and nitrogen cycles); this also implies extraterrestrial intelligence (ETI) to be significantly rarer still. Our results illustrate that not every star can host EHs nor can each rocky exoplanet within the HZCL evolve such that it might be able to host complex animal-like life or even ETIs. The Copernican Principle of Mediocrity therefore cannot be applied to infer that such life will be common in the galaxy.

Seawater barium and sulfide removal improved marine habitability for the Cambrian Explosion of early animals

An increase in atmospheric pO2 has been proposed as a trigger for the Cambrian Explosion at ∼539-514 Ma but the mechanistic linkage remains unclear. To gain insights into marine habitability for the Cambrian Explosion, we analysed excess Ba contents (Baexcess) and isotope compositions (δ138Baexcess) of ∼521-Myr-old metalliferous black shales in South China. The δ138Baexcess values vary within a large range and show a negative logarithmic correlation with Baexcess, suggesting a major (>99%) drawdown of oceanic Ba inventory via barite precipitation. Spatial variations in Baexcess and δ138Baexcess indicate that Ba removal was driven by sulfate availability that was ultimately derived from the upwelling of deep seawaters. Global oceanic oxygenation across the Ediacaran-Cambrian transition may have increased the sulfate reservoir via oxidation of sulfide and concurrently decreased the Ba reservoir by barite precipitation. The removal of both H2S and Ba that are deleterious to animals could have improved marine habitability for early animals.

Physical constraints during Snowball Earth drive the evolution of multicellularity

Molecular and fossil evidence suggests that complex eukaryotic multicellularity evolved during the late Neoproterozoic era, coincident with Snowball Earth glaciations, where ice sheets covered most of the globe. During this period, environmental conditions-such as seawater temperature and the availability of photosynthetically active light in the oceans-likely changed dramatically. Such changes would have had significant effects on both resource availability and optimal phenotypes. Here, we construct and apply mechanistic models to explore (i) how environmental changes during Snowball Earth and biophysical constraints generated selective pressures, and (ii) how these pressures may have had differential effects on organisms with different forms of biological organization. By testing a series of alternative-and commonly debated-hypotheses, we demonstrate how multicellularity was likely acquired differently in eukaryotes and prokaryotes owing to selective differences on their size due to the biophysical and metabolic regimes they inhabit: decreasing temperatures and resource availability instigated by the onset of glaciations generated selective pressures towards smaller sizes in organisms in the diffusive regime and towards larger sizes in motile heterotrophs. These results suggest that changing environmental conditions during Snowball Earth glaciations gave multicellular eukaryotes an evolutionary advantage, paving the way for the complex multicellular lineages that followed.

Late Paleozoic oxygenation of marine environments supported by dolomite U-Pb dating

Understanding causal relationships between evolution and ocean oxygenation hinges on reliable reconstructions of marine oxygen levels, typically from redox-sensitive geochemical proxies. Here, we develop a proxy, using dolomite U-Pb geochronology, to reconstruct seawater U/Pb ratios. Dolomite samples consistently give U-Pb dates and initial 207Pb/206Pb ratios lower than expected from their stratigraphic ages. These observations are explained by resetting of the U-Pb system long after deposition; the magnitude of deviations from expected initial 207Pb/206Pb are a function of the redox-sensitive U/Pb ratios during deposition. Reconstructed initial U/Pb ratios increased notably in the late-Paleozoic, reflecting an increase in oxygenation of marine environments at that time. This timeline is consistent with documented shifts in some other redox proxies and supports evolution-driven mechanisms for the oxygenation of late-Paleozoic marine environments, as well as suggestions that early animals thrived in oceans that on long time scales were oxygen-limited compared to today.

Inorganic Hydrogel Based on Low-Dimensional Nanomaterials

Composed of three-dimensional (3D) nanoscale inorganic bones and up to 99% water, inorganic hydrogels have attracted much attention and undergone significant growth in recent years. The basic units of inorganic hydrogels could be metal nanoparticles, metal nanowires, SiO2 nanowires, graphene nanosheets, and MXene nanosheets, which are then assembled into the special porous structures by the sol-gel process or gelation via either covalent or noncovalent interactions. The high electrical and thermal conductivity, resistance to corrosion, stability across various temperatures, and high surface area make them promising candidates for diverse applications, such as energy storage, catalysis, adsorption, sensing, and solar steam generation. Besides, some interesting derivatives, such as inorganic aerogels and xerogels, can be produced through further processing, diversifying their functionalities and application domains greatly. In this context, we primarily provide a comprehensive overview of the current status of inorganic hydrogels and their derivatives, including the structures of inorganic hydrogels with various compositions, their gelation mechanisms, and their exceptional practical performance in fields related to energy and environmental applications.

New keratose sponges after the end-Permian extinction provide insights into biotic recoveries

We challenge the prevailing view that the end-Permian extinction impeded the Triassic evolution of sponges. Here, we report a deep-water community dominated by abundant keratose sponges in the lowest Triassic strata from Southwest China. The sponge fossils occur as dark elliptical imprints in mudstone with distinct oscula on their tops. The structure of preserved fibers suggests closest affinity with the extant Dictyoceratida, an aspiculate demosponge. The exceptional preservation plays a crucial role in retaining their exquisite structures. Sedimentary, taphonomic, pyrite framboid, and trace elemental analyses indicate that the sponges proliferated in an oxygen-poor habitat, demonstrating the high tolerance of sponges to severe conditions. Sponge proliferation is a signal of environmental upheaval but they also stabilized the ecosystem, driving the first phase of biotic recovery after the end-Permian extinction.

Divergence time estimates for the hypoxia-inducible factor-1 alpha (HIF1α) reveal an ancient emergence of animals in low-oxygen environments

Unveiling the tempo and mode of animal evolution is necessary to understand the links between environmental changes and biological innovation. Although the earliest unambiguous metazoan fossils date to the late Ediacaran period, molecular clock estimates agree that the last common ancestor (LCA) of all extant animals emerged ~850 Ma, in the Tonian period, before the oldest evidence for widespread ocean oxygenation at ~635-560 Ma in the Ediacaran period. Metazoans are aerobic organisms, that is, they are dependent on oxygen to survive. In low-oxygen conditions, most animals have an evolutionarily conserved pathway for maintaining oxygen homeostasis that triggers physiological changes in gene expression via the hypoxia-inducible factor (HIFa). However, here we confirm the absence of the characteristic HIFa protein domain responsible for the oxygen sensing of HIFa in sponges and ctenophores, indicating the LCA of metazoans lacked the functional protein domain as well, and so could have maintained their transcription levels unaltered under the very low-oxygen concentrations of their environments. Using Bayesian relaxed molecular clock dating, we inferred that the ancestral gene lineage responsible for HIFa arose in the Mesoproterozoic Era, ~1273 Ma (Credibility Interval 957-1621 Ma), consistent with the idea that important genetic machinery associated with animals evolved much earlier than the LCA of animals. Our data suggest at least two duplication events in the evolutionary history of HIFa, which generated three vertebrate paralogs, products of the two successive whole-genome duplications that occurred in the vertebrate LCA. Overall, our results support the hypothesis of a pre-Tonian emergence of metazoans under low-oxygen conditions, and an increase in oxygen response elements during animal evolution.

Sponge functional roles in a changing world

Sponges are ecologically important benthic organisms with many important functional roles. However, despite increasing global interest in the functions that sponges perform, there has been limited focus on how such functions will be impacted by different anthropogenic stressors. In this review, we describe the progress that has been made in our understanding of the functional roles of sponges over the last 15 years and consider the impacts of anthropogenic stressors on these roles. We split sponge functional roles into interactions with the water column and associations with other organisms. We found evidence for an increasing focus on functional roles among sponge-focused research articles, with our understanding of sponge-mediated nutrient cycling increasing substantially in recent years. From the information available, many anthropogenic stressors have the potential to negatively impact sponge pumping, and therefore have the potential to cause ecosystem level impacts. While our understanding of the importance of sponges has increased in the last 15 years, much more experimental work is required to fully understand how sponges will contribute to reef ecosystem function in future changing oceans.

Respiration kinetics and allometric scaling in the demosponge Halichondria panicea

BACKGROUND

The aquiferous system in sponges represents one of the simplest circulatory systems used by animals for the internal uptake and distribution of oxygen and metabolic substrates. Its modular organization enables sponges to metabolically scale with size differently than animals with an internal circulatory system. In this case, metabolic rate is typically limited by surface to volume constraints to maintain an efficient supply of oxygen and food. Here, we consider the linkeage between oxygen concentration, the respiration rates of sponges and sponge size.

RESULTS

We explored respiration kinetics for individuals of the demosponge Halichondria panicea with varying numbers of aquiferous modules (nmodules = 1-102). From this work we establish relationships between the sponge size, module number, maximum respiration rate (Rmax) and the half-saturation constant, Km, which is the oxygen concentration producing half of the maximum respiration rate, Rmax. We found that the nmodules in H. panicea scales consistently with sponge volume (Vsp) and that Rmax increased with sponge size with a proportionality > 1. Conversly, we found a lack of correlation between Km and sponge body size suggesting that oxygen concentration does not control the size of sponges.

CONCLUSIONS

The present study reveals that the addition of aquiferous modules (with a mean volume of 1.59 ± 0.22 mL) enables H. panicea in particular, and likely demosponges in general, to grow far beyond constraints limiting the size of their component modules and independent of ambient oxygen levels.

Recurrent photic zone euxinia limited ocean oxygenation and animal evolution during the Ediacaran

The Ediacaran Period (~635-539 Ma) is marked by the emergence and diversification of complex metazoans linked to ocean redox changes, but the processes and mechanism of the redox evolution in the Ediacaran ocean are intensely debated. Here we use mercury isotope compositions from multiple black shale sections of the Doushantuo Formation in South China to reconstruct Ediacaran oceanic redox conditions. Mercury isotopes show compelling evidence for recurrent and spatially dynamic photic zone euxinia (PZE) on the continental margin of South China during time intervals coincident with previously identified ocean oxygenation events. We suggest that PZE was driven by increased availability of sulfate and nutrients from a transiently oxygenated ocean, but PZE may have also initiated negative feedbacks that inhibited oxygen production by promoting anoxygenic photosynthesis and limiting the habitable space for eukaryotes, hence abating the long-term rise of oxygen and restricting the Ediacaran expansion of macroscopic oxygen-demanding animals.

Animal survival strategies in Neoproterozoic ice worlds

The timing of the first appearance of animals is of crucial importance for understanding the evolution of life on Earth. Although the fossil record places the earliest metazoans at 572-602 Ma, molecular clock studies suggest a far earlier origination, as far back as ~850 Ma. The difference in these dates would place the rise of animal life into a time period punctuated by multiple colossal, potentially global, glacial events. Although the two schools of thought debate the limitations of each other's methods, little time has been dedicated to how animal life might have survived if it did arise before or during these global glacial periods. The history of recent polar biota shows that organisms have found ways of persisting on and around the ice of the Antarctic continent throughout the Last Glacial Maximum (33-14 Ka), with some endemic species present before the breakup of Gondwana (180-23 Ma). Here we discuss the survival strategies and habitats of modern polar marine organisms in environments analogous to those that could have existed during Neoproterozoic glaciations. We discuss how, despite the apparent harshness of many ice covered, sub-zero, Antarctic marine habitats, animal life thrives on, in and under the ice. Ice dominated systems and processes make some local environments more habitable through water circulation, oxygenation, terrigenous nutrient input and novel habitats. We consider how the physical conditions of Neoproterozoic glaciations would likely have dramatically impacted conditions for potential life in the shallows and erased any possible fossil evidence from the continental shelves. The recent glacial cycle has driven the evolution of Antarctica's unique fauna by acting as a "diversity pump," and the same could be true for the late Proterozoic and the evolution of animal life on Earth, and the existence of life elsewhere in the universe on icy worlds or moons.

A diverse Ediacara assemblage survived under low-oxygen conditions

The Ediacaran biota were soft-bodied organisms, many with enigmatic phylogenetic placement and ecology, living in marine environments between 574 and 539 million years ago. Some studies hypothesize a metazoan affinity and aerobic metabolism for these taxa, whereas others propose a fundamentally separate taxonomic grouping and a reliance on chemoautotrophy. To distinguish between these hypotheses and test the redox-sensitivity of Ediacaran organisms, here we present a high-resolution local and global redox dataset from carbonates that contain in situ Ediacaran fossils from Siberia. Cerium anomalies are consistently >1, indicating that local environments, where a diverse Ediacaran assemblage is preserved in situ as nodules and carbonaceous compressions, were pervasively anoxic. Additionally, δ²³⁸U values match other terminal Ediacaran sections, indicating widespread marine euxinia. These data suggest that some Ediacaran biotas were tolerant of at least intermittent anoxia, and thus had the capacity for a facultatively anaerobic lifestyle. Alternatively, these soft-bodied Ediacara organisms may have colonized the seafloor during brief oxygenation events not recorded by redox proxy data. Broad temporal correlations between carbon, sulfur, and uranium isotopes further highlight the dynamic redox landscape of Ediacaran-Cambrian evolutionary events.

Breathless through Time: Oxygen and Animals across Earth's History

AbstractOxygen levels in the atmosphere and ocean have changed dramatically over Earth history, with major impacts on marine life. Because the early part of Earth's history lacked both atmospheric oxygen and animals, a persistent co-evolutionary narrative has developed linking oxygen change with changes in animal diversity. Although it was long believed that oxygen rose to essentially modern levels around the Cambrian period, a more muted increase is now believed likely. Thus, if oxygen increase facilitated the Cambrian explosion, it did so by crossing critical ecological thresholds at low O₂. Atmospheric oxygen likely remained at low or moderate levels through the early Paleozoic era, and this likely contributed to high metazoan extinction rates until oxygen finally rose to modern levels in the later Paleozoic. After this point, ocean deoxygenation (and marine mass extinctions) is increasingly linked to large igneous province eruptions-massive volcanic carbon inputs to the Earth system that caused global warming, ocean acidification, and oxygen loss. Although the timescales of these ancient events limit their utility as exact analogs for modern anthropogenic global change, the clear message from the geologic record is that large and rapid CO₂ injections into the Earth system consistently cause the same deadly trio of stressors that are observed today. The next frontier in understanding the impact of oxygen changes (or, more broadly, temperature-dependent hypoxia) in deep time requires approaches from ecophysiology that will help conservation biologists better calibrate the response of the biosphere at large taxonomic, spatial, and temporal scales.

Time Domains of Hypoxia Responses and -Omics Insights

The ability to respond rapidly to changes in oxygen tension is critical for many forms of life. Challenges to oxygen homeostasis, specifically in the contexts of evolutionary biology and biomedicine, provide important insights into mechanisms of hypoxia adaptation and tolerance. Here we synthesize findings across varying time domains of hypoxia in terms of oxygen delivery, ranging from early animal to modern human evolution and examine the potential impacts of environmental and clinical challenges through emerging multi-omics approaches. We discuss how diverse animal species have adapted to hypoxic environments, how humans vary in their responses to hypoxia (i.e., in the context of high-altitude exposure, cardiopulmonary disease, and sleep apnea), and how findings from each of these fields inform the other and lead to promising new directions in basic and clinical hypoxia research.

Adaptive specialization of a unique sponge body from the Cambrian Qingjiang biota

Sponge fossils from the Cambrian black shales have attracted attention from both palaeontologists and geochemists for many years in terms of their high diversity, beautiful preservation and perplexing adaptation to inhospitable living environments. However, the body shape of these sponges, which contributes to deciphering adaptive evolution, has not been scrutinized. New complete specimens of the hexactinellid sponge Sanshapentella tentoriformis sp. nov. from the Qingjiang biota (black shale of the Cambrian Stage 3 Shuijingtuo Formation, ca 518 Ma) allow recognition of a unique dendriform body characterized by a columnar trunk with multiple conical high peaks and distinctive quadripod-shaped dermal spicules that frame each high peak. The body shape of this new sponge along with other early Cambrian hexactinellids, is classified into three morpho-groups that reflect different levels of adaptivity to the environment. The cylindrical and ovoid bodies generally adapted to a large spectrum of environments; however, the dendriform body of S. tentoriformis was restricted to the relatively deep-water, oxygen-deficient environment. From a hindsight view, the unique body shape represents a consequence of adaptation that helps maintain an effective use of oxygen and a low energy cost in hypoxic conditions.

Disentangling compartment functions in sessile marine invertebrates

Sessile invertebrates are frequently sampled and processed whole for downstream analyses. However, their apparent structural simplicity is deceptive as these organisms often harbour discrete compartments. These compartments have physicochemical conditions that differ markedly from neighbouring tissues, and that have likely evolved to support specific functions. Here, we argue that such compartments should be specifically targeted when characterising sessile invertebrate biology and we use the coral gastrovascular cavity to support our argument. This complex compartment displays steep and dynamic chemical gradients, harbours distinct microorganisms, and presumably plays a key role in coral biology. Disentangling the functions played by (and amongst) compartments will likely provide transformative insight into the biology of sessile invertebrates and their future under environmental change.

Molecular dating of the blood pigment hemocyanin provides new insight into the origin of animals

The Neoproterozoic included changes in oceanic redox conditions, the configuration of continents and climate, extreme ice ages (Sturtian and Marinoan), and the rise of complex life forms. A much-debated topic in geobiology concerns the influence of atmospheric oxygenation on Earth and the origin and diversification of animal lineages, with the most widely popularized hypotheses relying on causal links between oxygen levels and the rise of animals. The vast majority of extant animals use aerobic metabolism for growth and homeostasis; hence, the binding and transportation of oxygen represent a vital physiological task. Considering the blood pigment hemocyanin (Hc) is present in sponges and ctenophores, and likely to be present in the common ancestor of animals, we investigated the evolution and date of Hc emergence using bioinformatics approaches on both transcriptomic and genomic data. Bayesian molecular dating suggested that the ancestral animal Hc gene arose approximately 881 Ma during the Tonian Period (1000-720 Ma), prior to the extreme glaciation events of the Cryogenian Period (720-635 Ma). This result is corroborated by a recently discovered fossil of a putative sponge ~890 Ma and modern molecular dating for the origin of metazoans of ~1,000-650 Ma (but does contradict previous inferences regarding the origin of Hc ~700-600 Ma). Our data reveal that crown-group animals already possessed hemocyanin-like blood pigments, which may have enhanced the oxygen-carrying capacity of these animals in hypoxic environments at that time or acted in the transport of hormones, detoxification of heavy metals, and immunity pathways.

Actual and Model-Predicted Growth of Sponges—With a Bioenergetic Comparison to Other Filter-Feeders

Sponges are one of the earliest-evolved and simplest groups of animals, but they share basic characteristics with more advanced and later-evolved filter-feeding invertebrates, such as mussels. Sponges are abundant in many coastal regions where they filter large amounts of water for food particles and thus play an important ecological role. Therefore, a better understanding of the bioenergetics and growth of sponges compared to other filter-feeders is important. While the filtration (pumping) rates of many sponge species have been measured as a function of their size, little is known about their rate of growth. Here, we use a bioenergetic growth model for demosponges, based on the energy budget and observations of filtration (F) and respiration rates (R). Because F versus dry weight (W) can be expressed as F = a1Wb1 and the maintenance respiratory rate can be expressed as Rm = a2Wb2, we show that if b1~ b2the growth rate can be expressed as: G = aWb1, and, consequently, the weight-specific growth rate is µ = G/W = aWb1−1 = aWb where the constant a depends on ambient sponge-available food particles (free-living bacteria and phytoplankton with diameter < ostia diameter). Because the exponent b1 is close to 1, then b ~ 0, which implies µ = a and thus exponential growth as confirmed in field growth studies. Exponential growth in sponges and in at least some bryozoans is probably unique among filter-feeding invertebrates. Finally, we show that the F/R-ratio and the derived oxygen extraction efficiency in these sponges are similar to other filter-feeding invertebrates, thus reflecting a comparable adaptation to feeding on a thin suspension of bacteria and phytoplankton.

Metagenomic data for Halichondria panicea from Illumina and nanopore sequencing and preliminary genome assemblies for the sponge and two microbial symbionts

OBJECTIVES

These data were collected to generate a novel reference metagenome for the sponge Halichondria panicea and its microbiome for subsequent differential expression analyses.

DATA DESCRIPTION

These data include raw sequences from four separate sequencing runs of the metagenome of a single individual of Halichondria panicea-one Illumina MiSeq (2 × 300 bp, paired-end) run and three Oxford Nanopore Technologies (ONT) long-read sequencing runs, generating 53.8 and 7.42 Gbp respectively. Comparing assemblies of Illumina, ONT and an Illumina-ONT hybrid revealed the hybrid to be the 'best' assembly, comprising 163 Mbp in 63,555 scaffolds (N50: 3084). This assembly, however, was still highly fragmented and only contained 52% of core metazoan genes (with 77.9% partial genes), so it was also not complete. However, this sponge is an emerging model species for field and laboratory work, and there is considerable interest in genomic sequencing of this species. Although the resultant assemblies from the data presented here are suboptimal, this data note can inform future studies by providing an estimated genome size and coverage requirements for future sequencing, sharing additional data to potentially improve other suboptimal assemblies of this species, and outlining potential limitations and pitfalls of the combined Illumina and ONT approach to novel genome sequencing.

Marine sponges in the once and future ocean

Animals originated under hypoxia-low-oxygen conditions that are currently expanding throughout the global ocean. How marine sponges respond to hypoxia is both relevant to reconstructing early animal evolution and for forecasting the fate of modern marine ecosystems. In a new effort, multiple sponge species from two different oceans were found to tolerate hypoxia in the lab, revealing a more general capacity for hypoxia tolerance across sponges with implications for both the deep past and near future of animal life.

Adaptive strategies of sponges to deoxygenated oceans

Ocean deoxygenation is one of the major consequences of climate change. In coastal waters, this process can be exacerbated by eutrophication, which is contributing to an alarming increase in the so-called 'dead zones' globally. Despite its severity, the effect of reduced dissolved oxygen has only been studied for a very limited number of organisms, compared to other climate change impacts such as ocean acidification and warming. Here, we experimentally assessed the response of sponges to moderate and severe simulated hypoxic events. We ran three laboratory experiments on four species from two different temperate oceans (NE Atlantic and SW Pacific). Sponges were exposed to a total of five hypoxic treatments, with increasing severity (3.3, 1.6, 0.5, 0.4 and 0.13 mg O₂  L^-1 , over 7-12-days). We found that sponges are generally very tolerant of hypoxia. All the sponges survived in the experimental conditions, except Polymastia crocea, which showed significant mortality at the lowest oxygen concentration (0.13 mg O₂  L^-1 , lethal median time: 286 h). In all species except Suberites carnosus, hypoxic conditions do not significantly affect respiration rate down to 0.4 mg O₂  L^-1 , showing that sponges can uptake oxygen at very low concentrations in the surrounding environment. Importantly, sponges displayed species-specific phenotypic modifications in response to the hypoxic treatments, including physiological, morphological and behavioural changes. This phenotypic plasticity likely represents an adaptive strategy to live in reduced or low oxygen water. Our results also show that a single sponge species (i.e., Suberites australiensis) can display different strategies at different oxygen concentrations. Compared to other sessile organisms, sponges generally showed higher tolerance to hypoxia, suggesting that sponges could be favoured and survive in future deoxygenated oceans.

A revised lower estimate of ozone columns during Earth's oxygenated history

The history of molecular oxygen (O₂) in Earth's atmosphere is still debated; however, geological evidence supports at least two major episodes where O₂ increased by an order of magnitude or more: the Great Oxidation Event (GOE) and the Neoproterozoic Oxidation Event. O₂ concentrations have likely fluctuated (between 10^-3 and 1.5 times the present atmospheric level) since the GOE ∼2.4 Gyr ago, resulting in a time-varying ozone (O₃) layer. Using a three-dimensional chemistry-climate model, we simulate changes in O₃ in Earth's atmosphere since the GOE and consider the implications for surface habitability, and glaciation during the Mesoproterozoic. We find lower O₃ columns (reduced by up to 4.68 times for a given O₂ level) compared to previous work; hence, higher fluxes of biologically harmful UV radiation would have reached the surface. Reduced O₃ leads to enhanced tropospheric production of the hydroxyl radical (OH) which then substantially reduces the lifetime of methane (CH₄). We show that a CH₄ supported greenhouse effect during the Mesoproterozoic is highly unlikely. The reduced O₃ columns we simulate have important implications for astrobiological and terrestrial habitability, demonstrating the relevance of three-dimensional chemistry-climate simulations when assessing paleoclimates and the habitability of faraway worlds.

Triple oxygen isotope constraints on atmospheric O2 and biological productivity during the mid-Proterozoic

Reconstructing the history of biological productivity and atmospheric oxygen partial pressure (pO₂) is a fundamental goal of geobiology. Recently, the mass-independent fractionation of oxygen isotopes (O-MIF) has been used as a tool for estimating pO₂ and productivity during the Proterozoic. O-MIF, reported as Δ'¹⁷O, is produced during the formation of ozone and destroyed by isotopic exchange with water by biological and chemical processes. Atmospheric O-MIF can be preserved in the geologic record when pyrite (FeS₂) is oxidized during weathering, and the sulfur is redeposited as sulfate. Here, sedimentary sulfates from the ∼1.4-Ga Sibley Formation are reanalyzed using a detailed one-dimensional photochemical model that includes physical constraints on air-sea gas exchange. Previous analyses of these data concluded that pO₂ at that time was <1% PAL (times the present atmospheric level). Our model shows that the upper limit on pO₂ is essentially unconstrained by these data. Indeed, pO₂ levels below 0.8% PAL are possible only if atmospheric methane was more abundant than today (so that pCO₂ could have been lower) or if the Sibley O-MIF data were diluted by reprocessing before the sulfates were deposited. Our model also shows that, contrary to previous assertions, marine productivity cannot be reliably constrained by the O-MIF data because the exchange of molecular oxygen (O₂) between the atmosphere and surface ocean is controlled more by air-sea gas transfer rates than by biological productivity. Improved estimates of pCO₂ and/or improved proxies for Δ'¹⁷O of atmospheric O₂ would allow tighter constraints to be placed on mid-Proterozoic pO₂.

HIF-1α Hydroxyprolines Modulate Oxygen-Dependent Protein Stability Via Single VHL Interface With Comparable Effect on Ubiquitination Rate

The basic molecular mechanism underlying mammalian oxygen-dependent regulation of hypoxia-inducible factor (HIF) via the von Hippel-Lindau E3 ubiquitin ligase is well established. The principal step in this critical cellular process is the hydroxylation of either or both of the two conserved proline residues P402 and P564 within the oxygen-dependent degradation domain (ODD) of HIF-1α subunit via prolyl hydroxylases, which is necessary for binding VHL. However, the significance of the two prolines has remained unclear considering that only one hydroxyproline is sufficient for the recruitment of VHL. Here, we show using biophysical analyses that both hydroxyprolines bind to the same interface on VHL with similar affinity; VHL binding affinity to HIF-1α ODD remains relatively unchanged regardless of whether the ODD contains one or two hydroxyprolines; ODD with two hydroxyprolines can accommodate two VHLs; and the rate of in vitro ubiquitination of ODD with one hydroxyproline via VHL E3 ligase is comparable to the rate observed with ODD containing two hydroxyprolines. However, the two hydroxyprolines show distinct contributions to the intracellular stability of HIF-1α ODD. These results demonstrate for the first time that the graduated HIF-1α stability profile observed over a range of oxygen tension is not attributed to the binding of or ubiquitination via VHL per se, but is likely due to the preceding events such as the efficacy of oxygen-dependent prolyl hydroxylase-mediated hydroxylation of HIF-1α.

Adaptation to a Viscous Snowball Earth Ocean as a Path to Complex Multicellularity

AbstractAnimals, fungi, and algae with complex multicellular bodies all evolved independently from unicellular ancestors. The early history of these major eukaryotic multicellular clades, if not their origins, co-occur with an extreme phase of global glaciations known as the Snowball Earth. Here, I propose that the long-term loss of low-viscosity environments due to several rounds global glaciation drove the multiple origins of complex multicellularity in eukaryotes and the subsequent radiation of complex multicellular groups into previously unoccupied niches. In this scenario, life adapts to Snowball Earth oceans by evolving large size and faster speeds through multicellularity, which acts to compensate for high-viscosity seawater and achieve fluid flow at sufficient levels to satisfy metabolic needs. Warm, low-viscosity seawater returned with the melting of the Snowball glaciers, and with it, by virtue of large and fast multicellular bodies, new ways of life were unveiled.

Vulnerability of Temperate Mesophotic Ecosystems (TMEs) to environmental impacts: Rapid ecosystem changes at Lough Hyne Marine Nature Reserve, Ireland

Temperate Mesophotic Ecosystems (TMEs) are stable habitats, usually dominated by slow-growing, long-lived sessile invertebrates and sciaphilous algae. Organisms inhabiting TMEs can form complex three-dimensional structures and support many commercially important species. However, TMEs have been poorly studied, with little known about their vulnerability to environmental impacts. Lough Hyne Marine Nature Reserve (Ireland) supports TMEs in shallower waters (12-40 m) compared with other locations (30-150+ m) as a result of the unusual hydrodynamic conditions. Here, we report changes that have occurred on the sponge-dominated cliffs at Lough Hyne between 1990 and 2019, providing insights into TME long-term stability and vulnerability to environmental impacts. Our main finding was a marked decline in most three-dimensional sponges at the internal sites of the lough. This was likely the result of one or more mass mortality events that occurred between 2010 and 2015. We also found an increase in ascidians, which might have been more tolerant and benefited from the space freed by the sponge mortality. Finally, in the most recent surveys, we found a high abundance of sponge recruits, indicating that a natural recovery may be underway. The possible factors involved in these community changes include eutrophication, increased temperature, and a toxic event due to an anomaly in the oxycline breakdown. However, the absence of comprehensive monitoring of biotic and abiotic variables makes it impossible to identify the cause with certainty. Our Lough Hyne example shows the potential vulnerability of TMEs to short-term disturbance events, highlighting the importance of monitoring these habitats globally to ensure they are appropriately conserved.

Cell biology of photosynthesis over geologic time

Flamholz and Shih explain how photosynthetic organisms on earth have evolved carbon dioxide concentrating mechanisms to contend with an increased abundance of oxygen in the atmosphere. This critical innovation has occurred numerous times and comes in two basic flavors.

Evolution of a key enzyme of aerobic metabolism reveals Proterozoic functional subunit duplication events and an ancient origin of animals

The biological toolkits for aerobic respiration were critical for the rise and diversification of early animals. Aerobic life forms generate ATP through the oxidation of organic molecules in a process known as Krebs' Cycle, where the enzyme isocitrate dehydrogenase (IDH) regulates the cycle's turnover rate. Evolutionary reconstructions and molecular dating of proteins related to oxidative metabolism, such as IDH, can therefore provide an estimate of when the diversification of major taxa occurred, and their coevolution with the oxidative state of oceans and atmosphere. To establish the evolutionary history and divergence time of NAD-dependent IDH, we examined transcriptomic data from 195 eukaryotes (mostly animals). We demonstrate that two duplication events occurred in the evolutionary history of NAD-IDH, one in the ancestor of eukaryotes approximately at 1967 Ma, and another at 1629 Ma, both in the Paleoproterozoic Era. Moreover, NAD-IDH regulatory subunits β and γ are exclusive to metazoans, arising in the Mesoproterozoic. Our results therefore support the concept of an ''earlier-than-Tonian'' diversification of eukaryotes and the pre-Cryogenian emergence of a metazoan IDH enzyme.

Oxygenation, Life, and the Planetary System during Earth's Middle History: An Overview

The long history of life on Earth has unfolded as a cause-and-effect relationship with the evolving amount of oxygen (O₂) in the oceans and atmosphere. Oxygen deficiency characterized our planet's first 2 billion years, yet evidence for biological O₂ production and local enrichments in the surface ocean appear long before the first accumulations of O₂ in the atmosphere roughly 2.4 to 2.3 billion years ago. Much has been written about this fundamental transition and the related balance between biological O₂ production and sinks coupled to deep Earth processes that could buffer against the accumulation of biogenic O₂. However, the relationship between complex life (eukaryotes, including animals) and later oxygenation is less clear. Some data suggest O₂ was higher but still mostly low for another billion and a half years before increasing again around 800 million years ago, potentially setting a challenging course for complex life during its initial development and ecological expansion. The apparent rise in O₂ around 800 million years ago is coincident with major developments in complex life. Multiple geochemical and paleontological records point to a major biogeochemical transition at that time, but whether rising and still dynamic biospheric oxygen triggered or merely followed from innovations in eukaryotic ecology, including the emergence of animals, is still debated. This paper focuses on the geochemical records of Earth's middle history, roughly 1.8 to 0.5 billion years ago, as a backdrop for exploring possible cause-and-effect relationships with biological evolution and the primary controls that may have set its pace, including solid Earth/tectonic processes, nutrient limitation, and their possible linkages. A richer mechanistic understanding of the interplay between coevolving life and Earth surface environments can provide a template for understanding and remotely searching for sustained habitability and even life on distant exoplanets.

Possible poriferan body fossils in early Neoproterozoic microbial reefs

Molecular phylogeny indicates that metazoans (animals) emerged early in the Neoproterozoic era¹, but physical evidence is lacking. The search for animal fossils from the Proterozoic eon is hampered by uncertainty about what physical characteristics to expect. Sponges are the most basic known animal type^2,3; it is possible that body fossils of hitherto-undiscovered Proterozoic metazoans might resemble aspect(s) of Phanerozoic fossil sponges. Vermiform microstructure^4,5, a complex petrographic feature in Phanerozoic reefal and microbial carbonates, is now known to be the body fossil of nonspicular keratosan demosponges^6-10. This Article presents petrographically identical vermiform microstructure from approximately 890-million-year-old reefs. The millimetric-to-centimetric vermiform-microstructured organism lived only on, in and immediately beside reefs built by calcifying cyanobacteria (photosynthesizers), and occupied microniches in which these calcimicrobes could not live. If vermiform microstructure is in fact the fossilized tissue of keratose sponges, the material described here would represent the oldest body-fossil evidence of animals known to date, and would provide the first physical evidence that animals emerged before the Neoproterozoic oxygenation event and survived through the glacial episodes of the Cryogenian period.

Carbon cycle inverse modeling suggests large changes in fractional organic burial are consistent with the carbon isotope record and may have contributed to the rise of oxygen

Abundant geologic evidence shows that atmospheric oxygen levels were negligible until the Great Oxidation Event (GOE) at 2.4-2.1 Ga. The burial of organic matter is balanced by the release of oxygen, and if the release rate exceeds efficient oxygen sinks, atmospheric oxygen can accumulate until limited by oxidative weathering. The organic burial rate relative to the total carbon burial rate can be inferred from the carbon isotope record in sedimentary carbonates and organic matter, which provides a proxy for the oxygen source flux through time. Because there are no large secular trends in the carbon isotope record over time, it is commonly assumed that the oxygen source flux changed only modestly. Therefore, declines in oxygen sinks have been used to explain the GOE. However, the average isotopic value of carbon fluxes into the atmosphere-ocean system can evolve due to changing proportions of weathering and outgassing inputs. If so, large secular changes in organic burial would be possible despite unchanging carbon isotope values in sedimentary rocks. Here, we present an inverse analysis using a self-consistent carbon cycle model to determine the maximum change in organic burial since ~4 Ga allowed by the carbon isotope record and other geological proxies. We find that fractional organic burial may have increased by 2-5 times since the Archean. This happens because O₂ -dependent continental weathering of ¹³ C-depleted organics changes carbon isotope inputs to the atmosphere-ocean system. This increase in relative organic burial is consistent with an anoxic-to-oxic atmospheric transition around 2.4 Ga without declining oxygen sinks, although these likely contributed. Moreover, our inverse analysis suggests that the Archean absolute organic burial flux was comparable to modern, implying high organic burial efficiency and ruling out very low Archean primary productivity.

Petrographic carbon in ancient sediments constrains Proterozoic Era atmospheric oxygen levels

Oxygen concentration defines the chemical structure of Earth's ecosystems while it also fuels the metabolism of aerobic organisms. As different aerobes have different oxygen requirements, the evolution of oxygen levels through time has likely impacted both environmental chemistry and the history of life. Understanding the relationship between atmospheric oxygen levels, the chemical environment, and life, however, is hampered by uncertainties in the history of oxygen levels. We report over 5,700 Raman analyses of organic matter from nine geological formations spanning in time from 742 to 1,729 Ma. We find that organic matter was effectively oxidized during weathering and little was recycled into marine sediments. Indeed, during this time interval, organic matter was as efficiently oxidized during weathering as it is now. From these observations, we constrain minimum atmospheric oxygen levels to between 2 to 24% of present levels from the late Paleoproterozoic Era into the Neoproterozoic Era. Indeed, our results reveal that eukaryote evolution, including early animal evolution, was not likely hindered by oxygen through this time interval. Our results also show that due to efficient organic recycling during weathering, carbon cycle dynamics can be assessed directly from the sediment carbon record.

Oxygen suppression of macroscopic multicellularity

Atmospheric oxygen is thought to have played a vital role in the evolution of large, complex multicellular organisms. Challenging the prevailing theory, we show that the transition from an anaerobic to an aerobic world can strongly suppress the evolution of macroscopic multicellularity. Here we select for increased size in multicellular 'snowflake' yeast across a range of metabolically-available O2 levels. While yeast under anaerobic and high-O2 conditions evolved to be considerably larger, intermediate O2 constrained the evolution of large size. Through sequencing and synthetic strain construction, we confirm that this is due to O2-mediated divergent selection acting on organism size. We show via mathematical modeling that our results stem from nearly universal evolutionary and biophysical trade-offs, and thus should apply broadly. These results highlight the fact that oxygen is a double-edged sword: while it provides significant metabolic advantages, selection for efficient use of this resource may paradoxically suppress the evolution of macroscopic multicellular organisms.

The origin of animals: an ancestral reconstruction of the unicellular-to-multicellular transition

How animals evolved from a single-celled ancestor, transitioning from a unicellular lifestyle to a coordinated multicellular entity, remains a fascinating question. Key events in this transition involved the emergence of processes related to cell adhesion, cell–cell communication and gene regulation. To understand how these capacities evolved, we need to reconstruct the features of both the last common multicellular ancestor of animals and the last unicellular ancestor of animals. In this review, we summarize recent advances in the characterization of these ancestors, inferred by comparative genomic analyses between the earliest branching animals and those radiating later, and between animals and their closest unicellular relatives. We also provide an updated hypothesis regarding the transition to animal multicellularity, which was likely gradual and involved the use of gene regulatory mechanisms in the emergence of early developmental and morphogenetic plans. Finally, we discuss some new avenues of research that will complement these studies in the coming years.

Effects of Seasonal Anoxia on the Microbial Community Structure in Demosponges in a Marine Lake in Lough Hyne, Ireland

Climate change is expanding marine oxygen minimum zones (OMZs), while anthropogenic nutrient input depletes oxygen concentrations locally. The effects of deoxygenation on animals are generally detrimental; however, some sponges (Porifera) exhibit hypoxic and anoxic tolerance through currently unknown mechanisms. Sponges harbor highly specific microbiomes, which can include microbes with anaerobic capabilities. Sponge-microbe symbioses must also have persisted through multiple anoxic/hypoxic periods throughout Earth's history. Since sponges lack key components of the hypoxia-inducible factor (HIF) pathway responsible for hypoxic responses in other animals, it was hypothesized that sponge tolerance to deoxygenation may be facilitated by its microbiome. To test this hypothesis, we determined the microbial composition of sponge species tolerating seasonal anoxia and hypoxia in situ in a semienclosed marine lake, using 16S rRNA amplicon sequencing. We discovered a high degree of cryptic diversity among sponge species tolerating seasonal deoxygenation, including at least nine encrusting species of the orders Axinellida and Poecilosclerida. Despite significant changes in microbial community structure in the water, sponge microbiomes were species specific and remarkably stable under varied oxygen conditions, which was further explored for Eurypon spp. 2 and Hymeraphia stellifera However, some symbiont sharing occurred under anoxia. At least three symbiont combinations, all including large populations of Thaumarchaeota, corresponded with deoxygenation tolerance, and some combinations were shared between some distantly related hosts. We propose hypothetical host-symbiont interactions following deoxygenation that could confer deoxygenation tolerance.IMPORTANCE The oceans have an uncertain future due to anthropogenic stressors and an uncertain past that is becoming clearer with advances in biogeochemistry. Both past and future oceans were, or will be, deoxygenated in comparison to present conditions. Studying how sponges and their associated microbes tolerate deoxygenation provides insights into future marine ecosystems. Moreover, sponges form the earliest branch of the animal evolutionary tree, and they likely resemble some of the first animals. We determined the effects of variable environmental oxygen concentrations on the microbial communities of several demosponge species during seasonal anoxia in the field. Our results indicate that anoxic tolerance in some sponges may depend on their symbionts, but anoxic tolerance was not universal in sponges. Therefore, some sponge species could likely outcompete benthic organisms like corals in future, reduced-oxygen ecosystems. Our results support the molecular evidence that sponges and other animals have a Neoproterozoic origin and that animal evolution was not limited by low-oxygen conditions.

The redox structure of Ediacaran and early Cambrian oceans and its controls

The rapid diversification of early animals during the Ediacaran (635-541 Ma) and early Cambrian (ca. 541-509 Ma) has frequently been attributed to increasing oceanic oxygenation. However, the pattern of oceanic oxygenation and its relationship to early animal evolution remain in debate. In this review, we examine the redox structure of Ediacaran and early Cambrian oceans and its controls, offering new insights into contemporaneous oceanic oxygenation patterns and their role in the coevolution of environments and early animals. We review the development of marine redox models which, in combination with independent distal deep-ocean redox proxies, supports a highly redox-stratified shelf and an anoxia-dominated deep ocean during the Ediacaran and early Cambrian. Geochemical and modeling evidence indicates that the marine redox structure was likely controlled by low atmospheric O₂ levels and low seawater vertical mixing rates on shelves at that time. Furthermore, theoretical analysis and increasing geochemical evidence, particularly from South China, show that limited sulfate availability was a primary control on the attenuation of mid-depth euxinia offshore, in contrast to the existing paradigm invoking decreased organic carbon fluxes distally. In light of our review, we infer that if oceanic oxygenation indeed triggered the rise of early animals, it must have done so through a shelf oxygenation which was probably driven by elevated oxidant availability. Our review calls for further studies on Ediacaran-Cambrian marine redox structure and its controls, particularly from regions outside of South China, in order to better understand the coevolutionary relationship between oceanic redox and early animals.

Algal origin of sponge sterane biomarkers negates the oldest evidence for animals in the rock record

The earliest fossils of animal-like organisms occur in Ediacaran rocks that are approximately 571 million years old. Yet 24-isopropylcholestanes and other C₃₀ fossil sterol molecules have been suggested to reflect an important ecological role of demosponges as the first abundant animals by the end of the Cryogenian period (>635 million years ago). Here, we demonstrate that C₃₀ 24-isopropylcholestane is not diagnostic for sponges and probably formed in Neoproterozoic sediments through the geological methylation of C₂₉ sterols of chlorophyte algae, the dominant eukaryotes at that time. These findings reconcile biomarker evidence with the geological record and revert the oldest evidence for animals back into the latest Ediacaran.

The origin and rise of complex life: progress requires interdisciplinary integration and hypothesis testing

Understanding of the triggers and timing of the rise of complex life ca 2100 to 720 million years ago has expanded dramatically in recent years. This theme issue brings together diverse and novel geochemical and palaeontological data presented as part of the Royal Society ‘The origin and rise of complex life: integrating models , geochemical and palaeontological data ’ discussion meeting held in September 2019. The individual papers offer prescient insights from multiple disciplines. Here we summarize their contribution towards the goal of the meeting; to create testable hypotheses for the differing roles of changing climate, oceanic redox, nutrient availability, and ecosystem feedbacks across this profound, but enigmatic, transitional period.

On the use of models in understanding the rise of complex life

Recently, several seemingly irreconcilably different models have been proposed for relationships between Earth system processes and the rise of complex life. These models provide very different scenarios of Proterozoic atmospheric oxygen and ocean nutrient levels, whether they constrained complex life, and of how the rise of complex life affected biogeochemical conditions. For non-modellers, it can be hard to evaluate which-if any-of the models and their results have more credence-hence this article. I briefly review relevant hypotheses, how models are being used to incarnate and sometimes test those hypotheses, and key principles of biogeochemical cycling models should embody. Then I critically review the use of biogeochemical models in: inferring key variables from proxies; reconstructing ancient biogeochemical cycling; and examining how complex life affected biogeochemical cycling. Problems are found in published model results purporting to demonstrate long-term stable states of very low Proterozoic atmospheric pO2 and ocean P levels. I explain what they stem from and highlight key empirical uncertainties that need to be resolved. Then I suggest how models and data can be better combined to advance our scientific understanding of the relationship between Earth system processes and the rise of complex life.

Reconciling proxy records and models of Earth's oxygenation during the Neoproterozoic and Palaeozoic

A hypothesized rise in oxygen levels in the Neoproterozoic, dubbed the Neoproterozoic Oxygenation Event, has been repeatedly linked to the origin and rise of animal life. However, a new body of work has emerged over the past decade that questions this narrative. We explore available proxy records of atmospheric and marine oxygenation and, considering the unique systematics of each geochemical system, attempt to reconcile the data. We also present new results from a comprehensive COPSE biogeochemical model that combines several recent additions, to create a continuous model record from 850 to 250 Ma. We conclude that oxygen levels were intermediate across the Ediacaran and early Palaeozoic, and highly dynamic. Stable, modern-like conditions were not reached until the Late Palaeozoic. We therefore propose that the terms Neoproterozoic Oxygenation Window and Palaeozoic Oxygenation Event are more appropriate descriptors of the rise of oxygen in Earth's atmosphere and oceans.

The rise and early evolution of animals: where do we stand from a trace-fossil perspective?

The trace-fossil record provides a wealth of information to track the rise and early evolution of animals. It comprises the activity of both hard- and soft-bodied organisms, is continuous through the Ediacaran (635-539 Ma)- Cambrian (539-485 Ma) transition, yields insights into animal behaviour and their role as ecosystem engineers, and allows for a more refined characterization of palaeoenvironmental context. In order to unravel macroevolutionary signals from the trace-fossil record, a variety of approaches is available, including not only estimation of degree of bioturbation, but also analysis of ichnodiversity and ichnodisparity trajectories, and evaluation of the occupation of infaunal ecospace and styles of ecosystem engineering. Analysis of the trace-fossil record demonstrates the presence of motile benthic bilaterians in the Ediacaran, mostly feeding from biofilms. Although Ediacaran trace fossils are simple and emplaced at or immediately below the sediment surface, an increase in ichnofossil complexity, predation pressure, sediment disturbance and penetration depth is apparent during the terminal Ediacaran. Regardless of this increase, a dramatic rise in trace fossil diversity and disparity took place during the earliest Cambrian, underscoring that the novelty of the Fortunian (539-529 Ma) cannot be underestimated. The Fortunian still shows the persistence of an Ediacaran-style matground ecology, but is fundamentally characterized by the appearance of new trace-fossil architectural plans reflecting novel ways of interacting with the substrate. The appearance of Phanerozoic-style benthic ecosystems attests to an increased length and connectivity of the food web and improved efficiency in organic carbon transfer and nutrient recycling. A profound reorganization of the infaunal ecospace is recorded in both high-energy sand-dominated nearshore areas and low-energy mud-dominated offshore environments, during the early Cambrian, starting approximately during Cambrian Age 2 (529-521 Ma), but continuing during the rest of the early Cambrian. A model comprising four evolutionary phases is proposed to synthetize information from the Ediacaran-Cambrian trace-fossil record. The use of a rich ichnological toolbox; critical, systematic and comprehensive evaluation of the Ediacaran-Cambrian trace-fossil record; and high-resolution integration of the ichnological dataset and sedimentological information show that the advent of biogenic mixing was an important factor in fully marine environments at the dawn of the Phanerozoic.

A miniature Ordovician hurdiid from Wales demonstrates the adaptability of Radiodonta

Originally considered as large, solely Cambrian apex predators, Radiodonta-a clade of stem-group euarthropods including Anomalocaris-now comprises a diverse group of predators, sediment sifters and filter feeders. These animals are only known from deposits preserving non-biomineralized material, with radiodonts often the first and/or only taxa known from such deposits. Despite the widespread and diverse nature of the group, only a handful of radiodonts are known from post-Cambrian deposits, and all originate from deposits or localities rich in other total-group euarthropods. In this contribution, we describe the first radiodont from the UK, an isolated hurdiid frontal appendage from the Tremadocian (Lower Ordovician) Dol-cyn-Afon Formation, Wales, UK. This finding is unusual in two major aspects: firstly, the appendage (1.8 mm in size) is less than half the size of the next smallest radiodont frontal appendage known, and probably belonged to an animal between 6 and 15 mm in length; secondly, it was discovered in the sponge-dominated Afon Gam Biota, one of only a handful of non-biomineralized total-group euarthropods known from this deposit. This Welsh hurdiid breaks new ground for Radiodonta in terms of both its small size and sponge-dominated habitat. This occurrence demonstrates the adaptability of the group in response to the partitioning of ecosystems and environments in the late Cambrian and Early Ordovician world.

On the co-evolution of surface oxygen levels and animals

Few topics in geobiology have been as extensively debated as the role of Earth's oxygenation in controlling when and why animals emerged and diversified. All currently described animals require oxygen for at least a portion of their life cycle. Therefore, the transition to an oxygenated planet was a prerequisite for the emergence of animals. Yet, our understanding of Earth's oxygenation and the environmental requirements of animal habitability and ecological success is currently limited; estimates for the timing of the appearance of environments sufficiently oxygenated to support ecologically stable populations of animals span a wide range, from billions of years to only a few million years before animals appear in the fossil record. In this light, the extent to which oxygen played an important role in controlling when animals appeared remains a topic of debate. When animals originated and when they diversified are separate questions, meaning either one or both of these phenomena could have been decoupled from oxygenation. Here, we present views from across this interpretive spectrum-in a point-counterpoint format-regarding crucial aspects of the potential links between animals and surface oxygen levels. We highlight areas where the standard discourse on this topic requires a change of course and note that several traditional arguments in this "life versus environment" debate are poorly founded. We also identify a clear need for basic research across a range of fields to disentangle the relationships between oxygen availability and emergence and diversification of animal life.

Food sources for the Ediacara biota communities

The Ediacara biota represents the first complex macroscopic organisms in the geological record, foreshadowing the radiation of eumetazoan animals in the Cambrian explosion. However, little is known about the contingencies that lead to their emergence, including the possible roles of nutrient availability and the quality of food sources. Here we present information on primary producers in the Ediacaran based on biomarker molecules that were extracted from sediments hosting Ediacaran macrofossils. High relative abundances of algal steranes over bacterial hopanes suggest that the Ediacara biota inhabited nutrient replete environments with an abundance of algal food sources comparable to Phanerozoic ecosystems. Thus, organisms of the Ediacara biota inhabited nutrient-rich environments akin to those that later fuelled the Cambrian explosion.