Amoebozoan testate amoebae illuminate the diversity of heterotrophs and the complexity of ecosystems throughout geological time

Heterotrophic protists are vital in Earth's ecosystems, influencing carbon and nutrient cycles and occupying key positions in food webs as microbial predators. Fossils and molecular data suggest the emergence of predatory microeukaryotes and the transition to a eukaryote-rich marine environment by 800 million years ago (Ma). Neoproterozoic vase-shaped microfossils (VSMs) linked to Arcellinida testate amoebae represent the oldest evidence of heterotrophic microeukaryotes. This study explores the phylogenetic relationship and divergence times of modern Arcellinida and related taxa using a relaxed molecular clock approach. We estimate the origin of nodes leading to extant members of the Arcellinida Order to have happened during the latest Mesoproterozoic and Neoproterozoic (1054 to 661 Ma), while the divergence of extant infraorders postdates the Silurian. Our results demonstrate that at least one major heterotrophic eukaryote lineage originated during the Neoproterozoic. A putative radiation of eukaryotic groups (e.g., Arcellinida) during the early-Neoproterozoic sustained by favorable ecological and environmental conditions may have contributed to eukaryotic life endurance during the Cryogenian severe ice ages. Moreover, we infer that Arcellinida most likely already inhabited terrestrial habitats during the Neoproterozoic, coexisting with terrestrial Fungi and green algae, before land plant radiation. The most recent extant Arcellinida groups diverged during the Silurian Period, alongside other taxa within Fungi and flowering plants. These findings shed light on heterotrophic microeukaryotes' evolutionary history and ecological significance in Earth's ecosystems, using testate amoebae as a proxy.

The paleoredox context of early eukaryotic evolution: insights from the Tonian Mackenzie Mountains Supergroup, Canada

Tonian (ca. 1000-720 Ma) marine environments are hypothesised to have experienced major redox changes coinciding with the evolution and diversification of multicellular eukaryotes. In particular, the earliest Tonian stratigraphic record features the colonisation of benthic habitats by multicellular macroscopic algae, which would have been powerful ecosystem engineers that contributed to the oxygenation of the oceans and the reorganisation of biogeochemical cycles. However, the paleoredox context of this expansion of macroalgal habitats in Tonian nearshore marine environments remains uncertain due to limited well-preserved fossils and stratigraphy. As such, the interdependent relationship between early complex life and ocean redox state is unclear. An assemblage of macrofossils including the chlorophyte macroalga Archaeochaeta guncho was recently discovered in the lower Mackenzie Mountains Supergroup in Yukon (Canada), which archives marine sedimentation from ca. 950-775 Ma, permitting investigation into environmental evolution coincident with eukaryotic ecosystem evolution and expansion. Here we present multi-proxy geochemical data from the lower Mackenzie Mountains Supergroup to constrain the paleoredox environment within which these large benthic macroalgae thrived. Two transects show evidence for basin-wide anoxic (ferruginous) oceanic conditions (i.e., high FeHR/FeT, low Fepy/FeHR), with muted redox-sensitive trace metal enrichments and possible seasonal variability. However, the weathering of sulfide minerals in the studied samples may obscure geochemical signatures of euxinic conditions. These results suggest that macroalgae colonized shallow environments in an ocean that remained dominantly anoxic with limited evidence for oxygenation until ca. 850 Ma. Collectively, these geochemical results provide novel insights into the environmental conditions surrounding the evolution and expansion of benthic macroalgae and the eventual dominance of oxygenated oceanic conditions required for the later emergence of animals.

Nitrate limitation in early Neoproterozoic oceans delayed the ecological rise of eukaryotes

The early Neoproterozoic Era witnessed the initial ecological rise of eukaryotes at ca. 800 Ma. To assess whether nitrate availability played an important role in this evolutionary event, we measured nitrogen isotope compositions (δ¹⁵N) of marine carbonates from the early Tonian (ca. 1000 Ma to ca. 800 Ma) Huaibei Group in North China. The data reported here fill a critical gap in the δ¹⁵N record and indicate nitrate limitation in early Neoproterozoic oceans. A compilation of Proterozoic sedimentary δ¹⁵N data reveals a stepwise increase in δ¹⁵N values at ~800 Ma. Box model simulations indicate that this stepwise increase likely represents a ~50% increase in marine nitrate availability. Limited nitrate availability in early Neoproterozoic oceans may have delayed the ecological rise of eukaryotes until ~800 Ma when increased nitrate supply, together with other environmental and ecological factors, may have contributed to the transition from prokaryote-dominant to eukaryote-dominant marine ecosystems.

Phylotranscriptomic insights into a Mesoproterozoic-Neoproterozoic origin and early radiation of green seaweeds (Ulvophyceae)

The Ulvophyceae, a major group of green algae, is of particular evolutionary interest because of its remarkable morphological and ecological diversity. Its phylogenetic relationships and diversification timeline, however, are still not fully resolved. In this study, using an extensive nuclear gene dataset, we apply coalescent- and concatenation-based approaches to reconstruct the phylogeny of the Ulvophyceae and to explore the sources of conflict in previous phylogenomic studies. The Ulvophyceae is recovered as a paraphyletic group, with the Bryopsidales being a sister group to the Chlorophyceae, and the remaining taxa forming a clade (Ulvophyceae sensu stricto). Molecular clock analyses with different calibration strategies emphasize the large impact of fossil calibrations, and indicate a Meso-Neoproterozoic origin of the Ulvophyceae (sensu stricto), earlier than previous estimates. The results imply that ulvophyceans may have had a profound influence on oceanic redox structures and global biogeochemical cycles at the Mesoproterozoic-Neoproterozoic transition.

“Ulvophyceae is a remarkably morphologically and ecologically diverse clade of green algae. Here, the authors reconstruct the Ulvophyceae phylogeny, showing that these algae originated earlier than expected and may have influenced biogeochemical cycles at the Mesoproterozoic-Neoproterozoic transition.”

A possible billion-year-old holozoan with differentiated multicellularity

Sediments of the Torridonian sequence of the Northwest Scottish Highlands contain a wide array of microfossils, documenting life in a non-marine setting a billion years ago (1 Ga).^1-4 Phosphate nodules from the Diabaig Formation at Loch Torridon preserve microorganisms with cellular-level fidelity,^5,6 allowing for partial reconstruction of the developmental stages of a new organism, Bicellum brasieri gen. et sp. nov. The mature form of Bicellum consists of a solid, spherical ball of tightly packed cells (a stereoblast) of isodiametric cells enclosed in a monolayer of elongated, sausage-shaped cells. However, two populations of naked stereoblasts show mixed cell shapes, which we infer to indicate incipient development of elongated cells that were migrating to the periphery of the cell mass. These simple morphogenetic movements could be explained by differential cell-cell adhesion.^7,8 In fact, the basic morphology of Bicellum is topologically similar to that of experimentally produced cell masses that were shown to spontaneously segregate into two distinct domains based on differential cadherin-based cell adhesion.⁹ The lack of rigid cell walls in the stereoblast renders an algal affinity for Bicellum unlikely: its overall morphology is more consistent with a holozoan origin. Unicellular holozoans are known today to form multicellular stages within complex life cycles,^10-13 so the occurrence of such simple levels of transient multicellularity seen here is consistent with a holozoan affinity. Regardless of precise phylogenetic placement, these fossils demonstrate simple cell differentiation and morphogenic processes that are similar to those seen in some metazoans today.

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.

A one-billion-year-old multicellular chlorophyte

Chlorophytes (representing a clade within the Viridiplantae and a sister group of the Streptophyta) probably dominated marine export bioproductivity and played a key role in facilitating ecosystem complexity before the Mesozoic diversification of phototrophic eukaryotes such as diatoms, coccolithophorans and dinoflagellates. Molecular clock and biomarker data indicate that chlorophytes diverged in the Mesoproterozoic or early Neoproterozoic, followed by their subsequent phylogenetic diversification, multicellular evolution and ecological expansion in the late Neoproterozoic and Palaeozoic. This model, however, has not been rigorously tested with palaeontological data because of the scarcity of Proterozoic chlorophyte fossils. Here we report abundant millimetre-sized, multicellular and morphologically differentiated macrofossils from rocks approximately 1,000 million years ago. These fossils are described as Proterocladus antiquus new species and are interpreted as benthic siphonocladalean chlorophytes, suggesting that chlorophytes acquired macroscopic size, multicellularity and cellular differentiation nearly a billion years ago, much earlier than previously thought.

The origin of animal body plans: a view from fossil evidence and the regulatory genome

The origins and the early evolution of multicellular animals required the exploitation of holozoan genomic regulatory elements and the acquisition of new regulatory tools. Comparative studies of metazoans and their relatives now allow reconstruction of the evolution of the metazoan regulatory genome, but the deep conservation of many genes has led to varied hypotheses about the morphology of early animals and the extent of developmental co-option. In this Review, I assess the emerging view that the early diversification of animals involved small organisms with diverse cell types, but largely lacking complex developmental patterning, which evolved independently in different bilaterian clades during the Cambrian Explosion.

Neoproterozoic origin and multiple transitions to macroscopic growth in green seaweeds

The Neoproterozoic Era records the transition from a largely bacterial to a predominantly eukaryotic phototrophic world, creating the foundation for the complex benthic ecosystems that have sustained Metazoa from the Ediacaran Period onward. This study focuses on the evolutionary origins of green seaweeds, which play an important ecological role in the benthos of modern sunlit oceans and likely played a crucial part in the evolution of early animals by structuring benthic habitats and providing novel niches. By applying a phylogenomic approach, we resolve deep relationships of the core Chlorophyta (Ulvophyceae or green seaweeds, and freshwater or terrestrial Chlorophyceae and Trebouxiophyceae) and unveil a rapid radiation of Chlorophyceae and the principal lineages of the Ulvophyceae late in the Neoproterozoic Era. Our time-calibrated tree points to an origin and early diversification of green seaweeds in the late Tonian and Cryogenian periods, an interval marked by two global glaciations with strong consequent changes in the amount of available marine benthic habitat. We hypothesize that unicellular and simple multicellular ancestors of green seaweeds survived these extreme climate events in isolated refugia, and diversified in benthic environments that became increasingly available as ice retreated. An increased supply of nutrients and biotic interactions, such as grazing pressure, likely triggered the independent evolution of macroscopic growth via different strategies, including true multicellularity, and multiple types of giant-celled forms.

Early Earth and the rise of complex life

The history of life on Earth progressed in parallel with the evolving oxygen state of the atmosphere and oceans, but the details of that relationship remain poorly known and debated. There is, however, general agreement that the first appreciable and persistent accumulation of oxygen in the oceans and atmosphere occurred around 2.3 to 2.4 billion years ago. Following this Great Oxidation Event, biospheric oxygen remained at relatively stable intermediate levels for more than a billion years. Much current research focuses on the transition from the intermediate conditions of this middle chapter in Earth history to the more oxygenated periods that followed - often emphasizing whether increasing and perhaps episodic oxygenation drove fundamental steps in the evolution of complex life and, if so, when. These relationships among early organisms and their environments are the thematic threads that stitch together the papers in this collection. Expert authors bring a mix of methods and opinions to their leading-edge reviews of the earliest proliferation and ecological impacts of eukaryotic life, the subsequent emergence and ecological divergence of animals, and the corresponding causes and consequences of environmental change.