A possible billion-year-old holozoan with differentiated multicellularity

Proterozoic microfossils continue to provide new insights into the rise of complex eukaryotic life

Eukaryotes have evolved to dominate the biosphere today, accounting for most documented living species and the vast majority of the Earth's biomass. Consequently, understanding how these biologically complex organisms initially diversified in the Proterozoic Eon over 539 million years ago is a foundational question in evolutionary biology. Over the last 70 years, palaeontologists have sought to document the rise of eukaryotes with fossil evidence. However, the delicate and microscopic nature of their sub-cellular features affords early eukaryotes diminished preservation potential. Chemical biomarker signatures of eukaryotes and the genetics of living eukaryotes have emerged as complementary tools for reconstructing eukaryote ancestry. In this review, we argue that exceptionally preserved Proterozoic microfossils are critical to interpreting these complementary tools, providing crucial calibrations to molecular clocks and testing hypotheses of palaeoecology. We highlight recent research on their preservation and biomolecular composition that offers new ways to enhance their utility.

Ichthyosporea: a window into the origin of animals

Ichthyosporea is an underexplored group of unicellular eukaryotes closely related to animals. Thanks to their phylogenetic position, genomic content, and development through a multinucleate coenocyte reminiscent of some animal embryos, the members of Ichthyosporea are being increasingly recognized as pivotal to the study of animal origins. We delve into the existing knowledge of Ichthyosporea, identify existing gaps and discuss their life cycles, genomic insights, development, and potential to be model organisms. We also discuss the underestimated diversity of ichthyosporeans, based on new environmental data analyses. This review will be an essential resource for researchers venturing into the study of ichthyosporeans.

Macroalgal deep genomics illuminate multiple paths to aquatic, photosynthetic multicellularity

Macroalgae are multicellular, aquatic autotrophs that play vital roles in global climate maintenance and have diverse applications in biotechnology and eco-engineering, which are directly linked to their multicellularity phenotypes. However, their genomic diversity and the evolutionary mechanisms underlying multicellularity in these organisms remain uncharacterized. In this study, we sequenced 110 macroalgal genomes from diverse climates and phyla, and identified key genomic features that distinguish them from their microalgal relatives. Genes for cell adhesion, extracellular matrix formation, cell polarity, transport, and cell differentiation distinguish macroalgae from microalgae across all three major phyla, constituting conserved and unique gene sets supporting multicellular processes. Adhesome genes show phylum- and climate-specific expansions that may facilitate niche adaptation. Collectively, our study reveals genetic determinants of convergent and divergent evolutionary trajectories that have shaped morphological diversity in macroalgae and provides genome-wide frameworks to understand photosynthetic multicellular evolution in aquatic environments.

Frameworks for Interpreting the Early Fossil Record of Eukaryotes

The origin of modern eukaryotes is one of the key transitions in life's history, and also one of the least understood. Although the fossil record provides the most direct view of this process, interpreting the fossils of early eukaryotes and eukaryote-grade organisms is not straightforward. We present two end-member models for the evolution of modern (i.e., crown) eukaryotes-one in which modern eukaryotes evolved early, and another in which they evolved late-and interpret key fossils within these frameworks, including where they might fit in eukaryote phylogeny and what they may tell us about the evolution of eukaryotic cell biology and ecology. Each model has different implications for understanding the rise of complex life on Earth, including different roles of Earth surface oxygenation, and makes different predictions that future paleontological studies can test.

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.

It’s time for a new type of type to facilitate naming the microbial world

Since January 1, 2001, the only acceptable nomenclatural type for species under the International Code of Nomenclature of Prokaryotes (ICNP) has been pure cultures. Here, we argue that this requirement is discordant with the more inclusive nature of nomenclatural types accepted under other codes of nomenclature and posit that the unique rigidity of the ICNP has failed to serve the broad research community and has stifled progress. This case is based on the axiom that many archaea and bacteria are interdependent in nature and therefore difficult, if not impossible, to grow, preserve, and distribute as pure cultures. As such, a large proportion of Earth's biodiversity cannot be named under the current system, which limits our ability to communicate about microbial diversity within and beyond the microbiology research community. Genome sequence data are now encouraged for valid publication of new taxa in microbial systematics journals, and metagenome-assembled genomes and single cell-amplified genomes are being generated rapidly from every biome on Earth. Thus, genome sequences are available for both cultivated and uncultivated microorganisms and can readily serve as a new category of nomenclatural type, allowing for a unified nomenclature for all archaea and bacteria, whether or not they are available as pure cultures. Ideally this would be under a single code of nomenclature but, as we review here, the newly established SeqCode will operate in parallel with the ICNP as a first step toward this goal.

Eukaryogenesis and oxygen in Earth history

The endosymbiotic origin of mitochondria during eukaryogenesis has long been viewed as an adaptive response to the oxygenation of Earth's surface environment, presuming a fundamentally aerobic lifestyle for the free-living bacterial ancestors of mitochondria. This oxygen-centric view has been robustly challenged by recent advances in the Earth and life sciences. While the permanent oxygenation of the atmosphere above trace concentrations is now thought to have occurred 2.2 billion years ago, large parts of the deep ocean remained anoxic until less than 0.5 billion years ago. Neither fossils nor molecular clocks correlate the origin of mitochondria, or eukaryogenesis more broadly, to either of these planetary redox transitions. Instead, mitochondria-bearing eukaryotes are consistently dated to between these two oxygenation events, during an interval of pervasive deep-sea anoxia and variable surface-water oxygenation. The discovery and cultivation of the Asgard archaea has reinforced metabolic evidence that eukaryogenesis was initially mediated by syntrophic H₂ exchange between an archaeal host and an α-proteobacterial symbiont living under anoxia. Together, these results temporally, spatially and metabolically decouple the earliest stages of eukaryogenesis from the oxygen content of the surface ocean and atmosphere. Rather than reflecting the ancestral metabolic state, obligate aerobiosis in eukaryotes is most probably derived, having only become globally widespread over the past 1 billion years as atmospheric oxygen approached modern levels.

The forensic utility of reworked geological materials in soil

Geological materials such as rock fragments, microfossils and mineral grains are continuously being entrained (i.e. reworked) into soil during natural weathering processes. Distinctive reworked rock types in soil, and specific components of them such as palynomorphs (organic microfossils), can prove extremely useful in forensic investigations, i.e. to connect (match) people to places. If the outcrop area of a unique rock is small and well mapped, it potentially has substantial evidentiary value in soil forensic studies. Furthermore, clay minerals, geochemical data and minerals may support the presence of a suspect at a crime scene. Modern pollen and spores extracted from soil samples in forensic investigations can be invaluable in linking suspects to crime scenes. This is because the majority of localities, especially those with natural vegetation, have characteristic (often unique) floral character. Reworked (i.e. largely pre-Quaternary) palynomorphs and other microscopic fossils may co-occur with the in situ (indigenous) pollen and spores. If these reworked forms have relatively short geological ranges, they can indicate the age of the bedrock, thereby further helping to place a person at a location. However, stratigraphically recycled palynomorphs in the soil can be somewhat rare and sporadic, and many rock units are entirely or virtually devoid of palynomorphs. Furthermore, glacial sediments such as till can provide highly mixed reworked palynomorph associations due to their typically heterogenous nature. These diverse assemblages are frequently highly distinctive hence can potentially provide very powerful forensic evidence. The potential of geological materials derived from bedrock in soil for forensic investigations is absolutely clear. Hence, the use of reworked microfossils, minerals and rock should be considered in any major crime where the evidence includes soil.

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.