Earliest history of eukaryotic life: Integration of Biology and Geology

From time to time, GEITP ponders various aspects of the earliest evolution of Life Forms on Earth. The attached article discusses the early evolution of eukaryotes — which is arguably one of the most important processes in the history of life on Earth [see Box 1 of attached pdf.]. Eukaryotes, as a clade or lineage, are defined in terms of cellular complexity. This complexity enables morphological diversity in both unicellular and multicellular lineages — including animals, fungi, algae, and plants. Eukaryotes have also played a role in shaping the physical landscape and bio-geochemical cycles on Earth, ever since their origin in the early Proterozoic Era (i.e., 2,500 million years ago, all the way up to 541 million years ago).

Because the origin and early evolution of eukaryotes is so ancient, and many of the features that define the group are unlikely to be preserved in the geologic record — it has been challenging to uncover their early evolutionary history. Traditionally, our knowledge of the history of a major lineage such as the eukaryotes would rely heavily on the fossil record. While the fossil record of early eukaryotes is indeed vital to telling their story, it is also a relatively sparse and patchy source of information. Many fields — including genomics, phylogenetics, organic geochemistry, and redox geochemistry — have added new layers to our understanding of this ancient history. There are now convincing data for the origin of the eukaryote clade in the early Proterozoic and compelling data for diversification and increased ecological importance in the late Proterozoic, but there is much less consensus on events happening during the vast middle of this important interval. [“A picture is worth a thousand words,” and these figures and diagrams [see attached] are very helpful.]

It has been estimated that total-group eukaryotes [i.e., first eukaryotic common ancestor (FECA)] evolved between 1.6 and 3.0 billion years ago (BYA). Once established, LECA must have had at least a cytoskeleton, nucleus, and mitochondria to be “a eukaryote.” The vast majority of eukaryotes perform respiration with oxygen as the electron acceptor (and, while some can perform fermentation and other forms of anaerobic respiration, their metabolic pathways for generating energy are rudimentary, when compared with bacteria and archaea).

The original plastids (i.e., chloroplasts) derived from a subsequent endosymbiotic event with a cyanobacteria and became the center of oxygenic photosynthesis in eukaryotes. However, most photosynthetic eukaryotes are best described as mixotrophs because both respiration and photosynthesis play a major role in their metabolisms. Throughout their complex evolutionary history, plastids also became a driver for evolution and diversification in many other clades within the eukaryotes—via subsequent endosymbiotic events. Despite agreement on the overall story, there is still controversy over the actual partners involved as well as the timing and mechanisms of the primary symbiotic events.

The first agreed-upon eukaryote fossils have a complex cell wall, but no other preserved features; thus, it is unclear if they represent stem or crown groups (Figure I inside Box 1). The first crown-group eukaryote in the fossil record is an Archaeplastida, and thus a product of two endosymbiotic events, and a few evolutionary steps from LECA. It is possible, or even likely, that unclassifiable stem groups and recognizable crown groups coexisted in Proterozoic ecosystems.

If oxygen did not change significantly during the Proterozoic, then what other factors may have led to eukaryotic evolution, and emergence and diversification of crown groups, in the Neoproterozoic Era? Some suggest that intrinsic ecological changes, such as the rise of eukaryvory and predation, as drivers for Neoproterozoic diversification. Others have called on changes in nutrient fluxes, subtle changes in oxygenation levels that impacted nutrient levels, and even the breakup of the supercontinent Rodinia.

When did FECA and, subsequently, crown-group eukaryotes, evolve? The best evidence we have for crown-group eukaryotes are photosynthetic clades, which are more derived than the most basal eukaryotic lineages. Could basal heterotrophic crown group lineages be preserved in the geologic record? These ideas can be tested in part by recalibrating clocks using updated geochronology and collecting and incorporating new well-dated fossils.

The transition between FECA and crown group eukaryotes is an important evolutionary process that is likely rooted in major environmental transitions. What external environmental and/or paleoecological events may have triggered the origin of eukaryotes and subsequent diversification within the clade? There is compelling evidence for connections between abiotic factors and biological events recorded in the geologic record, however, it is difficult to infer causation between external environmental and evolutionary events; this can be explored via continued collection and integration of phylogenetic, geochemical, and fossil data.

Whatever the drivers, it is clear that total-group eukaryotes are present early in the Proterozoic, but do not leave a taxonomically definitive and biodiverse record until the end of the Proterozoic Era. Our analysis shows that sometimes biomarkers, molecular clocks, and fossils agree, and sometimes they do not. Yet it is only by delving into these apparent disagreements that we can refine our interpretations of the many proxies used in studying the early evolutionary history of eukaryotes. Further integration and synthesis will lead towards a more nuanced understanding of the early history of this overwhelmingly successful clade of life. 😊

I would absolutely agree with that. John

Wish I could’ve been there. And I would conclude that P450 genes had already arisen WELL BEFORE the Early Proterozoic Era (2.5 BYA). 😊

COMMENT: Indeed, most interesting. I spoke about this — and its implications for the evolving and transferring P450 genes, down through many millions of years of evolution, at the Cytochrome P450 meeting in Washington DC in July. Close to your own heart…

Trends in Ecology & Evolution, Mar 2022; 37: 3 https://doi.org/10.1016/j.tree.2021.11.005

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