Did early eukaryotes really radiate in the Tonian?
14 October 2025
Last update: 15/10/25 14:30
https://doi.org/10.1017/pab.2024.33
Since the 1980s Precambrian palaeontologists believed that early eukaryotes (Domain: Eukarya, microorganisms with a membrane bound nucleus) underwent a dramatic diversification in the Tonian Period (1000-720Ma). This period is often associated with the rise of complex life, a key step towards the evolution of animals, and the modern marine ecosystem. Long have palaeontologists looked to the microfossil record of ancient shales and cherts to elucidate the tempo of eukaryotic evolution, all centring on the view that eukaryotes originated early in the Proterozoic Eon (~2.5 billion years ago) but remained ‘stable’ for a billion years before rising to dominance. These special types of rocks preserve the earliest evidence of ecologically important clades like those of algae and fungi, but also complex traits like sexual reproduction, and multicellularity. The Tonian is thought to be a foundational period in the history of life.
A new study by Porter et al (2025) revisits this canonical viewpoint with a new exhaustive dataset of Proterozoic microfossils from around the world. What they argue is that this apparent rise in the Tonian may not be a biological explosion at all, but rather a direct record of sampling. The authors elegantly demonstrate that diversity of eukaryotic fossils is tightly correlated with the number of fossil-bearing formations, rendering diversity signals heavily biased by sampling efforts. Quite simply, more rocks means more fossils!
This finding challenges long held assumptions about the trajectory of eukaryotic evolution through the lens of Paleontology. Porter and colleagues explore the microfossil record with respect to number of formations, preservational factors related to lithology, and how eukaryotes are identified and interpreted in the fossil record. Though they champion a robust argument that the fossil record is more influenced by sampling intensity that previously thought, they authors do not deny that eukaryotes diversified at all, but rather that the timing may be misrepresented. Nonetheless, the growing microfossil record continues to give insight into the history of complex life, albeit shaped by how and where we look for these tiny, spectacular, and enigmatic fossils.
Check out the new study in Paleobiology, as part of our 50th anniversary special issue!
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