Billion-Year Mystery Solved: Earliest Eukaryotes Were Benthic Aerobes, Not Plankton

*Translated and summarized from a zhichai.net forum post.*

Key points

• Paper: Lechte, M.A. et al. *Early fossil eukaryotes were benthic aerobes*. Nature (2026-05-20). DOI: 10.1038/s41586-026-10533-4
• The puzzle: Eukaryote body fossils appear ~1.75–1.4 billion years ago (e.g., in Australia and China), but eukaryote-specific molecular fossils (steranes) only become abundant ~800 million years ago — a gap of nearly a billion years.
• Method: Integrated paleontological, sedimentological, and geochemical analysis of drill cores from the McArthur and Birrindudu basins (northern Australia), spanning 1.75 billion–540 million years ago.
• Core finding: Eukaryote fossils occur almost exclusively in oxygenated shallow-marine/lagoonal sediments and are nearly absent from anoxic/euxinic and deep-water facies — even where prokaryote fossils are abundant in those same samples.
• Interpretation: If early eukaryotes were planktonic, their sinking carcasses should appear in both oxic and anoxic seafloor deposits. Their restriction to oxic bottoms implies they were benthic (seafloor-dwelling) and aerobic (oxygen-dependent).
• Explaining the molecular gap: Benthic eukaryotes died and decomposed in oxygen-rich environments, where oxidation destroyed their characteristic sterol lipids — so molecular fossils could not accumulate.
• The Neoproterozoic shift (~800 Ma): Eukaryotes expanded into the planktonic niche (possibly aided by rising oxygen, ecological innovation, or changing ocean structure). Their carcasses then sank into anoxic deep waters, preserving steranes — explaining the sudden abundance of molecular fossils.

The two competing hypotheses

1. "Misidentified fossils": The 1.75-Ga eukaryote-like fossils might be morphologically complex prokaryotes (some cyanobacteria form eukaryote-like spheres and tubes). 2. "Lost chemical traces": The fossils are genuine eukaryotes, but their lipids failed to preserve because they lived in oxygenated settings where organic molecules are rapidly degraded.

The Nature paper supports hypothesis 2 with a convincing mechanism.

The critical evidence pattern

| Environment | Redox state | Eukaryote fossils | Prokaryote fossils | |---|---|---|---| | Shallow seafloor | Oxic | Abundant | Abundant | | Lagoonal/restricted | Oxic | Abundant | Abundant | | Shallow seafloor | Anoxic/euxinic | Nearly absent | Abundant | | Deep water | Anoxic/euxinic | Nearly absent | Abundant |

Across 15+ fossil assemblages, eukaryote fossils systematically correlate with oxic depositional environments. The absence in anoxic facies is the key clue: it constrains *where* the organisms lived, not just when.

The logic: benthic, not planktonic

1. Eukaryote fossils occur only in oxic settings → early eukaryotes required oxygen (consistent with mitochondria-based respiration; fossil size and complexity also support this). 2. A planktonic organism would leave fossils in both oxic and anoxic deposits (sinking carcasses preserve either way). The observed absence in anoxic facies therefore means the organisms were benthic — they simply could not live on anoxic seafloors. 3. Benthic life on oxic bottoms → post-mortem oxidation destroyed sterols → no molecular fossils for nearly a billion years.

Broader significance

• Two-step colonization of the ocean: (1) rooted on oxic seafloors from ~1.75 Ga to ~800 Ma; (2) occupation of the full water column starting in the Neoproterozoic, coinciding with the molecular fossil surge and rising diversity.
• Possible links to Snowball Earth glaciations and post-glacial ocean chemistry changes are noted, though the paper does not explore them deeply.
• If the earliest eukaryotes were already aerobic benthos, eukaryogenesis (including mitochondrial endosymbiosis) likely predates their ecological expansion, and the benthic-to-planktonic transition may have been a prerequisite for later complexification and animal origins.

Limitations noted in the post

• Geographic scope limited mainly to Australian drill cores; global confirmation is needed.
• The identity of the oldest "eukaryote fossils" remains debated in paleontology; the paper's argument assumes they are genuine eukaryotes.
• Sterane sources are not fully unambiguous, as some bacteria produce sterol-like molecules.
• The specific drivers of the benthic-to-planktonic expansion (oxygenation, metabolic innovation, new niches) remain open questions.

References

• Lechte, M.A. et al. Early fossil eukaryotes were benthic aerobes. *Nature* (2026). DOI: 10.1038/s41586-026-10533-4
• Data repository: https://doi.org/10.5061/dryad.1vhhmgr8n
• Brocks, J.J. et al. The rise of algae in Cryogenian oceans and the emergence of animals. *Nature* 548, 578–581 (2017)
• Mills, D.B. et al. Eukaryogenesis and oxygen in Earth history. *Nat. Ecol. Evol.* 6, 520–532 (2022)
• Porter, S.M. & Riedman, L.A. Frameworks for interpreting the early fossil record of eukaryotes. *Annu. Rev. Microbiol.* 77, 173–191 (2023)