Early Life
3.7 billion – 541 million years ago
Stromatolites, the oxygenation of the atmosphere, and the first multicellular life.
Every claim in this period carries its evidence with it.
Microfossils, stromatolites, isotopic carbon signatures in ancient sediment, and the molecular record read backwards from living organisms.
- 01
Microbial life is present in the rock record far earlier than complex life, and for most of Earth's history it was the only life.
- 02
Oxygen accumulated in the atmosphere through photosynthetic microbial activity, recorded in banded iron formations and sulfur isotopes.
- 03
Eukaryotic cells arose through endosymbiosis, a claim supported independently by genetics and by organelle biology.
Each row states the method, what is actually measured, and — the part usually skipped — what the result does not settle.
| Method | Material | What it constrains |
|---|---|---|
| Carbon isotope fractionation | Organic carbon trapped in ancient sedimentary rock | Indicates biological processing of carbon. It identifies metabolism, not a species. |
| Stromatolite morphology and stratigraphy | Layered carbonate structures in Archean and Proterozoic sequences | Strong evidence of microbial mats where biogenicity is confirmed. Some structures have abiotic look-alikes. |
| Molecular phylogenetics | Conserved gene sequences across living lineages | Reconstructs branching order well; absolute dates depend on calibration against fossils. |
Which claimed earliest microfossils are genuinely biological?
Several famous specimens have been reinterpreted as mineral artefacts, then defended again.
How abrupt was the Great Oxidation Event?
Proxy records suggest oscillation before permanent rise rather than a single step.
“Life began in a warm little pond and we know how.”
Origin-of-life chemistry is an active laboratory field. No proposed pathway has been demonstrated end to end.
“Evolution was a steady climb toward complexity.”
Billions of years passed with microbial life alone; complexity is episodic, not a trend line.
- What was the sequence from prebiotic chemistry to self-replication?
- How early did eukaryotes actually appear, as opposed to first fossilise?
- Why the long delay between oxygenation and animal life?
Precambrian palaeobiology reviews
Nature / Annual Review of Earth and Planetary Sciences
Great Oxidation Event proxy studies
Geobiology / PNAS