Two chapters, the same questions asked of both
Put two Deep Time chapters next to each other — their evidence base, the dating methods behind them, the live debates, and the questions still open.
Earth Formation
Accretion of the planet and the isotopic dating methods that fix its age.
Open full chapter →Early Life
Stromatolites, the oxygenation of the atmosphere, and the first multicellular life.
Open full chapter →Meteorites, the oldest surviving terrestrial minerals, and lunar samples. No rock survives from the planet's first tens of millions of years, so the age of Earth is read from the wider solar system.
Microfossils, stromatolites, isotopic carbon signatures in ancient sediment, and the molecular record read backwards from living organisms.
| Method | What it constrains |
|---|---|
| Radiometric dating (uranium–lead, lead–lead)Calcium–aluminium inclusions in chondritic meteorites | Fixes the age of the earliest solids in the solar system. It dates the neighbourhood, not the planet's surface. |
| Zircon geochronologyDetrital zircon grains from the Jack Hills, Western Australia | Shows crustal material and liquid water existed early. It does not describe what the wider surface looked like. |
| Isotopic comparison of Earth and MoonApollo lunar samples against terrestrial mantle rock | Supports a shared origin for both bodies. The exact impact geometry remains modelled, not observed. |
| Method | What it constrains |
|---|---|
| Carbon isotope fractionationOrganic carbon trapped in ancient sedimentary rock | Indicates biological processing of carbon. It identifies metabolism, not a species. |
| Stromatolite morphology and stratigraphyLayered carbonate structures in Archean and Proterozoic sequences | Strong evidence of microbial mats where biogenicity is confirmed. Some structures have abiotic look-alikes. |
| Molecular phylogeneticsConserved gene sequences across living lineages | Reconstructs branching order well; absolute dates depend on calibration against fossils. |
How quickly did Earth cool into a habitable surface?
The 'cool early Earth' reading of the Jack Hills zircons is well supported but contested in degree, not in kind.
What exactly struck the proto-Earth?
Impactor size, speed and composition are still recovered from simulations tuned to match isotopic data.
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.
- Where did Earth's water come from, and in what proportions?
- How long did the magma ocean phase last?
- Was there a distinct late heavy bombardment, or a smoother declining flux?
- 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?