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.
Early Life
Stromatolites, the oxygenation of the atmosphere, and the first multicellular life.
Open full chapter →Dinosaurs
The Mesozoic record and the impact evidence for the end-Cretaceous extinction.
Open full chapter →Microfossils, stromatolites, isotopic carbon signatures in ancient sediment, and the molecular record read backwards from living organisms.
Body fossils, trace fossils, bone histology, and the global iridium-bearing boundary layer that marks the end of the era.
| 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. |
| Method | What it constrains |
|---|---|
| Biostratigraphy and radiometric dating of volcanic ashAsh beds bracketing fossil-bearing layers | Dates the sediment around a fossil rather than the animal itself, which is why fossil ages are ranges. |
| Bone histologyThin sections of fossil bone | Reveals growth rate and maturity. It supports inferences about metabolism without measuring it directly. |
| Impact geochemistryIridium anomaly, shocked quartz and spherules at the boundary layer | Demonstrates a global impact event coincident with the extinction. Coincidence is established; sole causation is argued. |
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.
How much did Deccan Traps volcanism contribute to the extinction?
Both events are real and near-simultaneous; the weighting between them is genuinely unsettled.
Were non-avian dinosaurs already in decline before the impact?
Answers depend heavily on how sampling bias in the late Cretaceous record is corrected.
- 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?
- What determined which lineages survived the boundary?
- How widespread was feathering across the major groups?
- How fast was ecosystem recovery in the first million years after impact?