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.
Ice Ages
Glacial cycles, sea-level change, and the landscapes early humans actually walked.
Open full chapter →Australopithecus
Bipedalism before big brains — Lucy, Laetoli, and what the fossils can support.
Open full chapter →Ice cores, deep-sea sediment cores, glacial landforms, and orbital mechanics calculated independently of the geological record.
Fragmentary hominin fossils, footprint trackways, and the volcanic layers that bracket them in East and southern Africa.
| Method | What it constrains |
|---|---|
| Oxygen isotope stratigraphyForaminifera shells in deep-sea sediment cores | Produces a continuous global ice-volume curve. Regional temperature must be inferred separately. |
| Ice core analysisAnnual layers, trapped air bubbles and dust in polar ice | Directly samples past atmosphere back several hundred thousand years. It does not reach earlier glaciations. |
| Orbital calculationComputed eccentricity, obliquity and precession cycles | Predicts the pacing independently of any sediment. Amplitude of response still requires feedbacks. |
| Method | What it constrains |
|---|---|
| Argon–argon dating of volcanic tuffsAsh layers above and below fossil horizons | Brackets a fossil between two dated eruptions. Precision depends on how close those layers are. |
| Functional morphologyPelvis, femur, foot and vertebral remains | Establishes locomotor capability. It does not tell us how much time was spent in trees. |
| Ichnology (trackway analysis)Preserved footprints in dated ash | Records gait directly. Attribution to a specific species is inferred from context. |
Why did cycles shift from roughly 41,000 to roughly 100,000 years?
The mid-Pleistocene transition is not fully explained by orbital forcing alone.
How abrupt were the warming events within glacial periods?
Ice core records show rapid shifts whose triggers and ocean-circulation role remain argued.
How arboreal were these species?
Upper-limb anatomy retains climbing features while lower-limb anatomy is committed to walking.
Which australopith lineage, if any, leads to Homo?
Candidates exist; the fossil sample is too sparse to settle descent.
- What sets the amplitude of each cycle?
- How much of the coastline record of early humans is now on drowned shelves?
- How stable are the feedbacks that ended each glacial phase?
- What selected for bipedalism in the first place?
- How many contemporary hominin species were there?
- How much of the record is missing outside the East African rift, where preservation favours discovery?