Australopithecus
4.2 – 2 million years ago
Bipedalism before big brains — Lucy, Laetoli, and what the fossils can support.
Every claim in this period carries its evidence with it.
Fragmentary hominin fossils, footprint trackways, and the volcanic layers that bracket them in East and southern Africa.
- 01
Bipedal walking is attested in the anatomy of the pelvis, knee and foot well before large brains appear.
- 02
The Laetoli trackways preserve bipedal locomotion directly, as behaviour rather than inference from bone.
- 03
Multiple hominin species overlapped in time; the record is a bush, not a ladder.
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 |
|---|---|---|
| Argon–argon dating of volcanic tuffs | Ash layers above and below fossil horizons | Brackets a fossil between two dated eruptions. Precision depends on how close those layers are. |
| Functional morphology | Pelvis, 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. |
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.
“Lucy is our direct ancestor.”
She belongs to a species in the relevant time and region. Direct ancestry is not demonstrable from the current sample.
“Bipedalism arose because of tool use.”
Bipedality precedes the earliest known stone tools by a long interval.
- 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?
Hadar and Laetoli site reports
Journal of Human Evolution
Cradle of Humankind excavation records
South African Journal of Science