Evidence for Human Evolution

Bipedalism means walking on two legs. A skeleton can show adaptations for upright walking while retaining features useful for climbing. Human-like features did not all appear at once.

Ardi: upright walking alongside climbing

Ardi, an Ardipithecus ramidus fossil from Ethiopia, lived 4.4 million years ago. She had a small brain, about 350 cm³, similar in size to a chimpanzee’s brain. Her long arms and relatively short legs were more ape-like than those of modern humans.

Ardi’s grasping big toe was suited to holding branches, suggesting that she climbed trees. However, her leg structure also provides evidence that she could walk upright. This combination is significant: an early member of the human lineage could show some adaptation to bipedalism without having a modern human body.

Lucy: better adapted to walking upright

Lucy, an Australopithecus afarensis fossil also found in Ethiopia, lived 3.2 million years ago. Her brain was slightly larger than Ardi’s, at about 430 cm³, but was still small compared with a modern human brain.

Lucy’s skeleton provides evidence of more efficient upright walking than Ardi’s. Evidence from her species includes arched feet without an ape-like grasping big toe, making the feet better suited to walking than gripping branches. Her arm and leg proportions were between those of apes and modern humans.

Lucy is important because she shows that effective upright walking developed before the much larger brains seen in later humans. Walking upright and increased brain size were not a single simultaneous change.

Richard Leakey’s discoveries: a more human-like body

Richard Leakey led fossil-searching expeditions in Kenya. His team’s discoveries included Turkana Boy, a Homo erectus skeleton dating from 1.6 million years ago.

This skeleton had relatively short arms and long legs, with feet and legs well adapted to upright walking. Its brain was much larger than Lucy’s, at around 1,000 cm³, although still smaller than that of a typical modern human.

Comparing Ardi, Lucy and Turkana Boy reveals a broad pattern: increasingly human-like adaptations for walking on two legs and an increase in brain size. The fossils provide dated physical evidence of change. They should not be treated as proof that these three particular individuals formed a direct parent-to-descendant sequence; the fossil record is incomplete.

Stone tools reveal changing skills

A stone tool is an artefact: an object deliberately made or modified by a human or human ancestor. Unlike a skeleton, it gives evidence about what its maker could do.

Early tools, from around 2.5 million years ago, included simple sharp flakes made by striking stones together. These could cut or scrape meat from bones. By around 1.6 million years ago, toolmakers were producing more carefully shaped hand axes, useful for tasks such as chopping and preparing food.

Later tools included sharper, more specialised points and blades, followed by finely worked arrowheads. Making these required more controlled shaping and planning than producing a simple flake. The general development towards more sophisticated tools provides evidence of changing skills and behaviour in the human lineage. Tool shape alone, however, does not give an exact age or directly measure the maker’s brain size.

Dating tools from their surroundings

Stone tools are often buried as sediment or volcanic ash accumulates. In an undisturbed sequence, lower layers were deposited before those above them. A tool in a lower layer is therefore usually older than one in a higher layer. This is relative dating: it establishes an order without necessarily giving an age in years.

For example, a simple flake below a shaped hand axe would support the conclusion that the flake was deposited earlier, provided the layers had not been disturbed. Scientists need to consider whether erosion or later movement has displaced an object before trusting its position.

Scientists can also use radiometric dating of suitable surrounding rock or volcanic layers. Radioactive isotopes decay over time, allowing the age of the material to be estimated. A tool between two dated layers can then be assigned an age range: younger than the layer below and older than the layer above.

For more recent finds, associated material that was once living, such as a wooden handle, may be dated using radiocarbon dating. The wood contains carbon; the stone does not contain the once-living carbon needed for this method. Radiocarbon dating is not suitable for dating the millions-of-years-old fossils discussed here.

Together, skeletal features, tool development and dates from the surrounding environment provide complementary evidence: what earlier humans were like, what they could do and when they lived.

• Ardi — 4.4 million years ago: small brain; long arms and short legs; grasping big toe for climbing; leg structure indicates upright walking.
• Lucy — 3.2 million years ago: slightly larger brain; more efficient upright walking; species had arched feet without a grasping big toe.
• Richard Leakey’s team’s discovery — 1.6 million years ago: Turkana Boy, Homo erectus; much larger brain; short arms, long legs and human-like adaptations for walking.

Overall pattern: greater adaptation to bipedalism and increased brain size. Upright walking developed before a large brain.

Stone-tool evidence

Simple sharp flakes → shaped hand axes → more specialised, finely worked points and blades.

Increasing sophistication provides evidence of changing toolmaking skills and behaviour.

Dating from the environment

• Relative dating: lower layers are usually older in an undisturbed sequence.
• Radiometric dating: suitable surrounding rock or volcanic layers give age estimates; layers above and below can bracket a tool’s age.
• Radiocarbon dating: can date suitable associated once-living material, such as wood, not the stone itself or these millions-of-years-old fossils.