Paranthropus boisei skull Click to enlarge image
A cast of a Paranthropus boisei skull OH 5. The skull was discovered in1959 by Mary Leakey in Olduvai Gorge, Tanzania and dates to about 1.8 million years old. This is the ‘type specimen’ or official representative of this species. It was originally classified as Zinjanthropus boisei and earned the nickname ‘Nutcracker Man’ because of its powerful jaws and large teeth. No lower jaw was found with this specimen. Image: Stuart Humphreys
© Australian Museum

Early ancestors

Evidence from fossils, proteins and genetic studies indicates that humans and chimpanzees had a common ancestor millions of years ago. Most scientists believe that the ‘human’ family tree (known as the sub-group hominin) split from the chimpanzees and other apes about five to seven million years ago.

What this common ancestor looked like is not known. Until recently it was widely believed that it looked much like a chimpanzee, with features such as a short back, arms and hands adapted for grasping and swinging in branches, and wrists and forelimbs that enabled knuckle-walking. This view was based on the beliefs that our ancestors probably passed through a proto-ape stage and that African apes are less specialised than humans so have changed less since diverging from this ancestor. However, this lacked supportive fossil evidence as there are almost no fossils of early chimps or gorillas and very few of early hominins.

Recent studies on the skeleton of the 4.4-million-year-old Ardipithecus ramidus have changed all this. This species dates to a critical time in hominin evolution as it is nearing the time when scientists believe hominins diverged from the ape branch of the family tree. The fact that A.ramidus has a number of physical features that differ significantly from chimpanzees (particularly those that show it was not a knuckle-walker) is crucial to our understanding of hominin and ape evolution. It is highly likely that A.ramidus preserves some of the characteristics of the last common ancestor, suggesting that some of its features (particularly in the limbs and hands) were more like those in living monkeys and early apes like Proconsul.

Millions of years of evolutionary change and natural selection meant that later hominin species were less apelike in appearance and behaviour than their early ancestors. The ancestral line that led to modern chimpanzees also changed, possibly with changes that were as dramatic as our own.


Our own species Homo sapiens is the result of four major evolutionary changes. These can be summarised as trends involving the development of:

  1. bipedalism (walking upright on two legs)
    2. shorter jaws with smaller teeth
    3. larger brains
    4. increasingly complex forms of technology

Fossil evidence shows that our ancestors became bipeds first, followed by changes to the teeth and jaws. It was only much later that our larger brains and more complex technology set us apart as Homo sapiens.