Sisters in evolution

Two hominid fossils discovered in the Lake Turkana region of Kenya directly challenge established theories about the linear succession of the genus Homo, an international group of researchers report in Nature this week.

The Koobi Fora Research Project group, led by the famous mother and daughter team of Meave and Louise Leakey, discovered the two fossils in 2000. One is an upper jaw bone of Homo habilis, which dates from 1.44 million years ago, and the other is "an exquisitely preserved" skull of Homo erectus, dated to about 1.55 million years ago, the paper's lead author, Fred Spoor, said.

"What is truly striking about this fossil is its size," Spoor, a professor of evolutionary anatomy at University College London, said in a statement.

"It is the smallest Homo erectus found thus far anywhere in the world."

Long-standing evolutionary theory has held that human evolution over the last two million years is a linear succession of H. habilis to H. erectus to H. sapiens.

The H. habilis fossil, however, is much more recent than previously known. The researchers say that this late survivor shows H. habilis and H. erectus lived side by side in eastern Africa for nearly half a million years.

"Their co-existence makes it unlikely that H. erectus evolved from H. habilis," Meave Leakey said.

"The fact that they stayed separate as individual species for a long time suggests that they had their own ecological niche, thus avoiding direct competition."

The researchers believe both species must have had their origins between two and three million years ago. Spoor said the new fossil jaw suggests that H. habilis was a sister species of H. erectus, rather than a mother species.

H. erectus is still considered the mother species of H. sapiens, possibly via an intermediate form, he said.

The find also sheds light on sexual dimorphism, in which the male of the species is significantly larger than the female, as with gorillas. Spoor said a reduction in the size difference between the sexes is considered a character acquired during human evolution.

"H. erectus was commonly thought of as a less dimorphic, more human-like species, but the discovery of the small skull size suggests that males and females of this species still differed substantially in size," he said.

This has implications for understanding behavioural evolution, as sexual dimorphism in primates is thought to relate to competition for mates. Species with significant differences in size, such as gorillas and baboons, are characterised by a dominant male and a harem of females.

Primates featuring monogamous behaviour, such as gibbons, show little difference in size and shape.

"That H. erectus may still have been very dimorphic suggests the possibility of a reproductive strategy that mostly was not monogamous - and this may have implications for understanding behaviour evolution, the size of groups that H. erectus may have lived in and so on," Spoor said.