Holding hands across 2m years, our missing link

Australopithecus sediba had a small brain, about the size of a grapefruit, but shaped more like that of a human than a chimpanzee. And the two fossils, which are about two million years old and were discovered in a South African cave in 2009, show the creature had a dextrous hand that may have fashioned tools and been used to climb trees.

It had modern-looking ankles, but its heel and shin-bone were mostly ape-like. Scientists believe the creature walked upright, but not in the same way as living humans.

The combination of features observed in the fossilised remains of an adult female and male child, possibly a mother and son, suggest Au. sediba may have been the direct ancestor of our own species.

Scientists know that modern humans – Homo sapiens – are descended from the species Homo erectus (upright man), which evolved in Africa 1.8 million years ago. But earlier human lineages are harder to trace. Until now it was believed that Homo habilis or Homo rudolfensis, which post-dated Au. sediba by several hundred thousand years, were the most likely ancestors.

The discoveries, described in five papers published in the journal Science, may make experts think again. Professor Lee Berger, from the Institute for Human Evolution at the University of the Witwatersrand in Johannesburg, who led the team that found Au. sediba, said: “The fossils demonstrate a surprisingly advanced small brain, a very evolved hand with a long thumb like humans, a very modern pelvis, but a foot and ankle shape never seen in any hominin species that combines features of both apes and humans in one anatomical package.

“The many very advanced features in the brain and body, and the earlier date, make it possibly the best candidate ancestor for our genus, the genus Homo, more so than previous discoveries, such as Homo habilis.”

Richard Potts, director of the human origins programme at the Smithsonian Institution, said: “It’s as if evolution is caught in one vital moment, a stop- action snapshot of evolution in action. It will take a lot of scrutiny of the papers and of the fossils but these analyses could well be ‘game-changers’ in understanding human evolution.”

The juvenile, aged about ten to 13 in modern human terms, had a well-preserved cranium that enabled scientists to study his brain shape in detail. X-ray analysis showed that, although only slightly larger than a chimpanzee’s brain, it was human-like in appearance.

The hands and feet of the adult female displayed a unique mix of modern and primitive features. Her hand was powerfully strong, hinting at tree-climbing, yet also had a long thumb and short fingers, which indicated an ability to grip with precision. The scientists believe Au. sediba could have been a tool-maker, although no direct evidence of this has yet been found.

Professor Chris Stringer, from the Natural History Museum, said: “This isn’t the end of the story. What may be happening is that there were several australopithecine forms, all evolving human-like features in parallel as they turned to meat-eating and tool-making and moving greater distances.

“The question now is to pull out of this mess which one is really the ancestor of the genus Homo. We know there are more remains to come from this incredible site. Let’s see if other individuals also show this mix of features.”