The ascent of man onto his feet

Fossil ear bone canals may hold the secret of when man first walked upright.Ayala Ochert reports aspart of a THES serieson young researchers

Palaeoanthropology, the study of human origins, got off to an inauspicious start with the finding of an ancient human skull and jaw bone in a gravel pit in Piltdown, Sussex in 1912. First hailed as a momentous find, confirming as it did contemporary theories of human evolution, suspicions were later roused when the "first Englishman" was found with cricket bat carved out of elephant bone. The episode was eventually recognised for a hoax.

Palaeoanthropologists have gradually restored their credibility, and now boast a high-tech approach to studying fossils. The latest technique, pioneered by Fred Spoor of University College London's department of anatomy, uses medical imaging scanners to help dispel more recent myths about our ancestors.

While studying for his PhD in Utrecht in the Netherlands, Spoor wondered whether images of the inner ear, which contains the organ of balance, might show adaptations to upright walking, a skill widely used to distinguish man from apes. Deep within the temporal bone, behind the ear, are three semi-circular canals. Without a tail to steady ourselves, and with small feet originally adapted for climbing, we need all the help we can get not to fall over. That help comes in the form of enlarged canals corresponding to movements up-and-down and back-and-forward, the sort of movements we would expect to make when walking, and especially running upright.

The possibility of scanning fossil ear bones eventually became a reality for Spoor. "I was most interested in finding when for the first time the organ of balance became totally adapted to the modern way of walking," he says. "And that happens with Homo erectus," he adds, confident that his findings end a long debate about how the primate australopithecine moved around. "What was elegant about my results was that there was a deadlock between one group who said that the australopithecines (the name means southern ape) adopted modern locomotion and another group who said that they didn't. Here was independent evidence; suddenly you didn't have to look at the pelvis, you didn't have to look at the finger bones - you could look at the skull," explains Spoor. The organ of balance in australopithecines was "completely great-ape-like", indicating that they would have spent most of their time in the trees.

With his new technique, Spoor has been much in demand. Alan Walker, joint winner of this year's Rhone-Poulenc science book prize, has been using Spoor's technique to look at much older primate fossils dating back 40 million years. But for Spoor Homo habilis remains his pi ce de resistance. The first species to qualify for the title Homo, it is normally placed between the australopithecines and Homo erectus. But there are new doubts about the status of Homo habilis as a true ancestor of humankind, and some have even suggested that it might be two species, neither of which may be part of the human lineage.

When the fossils at Piltdown were found, they were seized upon because they fitted with the then widespread view that our developed brain had a long history. Similarly, when, in the 1960s, Louis Leakey found Homo habilis buried with tools, it was assumed that it must be a human ancestor, because tools were seen as a hallmark of our species, a view since revised. But a scan of one Homo habilis ear bone has shown no signs of modern upright walking, adding impetus to moves to relegate it from the Homo league.

When Spoor examines the Leakeys' large collection of fossils in Kenya in September, he hopes to clarify whether Homo habilis was an ancestor of man. But there is also a growing feeling that we must avoid the temptation to see our own evolution differently from that of other animals. Gone is the textbook image of a progression from stooping ape to upright human, along with the idea that we represent the apotheosis of evolution. Instead, there is growing appreciation that climate change forced a diversification of hominid species several million years ago - some lineages ended in extinction, our species is all that remains.