A confusão e a incerteza sobre a evolução humana perdura há 50 anos...

segunda-feira, abril 07, 2014

Human evolution: Fifty years after Homo habilis

Bernard Wood

02 April 2014

Bernard Wood explains why the announcement of 'handy man' in April 1964 threw the field of hominin evolution into a turmoil that continues to this day.

Natural History Museum/Mary Evans Picture Library
The foot of 'handy man', Homo habilis.

Half a century ago, the British–Kenyan palaeoanthropologist Louis Leakey and his colleagues made a controversial proposal: a collection of fossils from the Great Rift Valley in Tanzania belonged to a new species within our own genus1. The announcement of Homo habilis was a turning point in palaeoanthropology. It shifted the search for the first humans from Asia to Africa and began a controversy that endures to this day. Even with all the fossil evidence and analytical techniques from the past 50 years, a convincing hypothesis for the origin of Homo remains elusive.

In 1960, the twig of the tree of life that contains hominins — modern humans, their ancestors, and other forms more closely related to humans than to chimpanzees and bonobos — looked remarkably straightforward. At its base was Australopithecus, the apeman that palaeoanthropologists had been recovering in southern Africa since the 1920s. This, the thinking went, was replaced by the taller, larger-brained Homo erectus from Asia, which spread to Europe and evolved into Neanderthals, which evolved into Homo sapiens. But what lay between the australopiths and H. erectus, the first known human?

Betting on Africa

Until the 1960s, H. erectus had been found only in Asia. But when primitive stone-chopping tools were uncovered at Olduvai Gorge in Tanzania, Leakey became convinced that this is where he would find the earliest stone-tool makers, who he assumed would belong to our genus. Maybe, like the australopiths, our human ancestors also originated in Africa.

In 1931, Leakey began intensive prospecting and excavation at Olduvai Gorge, 33 years before he announced the new human species. Now tourists travel to Olduvai on paved roads in air-conditioned buses; in the 1930s in the rainy season, the journey from Nairobi could take weeks. The ravines at Olduvai offered unparalleled access to ancient strata, but fieldwork was no picnic in the park. Water was often scarce. Leakey and his team had to learn to share Olduvai with all of the wild animals that lived there, lions included.

They found the first trace of the potential toolmaker, two hominin teeth, in 1955. But these were milk teeth, which are not as easy to link to taxa as permanent teeth. The team's persistence was rewarded in 1959, when archaeologist Mary Leakey, Louis's wife, recovered the cranium of a young adult. The specimen still boggles the mind because it is so strange: its small brain, large face, tiny canines and massive, thumbnail-sized chewing teeth were not at all like those of H. erectus. Its big molars earned it the nickname 'Nutcracker Man'.

Because Nutcracker Man was found in the same layers as the stone tools, the Leakeys assumed that it was the toolmaker, despite its odd appearance. But when Louis announced the discovery, he was not tempted to expand the definition of Homo. That would have eliminated any meaningful distinction between humans and australopiths. Instead he erected a new genus and species, Zinjanthropus boisei (now called Paranthropus boisei), to accommodate it (see 'Who was related to whom?').

Australopithecus: Sabena Jane Blackbird/Alamy; H. habilis: Human Origins Program/Smithsonian Institution; P. boisei: Natural History Museum/SPL; H. neanderthalensis: Javier Trueba/MSF/SPL