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Machines Like Us

New fossils confirm diversity was the rule for human evolution

Thursday, 09 August 2012
by Darren Curnoe

Image credit: Pavel Švejnar, via Wikimedia Commons

New fossils described in the journal Nature this week seem to close the door on a controversy that has raged for 40 years. They also confirm that the beginnings of the human genus more than 2m years ago began with a burst of biodiversity. These fossils place yet another nail into the coffin of the view we evolved in a gradual, step-like, progression, instead confirming the bushy nature of our evolutionary past.

Four big bursts

The 7m-year-or-so history of human evolution is characterised by four big bursts of biodiversity.

The first burst happened not long after we shared an evolutionary common ancestor with living chimpanzees some 7m years ago. This one produced the three very earliest two-footed ape (or hominin) groups unearthed by anthropologists in the last decade or so: Sahelanthropus from Chad, Orrorin recovered in Kenya and Ardipithecus from Ethiopia.

These groups hint at great diversity among the very early hominins and we can expect many more spectacular discoveries to be made in the future from around this murky time in our biological history.

The second big burst happened from around 4.5 million years ago and heralded the arrival of the well-known hominin Australopithecus (or southern ape). It was described in the 1920s by Australian anatomist Raymond Dart for the skull of the Taung child in South Africa. It was eventually extended to include famous specimens such as the Lucy skeleton from Ethiopia.

Today we recognise four species of Australopithecusanamensis, afarensis, africanus and sediba. The last of them was found by South African scientists only a couple of years ago. Together, they span roughly 4.5m to a little less than 2m years ago.

The southern ape is famous also for its role in fulfilling Darwin’s prediction that the heartland of human evolution was Africa, where gorillas and chimpanzees reside today.

The last two biodiversity bursts occurred coincidentally around 2.5m years ago and marked the arrival of Paranthropus and our own genus Homo.

A large-toothed, ecological specialist, Paranthropus seems to have disappeared by about 1m years ago, while Homo, of course, survives as us until today.

Paranthropus was made famous by Louis and Mary Leakey’s 1959 nut-cracker man. The genus contains three species – aethiopicus, boisei and robustus – inhabiting the savannas of both eastern and southern Africa.

Homo is by far the most diverse of all hominins we know about. Our genus contained at least ten species including we the sole-surviving and youngest, Homo sapiens.

A controversy settled?

The three new fossils announced by Meave Leakey and her team in Nature belong to perhaps the earliest member of Homo. Yet the species in question, Homo rudolfensis, has had a rocky road to acceptance.