A barren piece of desert in the heart of Ethiopia has proved once more why it deserves to be called the "cradle of mankind".
Scientists have unearthed a set of fossils in the Ethiopian Afar region that they believe is a "missing link" between a primitive ape-like creature that lived more than 4.4 million years ago and a later ape-man who became our own ancestor.
The discovery means that the region now boasts the discovery of the fossilised remains of eight distinct species that represent different stages in the evolutionary transition from ape to anatomically modern man.
The latest fossil find belongs to a species called Australopithecus anamensis, which lived about 4.2 million years ago, between the earlier Ardipithecus ramidus and the later Australopithecus afarensis.
Professor Tim White of the University of California, Berkeley, said that the discovery filled a million-year gap in the human fossil record between our ape-man ancestors - the australopithecines - who lived 3.5 million years ago and the even more primitive Ardipithecus. "This new discovery closes the gap between the fully blown Australopithecines and earlier forms we call Ardipithecus. We now know where Australopithecus came from before 4 million years ago," Professor White said.
Scientists found the latest fossils, which belong to at least eight individuals, in an area of Afar called the Middle Awash, which lies about 140 miles north-east of the capital, Addis Ababa. Fossils of ape-like creatures dating back to 5.7 million years had already been found at the site. The most recent fossils belong to some of the oldest anatomically modern humans, who lived there about 80,000 years ago.
"Here, in a single Ethiopian valley, we have nearly a mile-thick stack of superimposed sediments and 12 horizons [layers] yielding hominid fossils," Professor White said. "These discoveries confirm the Middle Awash study area as the world's best window on human evolution."
The latest fossils belong to a creature that was more ape than human but nevertheless walked on two legs. Australopithecus anamensis was thick-set and short with a small brain and big cheek teeth for grinding the toughest plants. It is not known whether Australopithecus was a direct descendant of the earlier Ardipithecus, who had smaller teeth, but the fact that they both lived in the same area at apparently different times suggests this is one possibility. "It is fair to say some species of Ardipithecus gave rise to Australopithecus," said Professor White, who led the study published today in the journal Nature.
"Australopithecus became a superior omnivore, able to eat tubers and roots with more fibre and grit, adapting it better to times of scarcity during periods of extended drought," he said.
"They may have been small brained, but they stuck around a long time, fully half of our zoological family's six-million-year existence on the planet."
Hundreds of other fossilised bones have been dated to the same period. Giday WoldeGabriel, a geologist at the Los Alamos National Laboratory in New Mexico and the study's co-leader, said the bones belonged to species such as colobus monkeys, kudus, pigs, birds, rodents, hyenas and big predatory cats. "The abundance of monkeys, kudus and other mammals, and petrified wood, shows that a closed, wooded-habitat type persisted over a long period in this part of the Afar and was favoured by hominids between four and six million years ago," he said.
The latest fossils are mostly of teeth, some of which had to be painstakingly reconstructed after shattering into many tiny pieces when they had been exposed to wind and erosion.
Scientists have now built up a catalogue of 246 separate hominid specimens from fossils unearthed from a dozen different layers of sediments in the ground which each represent a distinct period of evolutionary history.
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