The fossils, which are 4.4 million years old, predate any other human fossils so far found by about half a million years. They consist of 50 early hominid bone fragments and teeth from 17 to 20 individuals.
Among the discoveries by a joint American-Japanese-Ethiopian expedition in eastern Ethiopia are fragments of skulls, limbs and jaws dating from the period after the evolutionary split between apes and humans, 4.5 to 7 million years ago. They are the most important early human find for 20 years.
The discovery is announced today in Nature, the London-based scientific magazine. Professor Tim White, of the University of California at Berkeley, joint leader of the expedition, says the find is 'the oldest known link in the evolutionary chain that connected us to the common ancestor we share with modern African apes'.
The bones provide the world's earliest evidence of bipedalism - moving around on two legs instead of four - which is one of the first evolutionary steps marking the change from ape to human.
'The short cranial base and the hominid shapes of the canine teeth and elbow show us that this species had already split from the apes,' said another joint leader of the expedition, Dr Berhane Asfaw, of the Ethiopian Government's Paleoanthropology Laboratory.
Scientists are calling the newly discovered species Australopithecus ramidus after the local tribal word for root (ramid).
The ramidus individuals were mainly vegetarian and lived in a woodland environment about 1.2 million years, or 80,000 generations, before the birth of the most famous early hominid, Lucy.
They probably used stones and pieces of wood as tools, and may even have modified sticks to help with specific tasks, as chimpanzees still do. They would probably have slept high up in trees.
Mentally they were apes - but physically they had taken the first evolutionary steps along the human road.
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