An international team of anthropologists working in Ethiopia has found 50 bone fragments and teeth from 17 to 20 early hominids dating from 4.4 million years ago. They are 500,000 years older than any other human fossil to have been found.
However, the most fascinating and tantalising aspect of the discovery is the dental evidence which suggests that these early humans may be from the period immediately after the evolutionary split between apes and humans.
In recent years genetic evidence collected by scientists from modern human and ape populations has suggested that the ape/human split occurred between about 5 million and 9 million years ago.
The new evidence - published today in the science magazine Nature - suggests that the split occurred in or even after the latter part of that period - perhaps between 4.5 million and 5.5 million years ago.
At 4.4 million years, the new fossils, dubbed Australopithicus ramidus by scientists, are therefore likely to be at the beginning of human evolution - 1.9 million years before the earliest known stone tools, 2.6 million years before the emergence of hominids with a more recognisably human physique and 4.3 million years before the development of Homo sapiens sapiens - modern humans, the only hominids to have survived to the present day.
The 50 early human fossils found so far were discovered over the past two years near the village of Aramis in the middle Awash Valley, 230 kilometres north-east of Ethiopia's capital Addis Ababa. The discovery has been a strictly kept secret until today. Further finds are expected this autumn.
The first fossil was discovered by Dr Gen Suwa, of Tokyo University. As he walked across the desert in the scorching midday sun he saw the glint of a human tooth among the thousands of pebbles.
Although the area is desert, 4.4 million years ago it was woodland inhabited by monkeys, antelopes, hyenas, sabre-toothed cats, rhinos, elephants, bears, bats, rodents - and humans.
The human remains include 20 teeth from two individuals, a child's jaw, skull fragments from two individuals, an upper arm, a complete left arm and several other teeth. 'These fossils represent the oldest direct human ancestors ever found,' the joint leader of the international team, Dr Berhane Asfaw, of the Ethiopian government's palaeoanthropology laboratory, said.
Detailed research is still to be carried out on the diet and lifestyle of the newly discovered species.
It is likely that they were predominantly vegetarian. However, they would probably have scavenged meat and may have banded together to hunt small animals.
They almost certainly made no stone tools - but it is quite likely that they fashioned rudimentary temporary tools out of sticks and small branches.
For safety they would have slept in trees - but in the daytime they were probably equally adept on the ground as they were in the branches. However, although they walked upright - almost like modern humans - their mental abilities were at the ape stage.
In terms of modern primates, the species probably resembled a modern bonobo chimpanzee standing upright. However their faces would perhaps have been slightly flatter and more robust- looking - and, unlike bonobos, the males (about 4ft 6in tall and weighting 60 kilos) were probably substantially larger than the females.
The age of 4.4 million years was attained by measuring the decay of potassium in rock crystals laid down in a volcanic eruption which immediately pre-dates the bones.
Potassium decays into argon gas at a known rate over time and by measuring the amount of argon 'locked up' within a rock crystal, scientists can work out the date of deposition. The gas is liberated from the crystal with a laser beam - a new technique known as Single Crystal Laser Fusion (SCLF). The date was then confirmed by geological and comparative fauna dating methods.
The search for more bones in Ethiopia will intensify this autumn when the 20-strong international team starts a new season of exploration.
The complete arm which the team found was the only group of bones not gnawed by carnivores. Scientists suspect that the rest of the skeleton may await discovery.
Of particular interest will be any leg bone fragments, as these - together with dental evidence - will shed light on the nature of the great evolutionary split between apes and humans.
The famous Missing Link of popular tradition has been gradually uncovered over recent decades - but this discovery does fill a crucial gap in the evolutionary jigsaw.
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