Much has been made of the evidence from the so-called Adam and Eve genes, which support the notion that all modern humans alive today have descended from ancestors living in Africa within the past 200,000 years. A recent find of skeletons in Ethiopia, dated to 160,000 years ago, confirms the final transitions between pre-modern and anatomically modern humans in Africa. But the fine details and dates of early human explorers do not just come from advances in the study of genes and bones. Traces of a great natural disaster may allow us to pinpoint just when humans first left Africa.
The scene of the disaster is Lake Toba which today is a popular tourist spot in Sumatra. Toba is the largest lake in South-east Asia - 100km long and 31km wide - and, at 450m deep, it is one of the deepest in the world. Tourists may be unaware that Toba is also the world's largest active volcanic crater. About 74,000 years ago the volcanic eruption of Toba caused the biggest explosion of the past two million years. This "mega-bang", dwarfing the historic eruption of Krakatoa, caused a six-year "nuclear winter" and released ash in a huge plume that spread to the north-west, covering the Indian sub-continent in a blanket of ash between one and three metres deep. The Toba eruption is a valuable date mark, since the ash covered such a wide area, and can still be chemically identified today.
New genetic evidence, using genes only passed down through our mothers, suggests that the ancestors of all non-Africans left Africa, as a single group, via Aden as much as 80,000 years ago. If the date is correct, their descendants could have reached South-east Asia well before the Toba explosion, most likely by beachcombing along the coast of the Indian Ocean. Stone tools have been found covered by volcanic ash from Toba in Kota Tampan in the Malay Peninsula. Were these the tools of modern humans, therefore backing up the genetic evidence for modern humans reaching Sumatra before the Toba explosion? Or were they the tools of an earlier human species? Recent re-dating of the volcanic ash layer at Kota Tampan, which is several metres thick, has forced an extraordinary reappraisal of the significance of the site.
The Kota Tampan site with its evidence of a Palaeolithic human culture is located in the Lenggong Valley, two-thirds of the way from Africa to Australia. This culture was first identified by the find of large pebble-tools, fashioned on one side only. In the absence of skeletal remains at Kota Tampan, the tools were initially thought by archaeologists in the 1960s to be the work of an earlier human species. On the face of it, these were not sophisticated tools.
No one has done more research into Kota Tampan and the Lenggong Valley culture than archaeologist Professor Zuraina Majid, of the University of Science in Penang, Malaysia. Her extensive work at a number of sites in the Lenggong Valley suggests that the local pebble-tool culture may have persisted continuously right up until only 7,000 years ago. If so, it implies that the oval pebble-tools really were made by modern humans.
Professor Majid's trump card is the much-publicised finding by her team of "Perak Man" in the Lenggong Valley in 1990. Surrounded by the same class of pebble-tools, this complete skeleton of a modern human was dated to about 10,000 years ago. This clear recent association of the pebble-tool culture with modern humans undermines the argument that the Kota Tampan pebble-tools were too crude to be the work of modern humans.
Only with recent re-dating of the thick volcanic ash layer at Kota Tampan has the penny finally dropped. When it was first dated several decades ago, the result came out at 31,000 years old. More recently, several geologists, including the one who did the original dating, have agreed on the evidence that the ash overlying the tools belonged to the 74,000-year-old Toba eruption and carried the same geochemical fingerprint as Toba ash found throughout India.
The dating is critical. If the Kota Tampan pebble-tools were made by modern humans, they would be the oldest precisely dated evidence for modern humans outside Africa. It therefore looks as though the ancestors of the Australians left Africa and arrived in Malaysia on their beachcombing trail well before the great Toba explosion. Perhaps as important as the precision of the dating, this connection between stone tools and ashes in Malaysia puts the first Indian and Pakistani colonists in the direct path of the greatest natural calamity to ever befall any humans.
It is difficult to see how the first colonies in India could have survived. So, we could predict a broad human extinction zone in India placed between East and West Asia. Population recovery from the Indian extinction would have added fresh shoots to the Asian genetic tree with a discontinuity between East and West. This prediction is confirmed by a deep East-West genetic division, still clearly seen in Asia's genetic record. Although descended from the same root lines of the single exodus, Indian maternal branch genetic lines are completely different from those of the Far East and mostly different from those in the West.
Another prediction we can make from the scenario of Asian colonisation before Toba, is that genetic lines both inside Africa and in Asia should show the same tight genetic bottleneck - low diversity followed by dramatic expansion - dated to around 71,000 years ago - and probably the effects of worldwide extinctions resulting from the prolonged volcanic winter. This is indeed the case, providing further evidence that our ancestors had left Africa before the great Toba disaster.
Last but not least, there are also local genetic links near the Lenggong Valley with those first explorers out of Africa. With collaborators in England, Italy and Malaysia, I recently performed a genetic survey of the various aboriginal groups living in this part of the Malay Peninsula. Our results, which have just been completed, combined with dental studies, are consistent with the view that these groups have remained isolated in the jungles of the interior ever since the out-of-Africa trek and may have been descended from some of those first pebble-tool makers in the Peninsula.
One might ask how there could be any surviving links if their ancestors had been buried in volcanic ash. Luckily, Kota Tampan was just at the eastern edge of the great ash fall and there would have been survivors in the rest of the Malay Peninsula re-occupying the Lenggong Valley - bearing the torch of humanity into the modern age.
The archaeological, geographical and genetic evidence from this small region of Malaysia all help to pinpoint our ancient relatives as a part of a single small group who left Africa 80,000 years ago and then went on to populate the rest of the planet. So, from a diversity of early human species who used to occupy the globe, we have reduced dramatically to just one species of modern humans alive today.
'Out of Eden: The Peopling of the World', by Stephen Oppenheimer (Constable, £18.99) is published on 24 JulyReuse content