by Tony Joseph
Marine Isotope Stages
(MIS) Start date
('000 years ago)
MIS 1 14
MIS 2 29
MIS 3 57
MIS 4 71
MIS 5 130
MIS 6 191
MIS 7 243
MIS 8 300
MIS 9 337
MIS 10 374
MIS 11 424
MIS 12 478
MIS 13 528
MIS 14 563
MIS 15 621
MIS 16 676
MIS 17 712
MIS 18 761
MIS 19 790
MIS 20 814
MIS 21 866
The list continues to MIS 104, beginning 2.614 million years ago.
It is from this MIS data that we know that the climate changed around 57,000 years ago, making for a warmer, wetter world when the deserts of north Africa and the Arabian peninsula were transformed into lush green grasslands, inviting both herbivores and carnivores, including our own ancestors, from the refuges of sub-Saharan Africa. The period before that, 71,000 to 57,000 years ago, was a glacial age classified as MIS 4 when the greenery and the grasslands receded along with the herbivores and the carnivores and deserts returned to being deserts again. The period even before that, 71,000 to 130,000 years ago, was warm, but this mostly falls outside the broad genetic range for OoA. So if the OoA migration did take place during a warm, wet period, then we can say it happened roughly between 60,000 and 50,000 years ago.
You will have noticed by now that while the logic flows easily and there is an overall chronological range that seems robust, the specific times arrived at are fluid rather than firm, with enough room for adjustment. New archaeological or genetic discoveries could shift them this way or that, within the broad range, as geneticists adapt to newer and more granular information.
At the gates of Asia
So far we have been looking at the genetic and climatic sides of the OoA migration story. When we come to archaeology, however, we see things in a slightly different light for reasons explained earlier: genetics mostly focuses on people who left behind a lineage while archaeology looks at people who left behind archaeological evidence, even if not a lineage (this difference between the two disciplines could disappear soon though, as archaeologists and geneticists get their hands on more and more ancient DNA of humans who may or may not have living descendants).
In January 2018, for example, archaeologists announced that upper jaw teeth belonging to a modern human discovered in Misliya in north Israel had been dated to 180,000 years ago, making it the earliest human fossil found outside of Africa ever. The Misliya site was a rock shelter frequently used by archaic or extinct members of the Homo species for hundreds of thousands of years, something like an even older version of our own Bhimbetka. Before the Misliya find, the earliest modern human fossils found outside of Africa were dated to between 80,000 and 120,000 years ago and these were also discovered in Israel, from the nearby Skhul and Qafzeh caves. All these dates are much earlier than the estimated period of OoA migration – between 50,000 and 60,000 years ago.
And it is not just in the Levant (modern-day Syria, Jordan, Lebanon and Israel) that modern human fossils that old have turned up. In April 2018 a team of archaeologists announced they had discovered a modern human finger fossil in the Al Wusta prehistoric lake in north-eastern Saudi Arabia, dated to about 88,000 years ago. This area is a desert now, but it would have been a well-watered and inviting habitat 88,000 years ago, with hundreds of freshwater lakes. ‘We have found 10,000 ancient lakes in Arabia. We have visited about 200, and about 80 per cent have evidence of archaeology,’ said Michael Petraglia of the Max Planck Institute of Germany, whose team made the discovery, in an interview to National Geographic.
Based on current evidence, it is becoming clear that modern humans started emerging in Africa around 300,000 years ago and they began their forays into the Levant by at least 180,000 years ago and into Arabia by at least 88,000 years ago. But the question remains: if modern humans were pushing through the gates of Asia as early as 180,000 to 88,000 years ago, why were these initial forays not successful in terms of leaving behind a lineage that filled out our world? Why did they have to wait till around 60,000 years ago or so before that became possible? Especially when you consider that our evolutionary cousin, Homo erectus, managed to move out of Africa and spread as far as south-east Asia as early as around two million years ago.6 Probably other members of the Homo species also made their way out of Africa much before Homo sapiens did, leaving behind descendant species such as Neanderthals and Denisovans. So what stopped the first modern human explorers out of Africa in their tracks?
There are two likely answers to that question. One is, of course, climate cycles and the other, Neanderthals. The routes out of Africa into Asia probably closed or opened depending on whether the world was going through a cold, glacial period or a warm, interglacial period. The presence of Neanderthals is perhaps even more pertinent. The modern humans that moved into the Levant would almost certainly have come across the Neanderthals, the dominant species in Eurasia by then. There is plenty of archaeological evidence of their presence in the Levant around the same time as the modern humans were there.
Neanderthal remains have been found at the Tabun cave in Israel, close to Skhul, dated to 120,000 years ago, roughly the same period as when modern humans were there. They have also been found at the Kebara cave, not very far from the Skhul–Qafzeh caves, and have been dated to 61,000 to 48,000 years ago. So it is quite possible that modern humans found it difficult to progress into Eurasia from the Levant because they couldn’t prevail over the Neanderthals who were well adapted to the colder climate of Europe. Our ancestors ultimately did prevail over the Neanderthals tens of thousands of years later in Europe itself, but it should be noted that the first encounters between us perhaps didn’t go in our favour.
A proper understanding of what happened during the first attempts of the modern humans to break into Eurasia, however, is not possible without a grasp of both geography and climate. There were four possible paths for ancient modern humans out of Africa and into Eurasia – from Morocco in north-western Africa to Spain across the Strait of Gibraltar; from Tunisia into Sicily; from Egypt into the Sinai peninsula and on to the Levant; and from Eritrea in eastern Africa to Yemen and Saudi Arabia across the Bab el Mandeb at the southern tip of the Red Sea. Of these four possibilities, there is no evidence that the first or the second routes were ever used. But we have plenty of evidence that the third and fourth routes were used – whenever the climate allowed these routes to be open.
For much of what palaeoscientists call the Pleistocene (between 2.58 million and 11,700 years ago), the climate would have been very cold and the Sahara and the Sinai would have been deserts, making the route to the Levant from Egypt a difficult one. But thankfully, there were interglacials, or warmer periods (these are the oddly numbered MIS stages, as we discussed earlier), when the weather became wetter and the deserts became green and passable. During these periods, it would have been possible for both human and animal migrations to occur between Africa and Eurasia. The period between 243,000 and 191,000 years ago was an interglacial (MIS 7), a relatively warm period, and so was the period from 130,000 to 71,000 years ago (MIS 5). The recent Misliya fossil (180,000 years old) find comes broadly within the first period, while the Skhul–Qafzeh finds (80,000 to 120,000 years old) in Israel fall within the second period.
Whenever the warm interglacial periods ended and the climate cooled again, the Neanderthals already in central or northern Eurasia would perhaps have moved down to southern Eurasia in greater numbers in search of slightly warmer climes, putting pressure on the newly arrived Homo sapiens there. It is thus possible that the climate cycles and the presence of Neanderthals together are what scuppered the attempts of the first modern humans to colonize the rest of the world from Africa through the Levant.
While we are using modern-day geographical descriptions to exp
lain what happened, our ancient ancestors wouldn’t have been thinking in terms of moving from one continent to another at all. They would have been merely expanding or contracting the range of their movements, as climate patterns changed and along with it the regional spread of the flora and fauna on which they depended. Dry, cold periods would have shrunk their range and warm, wet periods would have invited them, along with the animals they were hunting to eat, to newer pastures.
We do not know whether these early modern human occupants of the Levant managed to get back to Africa before the Sinai and the Sahara turned to dry deserts again or whether they just got cut off from Africa by these expanding deserts as the cold wave swept in, while also being hemmed in by the Neanderthals moving southward, and thus perished. What we do know is that after these first pieces of evidence of modern human presence in the Levant, we had to wait more than 30,000 years to see the next evidence of their presence in the region again, around 50,000 years ago – and this time, obviously, they survived.
Breakthrough at the Gate of Tears
While this was playing out on the Egypt to Levant route, usually called the Northern Route to Asia from Africa, there was action on the Southern Route too, which goes from Eritrea through Bab el Mandeb at the tip of the Red Sea, to Yemen and Saudi Arabia. (Bab el Mandeb means Gate of Tears. One legend attributes it to the cries of the drowned as an earthquake clove apart Asia and Africa, while another legend says it is simply a warning to travellers of the dangers of trying to cross the sea as it was full of reefs.)
Unlike the Northern Route, which does not involve any crossing of the sea, the Southern Route involves the crossing of the Red Sea at Bab el Mandeb – a distance of about thirty kilometres currently. But during glacial periods, when the climate is cold and dry, the sea level recedes, thus reducing the distance across the Red Sea to a third, making the crossing somewhat easier. There is no archaeological proof that modern humans of this age had figured out how to build boats, but it is possible that they had, because there is evidence that they had been leading a beachcombing lifestyle for thousands of years, living off marine resources such as fish and shells. Boats would have been a nice and natural addition to that lifestyle.
Whatever that may be, what we do know is that the cold and dry glacial periods that made life impossible for modern humans in the Levant may have been a little kinder on those making a move through the Southern Route. Not only because it made the Red Sea crossing easier, but also because once they crossed over to Arabia, the monsoons may have compensated somewhat for the dry aridity of the glacial ages at least in the coastal areas. (The monsoons are a side effect of the existence of the Himalayas, which were formed as a consequence of the Indian tectonic plate pushing into the Eurasian plate, beginning some fifty million years ago.)
So even during the coldest periods, some archaeologists posit, modern humans may have been able to cross into Asia at Bab el Mandeb. As we will shortly see, there is reason to be sceptical of the climatically deduced OoA migration window between 50,000 and 60,000 years ago. In fact, some archaeologists have been pushing for a much earlier migration out of Africa. Michael Petraglia is one of them. In 2012 he decided to back his convictions with an ambitious research project called ‘Palaeodeserts’, which focused on Saudi Arabia and intended to uncover ancient connections between Africa and the rest of Eurasia. It was under this project that the 88,000-year-old finger fossil was discovered at the Al Wusta prehistoric lake in north-eastern Saudi Arabia. Talking about the discoveries that were being made, Petraglia said in an interview to Nature Asia, ‘The most amazing thing to me are the fossil finds. They say something about the kinds of animals that could migrate into Arabia. We have fossils of elephants; these are gigantic creatures, much larger than the African elephant. Amazingly, we also have hippos. These finds tell you something of how wet it really was. Because the hippos cannot survive in very arid and dry situations, so the environment had to be green for them to survive.’
Was the person whose finger was found in Al Wusta part of the successful migration that later went on to populate the rest of the world? Definitely not, because the dating of that fossil, at 88,000 years ago, puts it beyond the maximum range for OoA migration that geneticists have deemed possible. So it is likely that this early migrant was part of a group that perished, much like those in the Levant. Or to propose a more radical idea, perhaps a subsection of his group managed to escape the aridity of interior Arabia as the climate changed, made it to south Asia, and perhaps even managed to make it to south-east Asia and Australia over thousands of years, but all of them perished for a combination of reasons or perhaps mostly because of the Toba supervolcanic eruption, the most violent volcanic eruption in the past two million years, thus leaving behind no genetic trace in today’s populations.
What Petraglia’s findings in the Arabian desert prove is that the Southern Route was very much viable, and that it is almost certainly the route taken by those modern humans who went on to fill up the world at the time of the OoA migration.7
The Arabian chapter in the history of the first modern human migration is momentous for quite a different reason too. This is the most likely place where modern humans and Neanderthals first met, mated and left behind a genetic trail in all non-African modern human genomes that is still detectable today. All non-Africans carry about 2 per cent of Neanderthal genome.
There is no reason why this interbreeding couldn’t have happened in different regions repeatedly. But it is simpler to assume that at least one mixing happened near where the first migrants broke out of Africa and before they split to go their different ways because that is the easiest explanation as to how all non-Africans came to possess a similar amount of Neanderthal DNA.
When this discovery of Neanderthal genes in humans was first announced in 2010, the world was shocked because until then we had considered Neanderthals quite inferior to us, and as belonging to a different species that would not have been able to reproduce with us. Now, of course, we know that we interbred not just with Neanderthals but with Denisovans as well, and that this may not be the full story either. In Africa and elsewhere too, research is throwing up the increasing likelihood that there were more interbreeding events between modern humans and our genetic cousins, some of whom we may not even have identified yet.
Now that we have taken the story out of Africa and into Asia, this may be a good time to bring in evidence from another part of the world that allows us to get a clearer picture of when the OoA episode took place and what might have happened after it. That part of the world is Australia, and it is a crucial element of any story about the first migrants. In June 2018 a team of scientists led by the archaeologist Chris Clarkson of the University of Queensland, Australia, established that humans were in that continent by between 59,300 and 70,700 years ago, or if you take the midpoint, by 65,000 years ago.
The scientists did by this by careful dating of things left behind by modern humans in a cave in Madjedbebe in Australia’s Northern Territory – things such as mortars, pestles, ground-edge axes and painting material. Since Australia has never been populated by any member of the Homo species other than Homo sapiens, there is little doubt that this was the work of our own species. In July 2018 another study led by Kira Westway of Macquarie University, in Sydney, declared that two teeth discovered from the Lida Ajer cave in Indonesia’s Sumatra island a century ago had been securely dated to between 63,000 and 73,000 years ago, and that they belonged to modern humans. Together, these two studies have pushed back the date for modern human occupation of south-east Asia and Australia by 15,000 to 20,000 years and put severe constraints on both the timing of the OoA episode and the mode of migration.
Even if you take the lowest estimate of these two studies, it is clear that modern humans were in south-east Asia by at least 63,000 years ago. That means the OoA episode has to be outside the postulated range of 50,000 to 60,000 years ago. There are only two ways to square this circle. One, either the people who left behind
fossils and tools at Madjedbebe and Lida Ajer were part of an earlier wave of migrants from Africa who failed to leave behind a genetic lineage, or the OoA episode happened much earlier than previously believed, even if this means modern humans crossed over to Arabia during a cold and dry glacial period rather than a wetter, warmer interglacial period. This would push the date for OoA closer to the absolute limit of genetic possibility for OoA at 80,000 years ago. Let us consider this option and see where it goes.
The race through Asia
What is a reasonable amount of time for hunter-gatherers to walk down from, say, Yemen to Australia, taking into account that they would not be walking with the intention to reach Australia, but merely to hunt, gather, eat and survive? There is no easy way of telling, but we do know from archaeological evidence that it took the first migrants to America (about 16,000 years ago) only a couple of thousand years to reach the tip of South America from Alaska – a distance of about 21,000 kilometres. They probably moved down the coastal route on the western seaboard of the Americas. The beauty of a coastal route is that it makes the migration process faster and simpler in two ways: one, the migrating hunter-gatherers do not have to keep upgrading or changing their life skills dramatically, so their progress can be quick. Two, a coastal route gives an unintentional direction to their migration, taking them forward inexorably, unlike an inland route with its surprises, roundabouts and uncertainties.
Adapted from Stephen Oppenheimer, ‘Out-of-Africa, the Peopling of Continents and Islands: Tracing Uniparental Genes Across the Map’, Philosophical Transactions of the Royal Society B, The Royal Society Publishing, 2012