This pattern of geographical spread would fit well with our model of sources and sinks. If south-east Asia was, as we proposed back in 2000,10 a major source of human expansions then a Denisovan origin in south-east Asia and subsequent expansion northwards should not surprise us. This would not have been unusual either. When I looked at the pattern of distribution of Palaearctic birds11 I found that many diverse south-east Asian groups had contributed populations and species northwards. Some of these species had initially adapted from tropical to more temperate climates by moving up the slopes of the Himalayas.12 When cooler conditions shifted climate belts southwards there would have been moments when comparable altitudinal and latitudinal climatic belts would have merged, releasing the populations trapped in a particular altitude zone. Then, when the climate belts shifted north once again, they would have carried new species with them. Once established on the Eurasian plains many of these species (including such diverse groups as crows, bullfinches, and orioles), well adapted to the climatic conditions, rapidly spread westwards across vast distances that had few topographical barriers, many reaching all the way to western Europe. I called this the Himalayan launchpad and found many examples among bird species (Fig. 5).
The point that is important here is that these populations spread north during climate warming, but tracking temperate, not tropical, belts that they had adapted to on the slopes of the Himalayas.13 Once in northern Eurasia they did not have to adapt to new conditions—they were already used to them and had tracked them north. The beauty of this model is that it allows populations time to adapt to new conditions within stable source areas. We saw in Chapter 6 how populations of early humans in tropical south-east Asia had moved up mountains in search of water when conditions became drier in the lowlands so we have a good basis for our model and it is, once more, linked directly with water. So I propose that the Denisovans were one such population, one that spread northwards and then westwards following temperate belts, and it is no coincidence that we discover them on the slopes of the Altai Mountains, on the other side of the Himalayas, and associated mountain ranges.
FIGURE 5A. Sequence showing how the Himalayan launchpad works: 5a and 5b show an intitial, tropical population; 5C and 5D show the population having dispersed up mountain slopes, during a cool and arid phase, in search of humid locations. Comparable climatic environments are available, but are disconnected and empty, to the north; 5E and 5F show a cooling that brings the climatic belt and humans down the hills. The belt to the north spreads south and merges with the altitude band allowing humans to spread; 5G and 5H show a subsequent warming with the belts once again separated from each other. This time humans occupy the altitude and latitude belts. Once in the latitude belt, human populations (P) can spread horizontally across large areas of land while those in the south are confined to their mountain refuges.
FIGURE 5B.
FIGURE 5C.
FIGURE 5D.
FIGURE 5E.
FIGURE 5F.
The Densiovans contributed genes to present-day Melanesians but not to Chinese or Australians. How can we explain this disparity? If the Denisovans entered south-east Asia from southern Middle Earth during a cold and dry phase when the rainforest opened up, we would expect them to have colonized mountain slopes where they could find water. Later arrivals of rain chasers probably behaved in a similar manner allowing opportunities for genetic interchange. But we have also seen how earlier human populations had also skirted round the expanding rainforest during warm and wet conditions by concentrating on the coast. We can see how mountain slope and coastal populations would have become segregated during such times. So a population of coastal humans may have proceeded towards Australia having had no contact with the mountain Denisovans. And, if the Denisovans followed a specific climatic belt, this could have kept them apart from later waves of people that entered China.
What of the Neanderthal lineage—could they have entered Eurasia via a similar mountain launchpad route? I think so, except that they would have accessed Eurasia west, not east, of the Himalayas and the opportunities would have been limited in comparison. When they met the Denisovan lineage they would have been occupying a similar climatic belt and would have come into direct contact with each other; and they were similar enough still to have been able to exchange genes. Let us explore this a little bit further.
FIGURE 5G.
FIGURE 5H.
The Himalayan launchpad relies on the warm tropical conditions of its southern slopes. Here the Denisovans and other humans could go up during cold and dry events because it was wetter the higher you went and it was not too cold. Essentially they reached relatively humid and temperate conditions with increasing altitude, up to a point when it became too cold. In the west (Arabia, Turkey, Syria, Iraq, and Iran), going up mountains during cold and dry phases would have taken the Neanderthal lineage people towards wetter environments but conditions would have been much colder than in the south-east Asian tropics and subtropics. So the altitude belt that would have permitted survival in such mountain ranges as the Zagros or the Caucasus would have been much lower down, narrower, and restricted.
The result would have been that at many times populations became trapped in particular altitude zones and, if there was little contact with other populations or the periods of isolation were very long, they would have died out. The ideal places for survival would have been where mountains and valleys existed in close succession, affording opportunities for movement up and down with climate change. Turkey, with its high plateau, was probably unsuitable.14 The populations inhabiting mountain refuges in the Arabian Mountains15 to the south would have been too detached but the Caucasus16 and Zagros17 Mountains would appear to have some survival potential within limited altitude belts. The nature of these locations would have promoted regional cultures, evident in the boundaries between stone-tool technologies in the Caucasus.18 It is not surprising either, given the high risk of population isolation and the Neanderthal abhorrence of extremely continental climates, that they seem to have died out at an early stage in these mountains.19
So the Neanderthal lineage would have penetrated into central Asia in the first instance, probably via present-day Iran or across the Caucasus, by moving up and down altitude zones as climate alternated between cold and dry (moving up the slopes in search of water but limited by cold) and warm and wet (moving down slopes into inter-montane valleys and plateaux). The cold and dry periods would be spent in wet hillside refuges that were probably isolated from each other and the warm wet periods would have launched the populations northwards. Once in the northern mountains, such as the Siberian Altai, they would have kept to particular climate belts which would, again, have moved up and down with the vicissitudes of climate. Penetration of the cold steppes and deserts would have been extremely difficult for these populations.20
We do not know how many times the Himalayas and Caucasus-Zagros might have worked as launchpads. It probably happened frequently and with variable success. We will probably never discover the failures, only the successes: we have known of the Neanderthals for a long time but we have only just discovered the Denisovans. We have little information about these people who were the contemporaries of the Neanderthals. We know of their existence from analysis of their DNA from fragmentary and undiagnostic bones recovered from Denisova Cave in Siberia. They must have once had a widespread geographical range and their genetic signal persists today in a number of human populations, particularly Polynesians. There must have been others. One successful entry seems to have happened relatively recently, bringing a population of sapiens into central Asia around 45 thousand years ago.21 This entry happened during one of the coldest and driest periods of the Earth’s recent history, an apparent paradox.
There is a simple possible explanation. As would have happened many times before, a population of rain chasers living south of the Caucasus-Zagros launchpad was hit hard by arid conditions. Each time this happened the resultant move up into the hills for wat
er would have been magnified as some human populations became better at coping with water shortages. The biological and cultural improvements for dealing with water shortages would have allowed humans to survive, as the Neanderthal ancestors had previously done, in higher altitude belts. The climate was colder and drier than ever before but humans would have been better prepared than their predecessors, so much so that these rain chasers were even able to spread into the drier interiors and eventually reach central Asia.
Once in central Asia, the behaviour of the new human population arriving from southern Middle Earth is a classic example of the kind of ecological release that I observed among many species of birds. Once in the new latitude band, movement west and east would have been very fast, taking these humans to the coasts of the Atlantic and Pacific in a relatively brief period. The plains-adapted humans found a rich world of herds of herbivores, one that could be accessed readily by a highly mobile hunter with a correspondingly lightweight toolkit which, by 45 thousand years ago, included projectile technology as well as composite tools.22 Finding water may still have been a problem once in these high latitudes but now they had new opportunities and solutions. When not readily available, ice and fire would have been combined to generate the vital liquid. The adaptation for moving around over vast distances in search of water, the defining feature of the rain chasers, was now being put to good use in tracking down herds of reindeer and other migratory herbivores. Their ability to do what the bulky Neanderthals could not do so well was an outcome of having had a rain-chasing career. Importantly, it allowed them to break away from the restrictive altitude belts of the high mountains that ran from Iberia to China and onto the plains beyond.
In the process, they would have come across Neanderthals but not Denisovans. We know this from the genetic contribution of the Neanderthals to the European human genome and the absence of such a contribution from the Denisovans. This is a strong indication that the entry was from the Caucasus-Zagros launchpad into southern Siberia. Somewhere in between, before the entry into Europe or the uninhabited plains, Neanderthals and rain chasers exchanged genes. Once on the plains, the rain chasers were on their own, at least until they came into contact with populations of Neanderthals in the west where the cold mammoth-steppe was reaching the latter’s receding world. It was only in the oceanic west that the Neanderthals had penetrated the plains to the north of Middle Earth and as these Atlantic forests were becoming open steppe, the Neanderthals retreated and the newly arrived humans advanced. The Neanderthals survived in refuges, deep in the warmest and wettest part of Middle Earth which also happened to be the last places that the newcomers reached.
When we looked at the habitat of the Neanderthals using birds as indicators (see Chapter 8), we found the consistent trend that characterized Homo sapiens throughout its career: it lived in places with water, trees, and open spaces in combination. We also found that the Neanderthals were the first to exploit rocky habitats to the full. The pattern of substituting rocky places for trees as habitats is another feature that I identified in the success of a large number of bird species in the Palaearctic.23 Many species belonging to families of tree nesting birds had taken to nesting on rocky surfaces.24 The adaptation catapulted their east-west spread along the mountain chains of Middle Earth and, in some cases, south along the Rift Valley. The Neanderthal lineage behaved in similar fashion once it took to the world of rocks and caves, a world which would have characterized the mountain slopes that had marked their point of entry into Eurasia and their subsequent expansion with the continent.
We then looked at bird indicators of the habitat of the rain chasers in Eurasia. We found many similarities with the Neanderthals but also some very telling differences. The habitat specificity of the Neanderthals, so typical of all Homo sapiens, fizzled out when looking at the newcomers in Eurasia. Places with trees and open spaces were included but so were places without trees. Water was never forgotten and rocky habitats were also included but the ability to cut across vast expanses of openness was a new feature that characterized the rain chasers in Eurasia, just as it probably did where they originated, somewhere in the core of southern Middle Earth. It was made possible by 2 million years of evolution of a lightweight body built for endurance running and the development of portable projectile technology.
At the time that these events were happening in central Asia, between 45 and 40 thousand years ago, humans had been in Australia for at least 15–20 thousand years. That they should have taken so much longer to disperse from southern Middle Earth into nearby Eurasia than to distant Australia seems odd. My own view is that entering central Asia took these humans into a world of other humans, Neanderthals and Denisovans at least, who were on familiar home ground. If we take the view, which I advocate, that all these peoples were part of the large widespread Homo sapiens family, then we would not expect the locals to have allowed themselves to be overrun by the newcomers. That scenario would only occur if the immigrants had some kind of superiority over the residents. Now, if we take the view that the rain chasers were more intelligent than the Neanderthals, rapid takeover and domination might be expected, but that does not then explain why they should have taken so long to penetrate into Neanderthal territory. My own view is that the Neanderthals were doing very well on the slopes of the Zagros, Caucasus, Altai, and other ranges and they simply kept the newcomers at bay. This need not have been an active defence of territory, although some of that would have surely happened, but a simple process of preventing their demographic expansion by limiting their access to resources. Clearly they met along the way and interchanged genes in the process, as we have seen.
Those entering Australia had no such difficulty. Australia was uninhabited and, once in, there was no resistance of this kind. There were people along the way, in south-east Asia, where early human populations would have been around from the very beginning. Why did this particular expanding population of humans not meet resistance from the locals? I have already suggested how they would have, unconsciously, avoided the Denisovans who would have been occupying humid temperate belts in south-east Asia during cold and dry conditions. These would have been the conditions that would have opened up the rainforest and permitted an expansion of the rain chasers into south-east Asia. My hunch is that they may never have come across anyone at all or, at most, a small and highly scattered population that could not have had the impact on the immigrants that the Neanderthals did in Eurasia. The reason for that has to be the impact of the Toba Volcano eruption.25
The Toba super-eruption in Sumatra of 74 thousand years ago would have had a devastating effect on populations that were living in the south-east Asian heartland. The event was one of the world’s largest known eruptions and is thought to have generated a volcanic winter that may have lasted up to 10 years, causing also a 1,000-year-long global climate-cooling. In a recent paper Mike Petraglia at Oxford has downplayed the impact of Toba on people living in India and further away from Toba.26 While it makes sense that at least some populations away from the eruption may have survived, the situation closer to home would have been far different especially when the post-eruption effects, notably climate cooling and aridification, are considered.27 The consequent opening up of the rainforest let the rain chasers in. Their adaptability to arid environments would have enabled them to survive the super-eruption and subsequent aridity in southern Middle Earth and stage an early recovery. When the gates of south-east Asia opened up, humans were ready to use this new launchpad28 to catapult themselves in.
We have little evidence of the rain chasers in south-east Asia, other than at Niah Cave on the island of Borneo. Here people exploited a variety of environments including, unusually, the rainforest itself; but they seem to have done so from the river networks and there is evidence that suggests that these people started to open up the landscape with the use of fire. This would be the first case in our history of habitat management, curiously and significantly turning dense forest into places that combined the an
cient, tried-and-tested formula of water/trees/open-spaces. The human presence at Niah Cave would be quite late at 45 thousand years ago but there is increasing evidence that humans may have been in the region by 67 thousand years ago,29 after the super-eruption.
The nature of the geography, climate, and vegetation of southeast Asia is such that it would have promoted life along rivers and coasts during warm and wet phases when rainforest engulfed the land. We saw this happening to humans in the early stages of our evolution, with the first recorded sea-crossing to Flores. It is probably no coincidence that a similar way of responding to such conditions by the new populations got them to new worlds. The arrival of humans in Australia was probably the consequence of a population expansion along rivers and coasts by people used to living in arid lands, and its ultimate cause was a giant volcanic explosion.
The thrust of this book so far has been that there has been a long interrelationship between climate change and human evolution and that the main driver behind this story has been water. The world has been getting progressively more arid and a lineage that led to all of us in the world today managed to survive by constantly adapting to this changing world. These ideas can be drawn together in what I have called the Water Optimization Hypothesis.30
The Improbable Primate Page 12
