The Improbable Primate

by Finlayson, Clive

  When I was starting to write about human evolution, I wrote a paper with my wife Geraldine and my friend and colleague at the Gibraltar Museum, Darren Fa. The paper was published in 2000 and it summarized our views on human geography at the time.23 The three of us were ecologists and biogeographers and we were unsatisfied with the existing models of human dispersal. In that paper, and again later in my book Neanderthals and Modern Humans,24 we put forward a model of sources and sinks in human evolution. In it we proposed that the persistence of populations of Homo (including erectus) was expected to be highest in tropical areas, which we defined as sources, and lowest in higher latitudes, which we defined as sinks.25 Tropical Africa was a major source area and we argued that south-east Asia became a source area once colonized. At the time we were working with the established paradigm that Homo erectus had colonized Asia from Africa. We argued that mid-latitude areas, such as the Mediterranean, were secondary sources where populations could sometimes hang on during bad times, which fed populations at higher latitudes. This model fits with the idea that populations on the northern edge of Middle Earth kept going locally extinct and subsequently restocked from the south.

  There is another, very important, factor that we need to add to this north-south interpretation of human survival based on temperature and day length. That factor is water, and its impact was huge at a time when the world was becoming much drier. In particular, the expansion of the central Asian deserts and the Sahara introduced major barriers to the movement of the water-dependent humans. Like Arabia, the Sahara alternated from barren desert to a land of lakes26 when humans would have thrived along lakesides and riversides just as Sahelanthropus and the australopithecines had done millions of years earlier. The Sahara was part-time paradise and part-time hell. To understand the implications of these gigantic desert barriers I will draw from the world of birds. If we compare the diversity of birds in the west and the east of Eurasia, Europe, and China respectively, we find that China wins easily.27 Species of tropical families are also better represented in China than in Europe and we can find similar patterns among other groups. Part of the reason for the difference, at least, lies in the climatic continuity between China and tropical regions to the south while Europe is severed from tropical regions to the south by the Sahara Desert. This means that extinction would have been higher in Europe than in China during glacial periods. In Europe populations became trapped in the Mediterranean cul-de-sac but in China they could shift their ranges further south. Replenishment from the south during warmer periods was also easier in China than in Europe. One consequence, in birds at least, is that the European fauna has stronger affinities with the east than with the immediate south.

  In the Middle Pleistocene, the Saharan, Arabian, and central Asian deserts had the combined impact of disconnecting the human populations of tropical Africa from Europe and both populations from those of the Far East. The absence of water was contributing to a biogeographical pattern of isolation. If to this we add the effect of the glaciations in the northern hemisphere we have to reach the conclusion that the populations that fared worst must have been those from Europe and adjacent regions because they were repeatedly pegged back by glacial conditions and were kept isolated from populations to the south and east by the great deserts. The populations that fared best would have probably been those of eastern Asia. Cold and arid conditions in the north made it unliveable, as we have seen, but they were countered by the opening up of rainforest further south and this generated favourable conditions for humans. The cold conditions also lowered sea levels and exposed additional land that could be colonized or that could act as bridges to other land masses. Tropical Africa had the benefit of latitude which meant that the full impact of the cold did not compare with the higher latitudes but it was also less wet than south-east Asia so dry periods intensified desertification and reduced the surface area of habitable land. India would have had connection with south-east Asia during dry periods, when the rainforests opened up, and with Arabia and the Middle East during wet periods, but the Himalayan wall to the north always prevented north-south movement.

  From 600 thousand years ago onwards, many authors refer to the living Homo populations as belonging to a new species, Homo heidelbergensis.28 The main reason is a perceived step in brain size at this time which has been considered to represent a speciation event, the formation of a new species of hominid. According to this view, Homo erectus continued in east and south-east Asia and heidelbergensis occupied western and central Eurasia and Africa. Others prefer to separate the Eurasian and African populations, heidelbergensis in Eurasia and rhodesiensis in Africa. On the one hand it seems clear that connections between intercontinental populations of humans became severed during the Middle Pleistocene. The fragmentation of intracontinental populations also seems to have been important, especially in western and central Eurasia. This repeated segregation would have promoted differences between populations through genetic isolation so it should not surprise us that some researchers have even tried to turn Eurasian heidelbergensis into several species. I am taking a step in the opposite direction by considering all populations after 1.8 million years ago, including the Middle Pleistocene ones, as belonging to Homo sapiens.

  The reasons are very clear to me. On the one hand, I would argue that the supposed sharp increase in brain size at 600 thousand years ago is not real but an artefact of the small samples available to us. We can explain the increase in brain volume, instead, as one of exponential growth, which would appear as a practically imperceptible change in size for a long time followed by a sharp increase. So there would then be no sharp break that would allow us to separate heidelbergensis from erectus. What about intercontinental isolation? The timing of the split between modern human and Neanderthal lineages coincides with the Middle Pleistocene separation of western Eurasia from Africa and the two lineages remained separate for around half-a-million years. Yet, we now know that when they met they exchanged genes and therefore behaved as members of one species. If half-a-million years of isolation were insufficient to turn modern humans and Neanderthals into separate species, then it is highly improbable that a shorter time of isolation between populations of Homo sapiens (or erectus or heidelbergensis as others would prefer) would have achieved that result.

  After a million years of occupation of Middle Earth under comparatively mild conditions, the drying world of the Middle Pleistocene, particularly after 800 thousand years ago, was responsible for the breakup of populations between continental land masses and within continents too. For a species that had become intimately tied to water, aridity spelt disaster. We should not forget that the Middle Pleistocene was a world of climatic swings. These breakups were mitigated, in some places at least, by reinvasion of lost territory when conditions improved. Reinvasion would have followed the linear watery world of rivers, punctuated by the bonanzas provided by lakes, swamps, and marshes. Such reinvasions, many of which we may not have the resolution to pick up with the archaeology, would have remixed genes. The result was that the unity of Homo sapiens as a polytpic species29 was maintained throughout.

  7

  The Rain Chasers—Solutions in a Drying World

  400 TO 70 THOUSAND YEARS AGO

  Homo erectus, or as interpreted here, Homo sapiens erectus, had the beginnings of a body built for endurance running and walking, a body that would become increasingly refined in some populations as time went by.1 Some human populations sacrificed muscular power for lightness that enabled an efficient way of getting across large tracts of open landscapes. Other than the oft-quoted advantages in the pursuit of prey or in the early arrival at carcasses, lightness of build would have provided a key advantage in a water-limited world. This advantage facilitated the seekingout of ephemeral and widely scattered sources of water, the tracking of seasonal flushes of grass, and the gatherings of herbivorous mammals that followed. This is the basis of the rain chasers, the human populations living in southern Middle Earth that increasingly adap
ted to cope with the opening-up of the landscape as water became an increasingly limiting and limited resource.

  In Chapter 6 I argued against a speciation event around 600 thousand years ago based on a rapid increase in brain size, proposing instead a steady and gradual increase in brain size in Homo sapiens populations. As this evolution was gradual I could see no case in favour of renaming humans after 600 thousand years ago. Now let us move forward to seek out human populations in southern Middle Earth, after 400 thousand years ago, when the world’s climate had deteriorated even further than before -450 thousand years ago. The 100-thousand-year cycles of warm and cold, that started around 800 thousand years ago, continued but with important differences. The extremes became greater than before and the length of the warmest intervals shortened. The changes in climate also became more abrupt, with transitions between cold and warm being very rapid. In Africa, the cold-warm swings were translated into wet-dry oscillations. Dry periods became increasingly prevalent. Africa was drying up even more quickly than before.2

  These climate changes not only isolated humans in different continents, as we saw in Chapter 6, but they also cut off populations within continents. In southern Middle Earth the fragmentation was driven by water shortage and these isolated populations experienced severe genetic bottlenecks3 which in the worst moments reduced numbers to a few thousand individuals. My argument here is that it was the shortage and scattered nature of water resources that drove natural selection to favour individuals that could best cope in this kind of water-limited world. The period after 450 thousand years ago was no different from the preceding 1.5 million years in that respect, except that the severity of the climatic conditions was far worse. The human populations simply had to do what they had been doing for a long time but each time they had to get better at doing so as the climatic signals got stronger.

  What did these sapiens living in southern Middle Earth after 450 thousand years ago look like? We have a reasonably good image of their appearance, which contrasts with that of the Neanderthals who were their contemporaries in Eurasia (Chapter 8). The sapiens had long legs, a linear physique, narrow pelvis, and low body mass for their stature.4 In other words they were slender, streamlined, and lightweight. Their anatomy, including the well-developed Achilles tendon, indicates that they were endurance runners and long-distance walkers.5 The humans of after 450 thousand years ago were fine-tuned versions of the earlier models, all of whom from their origins 1.8 million years ago had been selected for the ability to cover huge areas of ground quickly on two legs. We have also seen how large brains would have been useful hard disks that stored masses of data accumulated during the life of each individual person. These hard disks also had a good RAM, which allowed easy retrieval of the stored information. This meant that sources of water could be picked up in a dry landscape from memory, just as the earliest ancestors had retained information about where and when fruiting trees were in season. The same ability was working in favour of humans in southern Middle Earth. Not only was the post-cranial anatomy constantly being tweaked for improved performance but the cranium and its soft contents were also the subject of focused attention by natural selection. The fashion was towards an increasingly slender and large-brained human and this trend was continuous through time. It did not, in my view, involve any sudden step, implying the emergence of a new species, as has been repeatedly suggested.

  In the same way that the arrival of the new Homo heidelbergensis has been claimed for around 600 thousand years ago, so the entrance of Homo sapiens is marked at around 200 thousand years ago by the oldest known fossil with characteristics of our species.6 Yet, when we look at Homo sapiens sapiens, especially at these initial stages of its career, we tend to find a hominid that resembles us but that also retains many features from its ancestors. This has led palaeoanthropologists to introduce such paradoxical terms as ‘archaic modern humans’ to define these populations. They exhibit a mosaic of features that illustrates that we are observing a gradually evolving lineage. In the drying world of Africa and adjacent regions of southern Middle Earth after 450 thousand years ago, populations were becoming isolated and were then reuniting as climatic conditions changed. In this situation of instability, isolated populations may have developed distinctive anatomical features which may have subsequently become intermixed with those of other populations when they reunited, creating the impression of the evolution of a medley of characters.

  It is easy to fall into the trap of looking at ourselves today and the first populations (H. s. erectus) and seeing the differences that convince us that we are looking at two different species. It is quite possible that they are linked by intermediate forms which unite the lineage, just as intermediates unite geographically disparate forms in a polytypic species (see Chapter 6 note 29). This interpretation has been championed by Milford Wolpoff at the University of Michigan who has argued in favour of ‘a geographically dispersed polytypic species, Homo erectus, evolving into a geographically dispersed polytypic species, Homo sapiens’ and that ‘Taxonomically, this means that there is but one species of Homo, Homo sapiens’.7 This does not mean that there was no branching of separate lineages during the long course of our evolution. A case in point is the divergence of lineages that led to the Neanderthals and an African family, which is the one we are discussing in this chapter. These lineages probably spawned many other lines that had their own genetic identity, without necessarily achieving the distinctness that would warrant taxonomic separation as species.8

  The southern Middle Earth—northern Africa, Arabia, and India—human populations reveal a great capacity for technological innovation, which is hardly surprising given their large brains and the stressful conditions in which they were evolving. It was in these marginal populations that the pressure to invent new ways of dealing with their environment was greatest. Technological innovation would have started with the first stone tools and was later refined with the invention of the Acheulian technology. The new pressures after 450 thousand years ago stimulated further novelties.

  Technological fashion appears to match the anatomical trend, which is towards the invention of lightweight and portable tools. This makes sense if the pressures on people in the drying world of southern Middle Earth were towards larger and larger home ranges, rapid location of highly dispersed and ephemeral sources of water, and the ability to track rain fronts. The last would be observable at long range on the African plains and humans would have been keen to reach these quickly, not just for the water but also because the flushes of grass that would result from the downpours would attract the herds of migrating herbivores. Rain meant food as well as water. In these situations, the rain might not fall where there were sources of stone to make tools from so it was best to carry the stone to the water. Acheulian tools were portable and people continued to use this technology for a long time but, as new techniques were invented, they were probably used alongside the Acheulian or even in preference to the tried-and-tested ancient technology. There is an obvious parallel with the earlier discussed use of Oldowan and Acheulian tools.

  The most noteworthy novelty that post-dated the Acheulian was the invention of hafted tools and the production of small flakes from previously prepared stone cores.9 The earliest evidence of this new technology dates to around 300 thousand years ago but there are glimpses, requiring further study, which might take the start back further, possibly beyond 340 thousand years ago. This has been interpreted to mean that this new technology pre-dated sapiens by at least 100 thousand years and would have been invented by erectus or heidelbergensis, considered as separate species. The lack of correlation between the different perceived species of Homo and technological novelty has been a source of concern for archaeologists who, in order to explain the mess, have even suggested that different species of Homo might have coexisted.

  If we adopt the view that there was only a single species—Homo sapiens—then the problem is instantly resolved. What we have are groups of large-brained, intellig
ent Homo sapiens capable of improvising and inventing new solutions for dealing with their environment and transmitting this information to other groups. Invention has always been a cocktail of intelligence, improvisation, need, and chance, so, for a large-brained hominid, new ways of dealing with the world could have come up anywhere and at any point, which is what we find in the archaeological record.10 Technology in humans owes its existence to the pressures of finding dispersed sources of fruit in the forest, later water in a drying world, coupled with the need to maintain complex social relationships within and between groups.11

  Hafted technology conferred huge advantages to its nomadic makers. It was economical on stone as more flakes could be extracted from a core than with the Acheulian tools; the haft could be reused when the stone point was spent, by simply replacing it; energy was saved as less force was required by the leverage provided by the extended arm of the haft; and it reduced the risk of injury to the bearer by putting distance between hunter and hunted. Hafted tools could also be carried more easily because they were lightweight in comparison to the Acheulian hand axes, so they made the life of the plains-wanderer easier all round.

  Composite technology, as in hafting, in which the tool or weapon was composed from distinct items, meant that humans could respond quickly to rapidly shifting resources such as water and migrating herds of herbivores. The improvements to mobility that were triggered by having to move between water sources were now coming to the fore in the context of chasing highly mobile animals; this ability to hunt mobile prey was a by-product, I would argue, of adaptations resulting from the need to search for highly dispersed water. The new technology was especially useful in highly seasonal environments where food was constantly on the move; with it humans could make specialized tools that allowed them to process abundant and short-lived resources such as fish, and also to target large, wide-ranging, and potentially dangerous prey.