Humans were also significantly heavier than habilis.7 The body weight increase from a male habilis to a male human from this period may have been of the order of at least 20 per cent but it is in the females that we observe the sharpest weight increase, which was of the order of at least 50 per cent. Prehuman males increased slightly in size through time within the range 40–52 kilograms (88–114 pounds) while females did not and stayed in the range 29–34 kilograms (64–75 pounds) throughout. So humans marked a step in terms of body-size increase and it was most pronounced among the females of the species. Consequently, the sexual dimorphism in body size, so evident in the australopithecines, was significantly reduced in humans.
Large size has important implications in ecology. I am interested here in the effect of size on the movement of animals. It has a direct bearing on net energy expended in travel, net transport costs8 decreasing with increasing size. It is interesting that body size has the opposite effect on climbing animals,9 which may have restricted body size in hominids until they committed to walking on flat surfaces. Larger animals also reach higher maximum velocities than smaller ones when running; they move at faster optimal speed (the normal speed of movement) and they therefore cover greater distances in the same amount of time. This means that humans would have done better than habilis in terms of energy efficiency as they walked across their territory and they could have moved faster if required to do so. They could also have had a larger home range.
Hind limb proportions also improved the efficiency with which humans moved. Increase in body size meant that the total cost of locomotion also increased, even if the net cost (energy expended per kilogram) decreased. When the daily cost of movement was calculated for a human from this period with its elongated hind limbs, travel costs were 50 per cent lower than for a hypothetical human with the hind limb proportions of an australopithecine.10 The elongated limbs of humans mitigated to some degree the increased cost of locomotion due to its larger size, making walking more economical than in the australopith-ecines.11 Longer hind limbs in humans have also been found to reduce running costs.12 The lengthening of the hind limbs in humans improved their efficiency not only as walkers but also as runners. Australopithecus habilis, of similar size to the australo-pithecines and hence with a more economical, though less efficient, walk than humans, would have had its own advantage over other Australopithecus, by virtue of having longer hind limbs. We can see how, after a long period when hind limb proportions remained quite constant among the australopithecines, the increase that we first pick up in habilis and which really took off in Homo sapiens, especially in the earliest erectus humans, would have dramatically augmented the daily distances that could be covered. We have seen that the habitat requirements of early humans in tropical Africa were essentially those of their predecessors. What really changed in humans was the ability to cover much more ground in a day and to roam across larger home ranges than any of their predecessors. Such a change was the result of an increase in body size and the length of the hind limbs. Once committed to this new road, any further increases in these features would have been favoured by natural selection. Standing tall in the East African landscape 1.8 million years ago were the first humans. From an ancestral, fruit-eating, forest-canopy primate millions of years of evolution assisted by climate change had finally produced this improbable primate.
Why were humans so improbable? The world of humans and their predecessors had been dominated for millions of years by a wide range of mammalian herbivores13 and a guild of carnivores.14 The australopithecines were not alone among the primates in this world either, terrestrial baboons being noteworthy neighbours15 alongside pigs which were also omnivorous.16 All these species were perfectly capable of consuming meat, even the pigs,17 but they would have done so opportunistically or at a small scale relative to the dominant and specialized carnivores. They had reached a level of improbability, though not uniqueness, but a minor one in comparison to humans who seem to have barged into the carnivore guild in their own right. That, for a primate, was certainly an unexpected first.
What pushed the ancestors of humans in the direction of increased mobility and how did that generate the world’s first and only primate carnivore? It had to be water. As water sources became scarcer, more thinly scattered on the ground, and increasingly seasonal, hominids would have been faced with three choices. The first was to stay put close to water and ride out the bad times while eating what you could find around you—a good option if you were not too bothered about what you ate but one in which you might have to depend on low-quality fallback foods for a while. It carried the added risk that the water source might dry out altogether. Second, you could stay close to water but you could range more widely than in the first option, provided you were able to return to the water source. This option would certainly permit you to exploit a wider range of resources but you would still run the risk of the water source disappearing in extreme drought. The third option would be to become highly mobile and move between two or more water sources during the annual cycle. Such a strategy would seem the least risky and would offer many more dietary options. It would also put the users under intense pressure from natural selection, favouring any genetic novelty that promoted improved mobility.
It is time to introduce a new concept, one which has been discussed occasionally by ecologists and which is vitally important to our story—it is usually known as the Baldwin Effect.18 It happens when an animal learns a new behaviour, either by itself or from others, and this behaviour eventually becomes incorporated into the genome of future generations if it confers advantages. Such novel behaviours can place individuals in new environmental settings which may, in turn, expose them to new selective pressures. What we are looking at here are ways in which the behaviour of an animal could affect the course of evolution. I do not mean, of course, Lamarckism, the discarded view exemplified by the animal that stretched its neck each time further to reach the leaves on the highest branches and produced a giraffe after several generations. There are ways in which animals can position themselves in particular locations or even modify their environment. Patrick Bateson,19 the Cambridge animal behaviourist, usefully summarized these possible ways into four. The first was when the animal made an active choice of where to live; the second was by changing the physical or social conditions, through their behaviour, in which their offspring would live; the third was by changing their behaviour when conditions changed, thus avoiding death; and the fourth, especially relevant to us, was when animals exposed themselves to new conditions which could open up possibilities for evolutionary changes that would not otherwise have taken place.
Let us make what we are saying here relevant to humans. For example, let us assume that an ancestral population of this species, deliberately or because they had no choice, moved into empty territories that were not exploited because water sources were very distant from each other. If that population had genetic variability which had the potential to have larger individuals with longer limbs, then natural selection would favour the larger, longer-limbed individuals. In time a larger, longer-limbed population would be exploiting this territory and would have become more efficient at doing so. We can contrast this situation with a second population that stayed put, had the same genetic variability, but was not under pressure to move in that direction. By placing itself in the new conditions, the first population was unconsciously making itself available to evolutionary change. More often than not, such attempts at exploitation of marginal areas probably end in disaster and those are the myriad stories that we cannot tell. Occasionally, there is success and we have a tale. Our narrative is one of those few.
I have argued for water as the principal driver of the evolutionary changes that characterized the first humans because it was a constant requirement for their daily lives. Homo sapiens was an evolutionary response to the scattered distribution of water in space and time. Food and shelter would have been secondary drivers for the simple reason that the early ho
minids were not committed to particular food types and had, as we have seen, found shelter alternatives to trees in the world of rocks. What was not open to alternatives was water, which is why it had to have been the main limiting factor and the principal driver of our evolution. Improved terrestrial mobility was a response, first and foremost, to the need to quickly locate water sources in a drying world.
Where are we so far? We have a large, active, and highly mobile terrestrial biped that occupied similar habitats to its predecessors. It had a larger home range which must have exposed it to a wider range of situations than its predecessors who lived within the confines of a smaller territory. The movement of the first humans may not have been limited to a day range. For the first time in the history of this primate’s lineage, movement probably involved seasonal changes of location within an annual home range. Such movement would have further increased the probability of encountering a diversity of food sources. For a hominid that had already developed a taste for meat, marrow, and fat, its mobility would have increased the chances of encountering carcasses which would have been ideal alternative sources of nutrients. Mobility would have facilitated quick arrival at fresh carcasses, by observing the flight behaviour of vultures, and their large size and group behaviour may have allowed them to compete with the strong predators and scavengers that lived in these places.
We do not know when humans started to hunt as well as scavenge. Perhaps their predecessors already hunted but, if so, it may have been on a sporadic or opportunistic basis. We cannot tell simply from cut bones. I suspect that hunting herbivorous mammals became a regular feature of the repertoire of human behaviour at a very early stage. Hunting and scavenging for meat and related products were novel behaviours that would have been open to the Baldwin Effect. Hair loss and sweating were probably products of hunting and scavenging.
The areas that humans frequented were rife with powerful and dangerous predators. These predators would have been most active at dawn, dusk, and at night, just like those that live in tropical Africa and India today.20 One tactic that smaller predators use to avoid the more dangerous, larger predators is to shift the time when they are active in order to avoid encounters.21 Curiously, it is the smaller predators that hunt mainly by day22 which could be taken to mean that the dominant predators have taken up the prime time slots, which would be those that provide most cover and avoid the intense heat of the middle of the day. Cheetahs survive by living in what have been described as competition refuges—areas of low prey density that are unattractive to the larger predators. Combined with different feeding times, cheetahs, whose densities are directly controlled by the larger predators, are able to persist in heterogeneous environments that offer such competition refuges.23 If humans wanted to avoid the most dangerous predators, they would have been well advised to limit their activities to those times when the predators were resting or sleeping, that is, during the heat of the day, and to those places where they were least likely to encounter them.
Such a strategy would have been useful simply as a means of escaping being eaten and that may well have been how the behaviour started. Once behaving in this manner, humans would have found themselves in a position in which they had a good chance of taking food from carcasses, when the main competitors would have been vultures. After that, it would have been simply a matter of time before they started actively hunting. Predator and competition avoidance drove humans towards a life in the middle of the day. But such behaviour put this bipedal terrestrial primate in a position in which it was at risk of overheating. The selective pressure to deal with this would have been huge and any variability in the human population that alleviated the problem would have been seized upon. Becoming naked and adopting a cooling system that involved the production of sweat were the answer.24
The defenders of the Aquatic Ape Hypothesis argue that the system is inefficient because it means that a hominid in a hot environment needed to conserve water and would not have developed a sweat-production cooling mechanism. This is a fallacy that seems to assume that Nature is perfect. It is not. We have seen how much of what we observe is a by-product of other developments; organisms do the best with what they inherit, and compromises are frequent. A naked, sweaty hominid may not have been the best solution in an ideal world but it was the solution for the state of affairs of the world of 1.7 million years ago. Many other animals, including some primates, lived successfully in that world and sorted their overheating problems in other ways. Comparing the different ways and trying to find which was best is futile. They were all good because they allowed the different species to survive and reproduce. What happened to our bare and perspiring hominid was that, in order to be able to make a living in its hot world, it tied itself down to having to drink water frequently, and this need would have put further pressure on being highly mobile and able to move quickly from one water source to another. Natural selection drove a ratcheted linked response in which preventing overheating and preventing dehydration fed off each other.
I will return to nakedness in Chapter 5 in the context of the spread of the first humans away from tropical areas. In this chapter I have attempted to build a picture of how and why humans were a viable proposition 1.7 million years ago. Mobility was at the heart of this new primate solution to a drying world. It should not surprise us then that, after close to a million years of observing tools—known as Oldowan—that appear to have been of a kind that were readily made and discarded in situ, precisely at this juncture we should find a new technology—the Acheulian—which seems to have been the hallmark of mobility.25 From the very origins of this new technology, tools were either made with thick, pointed tips or were long with durable cutting edges. The first type seem to have been used as picks for tasks that needed bulk and weight while the latter provided stable cutting edges. These tools were probably transported between sites by humans and they would have had greater scope when roaming across their large home ranges. Carrying tools with the freed hands of a biped would have provided an instant match between newly discovered food and the ability to process it immediately. Prior to this new technology, early hominids would have often found themselves in situations with food but without tools that gave them access to it. Sometimes they may have been able to carry food back to quarries or stashes of tools with all its risks, such as meeting powerful competitors along the way. The ability to carry the tools with them was an important step in the new world of mobility.
Manufacturing tools of this type required advanced motor skills and cognition. The first humans clearly had brains that could deal with these new problems but a larger brain also brought with it new problems. In this chapter I have kept the focus on humans in their tropical African home at the very beginning of their emergence. In Chapter 5 I will widen the geographical and time frames to see what happened next. Part of that story will focus on the developing brain of this improbable primate that we can now identify as distinctly human. Not surprisingly, water remains central to that story.
5
Middle Earth: The Home of the First Humans
1.8 MILLION TO 800 THOUSAND YEARS AGO
In Chapter 4 we established that the earliest recorded African humans lived around 1.7 million years ago. In this chapter we look for their home in that period. Recent revisions place the actual date of these first Africans between 1.7 and 1.65 million years ago.1 This means that the earliest Africans recognizably belonging to Homo are, on current evidence, around 100 thousand years later than the earliest Asian specimen, which is dated to around 1.8 million years ago.2 These results, which could rapidly change with the discovery of new specimens, have put the location of the centre of origin of Homo sapiens in question:3 were they in Africa as traditionally held, or were they in south-east Asia instead, or indeed somewhere in between?4
Dmanisi in Georgia is a priori an odd locality in which to expect early hominids. Well north of the tropical belt, at 41oN, and at an altitude above 1,000 metres, we would hardly have expected to find homin
id fossils here, and certainly not of an age comparable to the earliest African humans. Yet, that is exactly what we have in Dmanisi—the remarkably well-preserved crania and associated remains of hominids that lived there around 1.7 million years ago.5 Were they humans? We do not know. I am not even sure whether they belong to the Homo-Australopithecus group with habilis and rudolfensis or the earlier Australopithecus (but see Preface xviii). Taxonomic issues aside, there is an important point that we need to take home at this stage: by 1.7 million years ago early hominids were widespread across a vast area of northern, eastern, and southern Africa, central and south-east Asia, and we presume they also occupied the vast area in between. I have called this the mid-latitude belt6 and Dennell and Roebroeks7 called it Savannahstan. Here I will refer to it as Middle Earth—the home of the first humans and their contemporaries.
The Improbable Primate Page 6