The robust australopithecines have been traditionally placed in a separate genus, Paranthropus. They appear later than Australopithecus in the fossil record and survive later too, overlapping for a long time with Homo. We can place the time frame between 2.8 and 1.4 million years ago.17 They had a wide eastern African geographical distribution, from Ethiopia right down to South Africa. Using the same sources of information as for Australopithecus, I was able to find 21 sites that provided useful information on Paranthropus habitat. There was little variation between these robust australopithecines and their gracile cousins in proportion of habitat elements, with one exception. Water was again a key factor in Paranthropus habitat (95.2 per cent of all sites had indicators of presence of water). Trees and open spaces were also important and bushland much less so, as in the case of Australo-pithecus.18 One feature stood out and may be significant. It was the number of sites recording open habitat and no trees. There were five such sites, which may seem few but represent almost a quarter of all the sites and contrasted with the Australopithecus sites that always had trees present. Does this mean that these later australopithecines were moving away from the trees? It seems so and bears out Reed’s19 conclusions that Australopithecus species existed in fairly wooded, well-watered regions while Paranthropus species lived in similar environs and also in more open regions, but always in habitats that included wetlands. Perhaps in habitat terms, we are seeing a parallel between Paranthropus and baboons.
These robust australopithecines may represent a lineage that ventured further into the open plains than any other before them. The southern African robustus seems to have had a wide and varied diet with clear evidence that it was capable of eating hard foods. It is possible that they only went for these as fallback foods when softer ones were scarce, in the same manner as some modern monkeys do.20 In contrast the East African boisei stands out among all early hominids in the high intake of C4 foods, in the region of 75–80 per cent of its diet, very similar to the grass-eating warthogs, hippos, and zebras. Such a large intake would certainly seem to fit a hominid that was venturing more into open grassland environments.
Paranthropus, literally meaning ‘beside man’, was a parallel lineage to humans which went extinct. It was not our ancestor. It illustrates the degree of evolutionary experimentation that was happening over a period of millions of years as forests shrank and grasslands expanded. Like Australopithecus its movements appear to have been tied to the proximity of water, but Paranthropus seems to have started on a path that moved it further away from trees, without abandoning them altogether. We now need to look at a third group which, like Paranthropus, were descendants of an ancestral lineage of Australopithecus. This third group is Homo-Australopithecus.
Homo-Australopithecus contains two species: habilis and rudolfen-sis. These hominids resembled the australopithecines in size, including brain volume, and some authors have considered them to be so similar that they have placed them in Australopithecus.21 Since they are generally regarded to be in our direct ancestry, we shall take a look at them separately from the other two groups. Their time frame is between 2.33 and 1.4 million years ago, overlapping almost completely with Paranthropus but not with Australopithecus, except the late surviving sediba. Their geographical distribution matches Paranthropus, from Ethiopia down to South Africa.
Nine sites provided information on where early habilis/rudolfen-sis lived and they appear to differ little from Australopithecus or Paranthropus. They too depended on water, trees, and open spaces.22 One of the nine sites did not have trees and another did not have water, suggesting very tentatively that, like Paranthro-pus, habilis/rudolfensis may have been venturing further away from cover and water sources. It may reflect greater mobility among these bipedal hominids, which would have allowed them to stray further and further from these key places in the knowledge that they had the capability of returning to them with relative ease. They may have still returned to the safety of trees at night but were prepared to venture into the open and away from water in search of food. Some died in these open, waterless places, as the discovery of their remains in these contexts indicates. But overall, trees and water would never have been very far away. The diet of habilis/rudolfensis seems to have been unspecialized and was not geared towards either hard or tough foods. Instead it seems that they had a fairly generalized diet that included C3 and C4 foods in similar proportions.
When we put all this together we can begin to assemble a possible chain of events which is the history of these early hominids. From the appearance of the first australopithecines to around 2.8 million years ago, when we find the first Paranthropus, the picture is not too different from the earlier one described in Chapter 2. It is one of hominids walking on two feet in areas with trees, open spaces, and water close by. They ate a range of plant foods but largely avoided hard nuts and similar items. Then, something seems to change after 2.8 million years ago.
The African climate took a shift towards becoming drier and more variable in the time interval between 2.9 and 2.4 million years ago, the first of three such shifts which were related to the start and strengthening of the influence of glacial cycles in the tropics.23 One consequence seems to have been the opening-up of areas of woody vegetation which became dominated by wooded grassland.24 The open grasslands, which we associate with present-day Africa, came much later25 which means that we cannot implicate them in these early steps in our evolution. The changes that climate generated around the 2.8-million-years-ago mark were in the direction of expansion of open spaces within a framework of woodland vegetation. In other words a patchwork or trees and open spaces was the outcome, exactly what the australopithecines had been exploiting for close to 1.5 million years. It would have generated a population boom which seems to have been accompanied by the start of the new Paranthropus line at 2.8 million years ago and habilis/rudolfensis at 2.33 million years ago. The different australopithecines were splitting the wooded grassland cake between them.
One additional consequence of these climate changes would have been a reduction of the area covered by wetlands which probably became more seasonal in character as well. We have already seen just how important water was to the australopithecines and new seasonal shortages may have given rise to stress and mortality which would, in turn, have intensified the pressure in favour of adaptations that improved survival under these new conditions. Improvements, for example in the hind limbs, that allowed hominids to move quickly and over greater distances across the landscape, would have been seized upon rapidly by natural selection. This was, in my opinion, the trigger to our evolution: adaptations which promoted mobility were spurred on, above all else, by the need for swift and efficient movement between ever-shrinking sources of water. As the pressure increased, so compromises had to be made at the expense of life in the trees, and the anatomy of the terrestrial biped was fine-tuned. The start was made in some tropical African wooded grassland at a point prior to 2.8 million years ago and it took off after this time. Once started, there was to be no return to the trees. Such unconditional commitment to life on the ground was not a feature of Australopithecus but it seems that Paranthropus and habilis/rudolfensis may have been edging closer to it.
I want to end this chapter with three aspects that were important during this initial period of arboreal emancipation: rocky places, stone tools, and meat. Let us start with rocky places. I looked for evidence of rocky habitats—caves, cliffs, screes—in the habitat descriptions of the australopithecines that I have used to build a picture in this chapter. I found evidence of such habitats in 18.2 per cent of the Australopithecus sites, 23.8 per cent of the Paranthropus sites, and 22.2 per cent of the habilis/rudolfensis sites. There was no evidence of rocky places in Sahelanthropus or Ardi-pithecus sites so it seems as if we have a new feature here, one that came with the australopithecines. The number of sites reflecting rocky habitats is small in comparison to other habitat elements—trees, open spaces, water, even bushes. Rocky places may not have been ess
ential to the australopithecines, which seem to have retained an ancestral desire to seek cover among the trees, but it is there and we should account for it.
A recent study has proposed that places in eastern and southern Africa that were subject to repeated tectonic or volcanic activity may have been favoured by early hominids such as Australopithecus because they offered situations in which heterogeneous or mosaic habitats were frequently found as a result of the abrupt changes in relief caused by geological processes.26 Of course, when they talk of heterogeneous or mosaic habitats or landscapes they really mean places in which a variety of habitat elements were present in close proximity and in varying proportions. We should be quite happy by now that such places were what the australopithecines actively sought out. They were not habitat mosaics in the eyes of the australopithecines but instead were what they perceived as their habitat. These places would have provided ample water, as streams and rivers cut into the newly exposed rocks. As these rivers flowed downstream, different geological settings would have generated lakes, swamps, and other wetlands. They would have also left behind cliffs of harder, uneroded rock. It is this latter aspect that interests me most because the rest of the key habitat elements could have been found in other areas which were not so geologically active. But venturing into these attractive areas of tectonic and volcanic activity brought australopithecines close to significant areas of exposed rock. In these areas there would have been caves.
We have established that the climate started to get drier after 2.8 million years ago. Water sources were more distant from each other than before and I have proposed that it was this increased patchiness in water sources that promoted early hominid mobility. But cover in which to sleep in relative safety at night became increasingly difficult to find, too, as trees gave way to open spaces. Cliffs and other rocky places would have provided alternative locations of relative safety from predators and would have also given the early hominids new options. Imagine a rich wetland, full of food and with plentiful water but in a situation that was far away from clusters of trees where the early hominids felt safe. Going to such a wetland would have meant taking big risks, moving over open areas in which they would have been exposed to dangerous predators. Then having to go back and forth between trees and water every day would have meant that a lot of time and energy was wasted. Imagine further that a line of cliffs overlooked the wetland. That would have solved the problem for an adaptable hominid that was able to take the unprecedented step of choosing to sleep in a place that had no trees. Cliffs, caves, and other rocky places permitted the australopithecines to exploit new territories that would have otherwise been out of reach. Rocky places were woodland substitutes.
Should it surprise us that the earliest known stone tools made by early hominids should date to 2.6 million years ago, precisely at the time of the first big drying event in Africa?27 The australopithecines must have been using tools long before this time, just as chimpanzees do,28 made from wood which they had readily accessible, and they may have used stones, too, when these were available judging from recently discovered marks left on butchered bones in Ethiopia which are estimated to date in the region of 3.42-3.24 million years ago.29 It is possible that, prior to 2.6 million years ago, they simply picked up isolated stones or they may have, instead, deliberately visited known quarries where they took particularly attractive and suitable pieces. We just do not know to what degree they shaped them into tools either. But by 2.6 million years ago they were certainly turning rocks into tools and visiting places where suitable rocks were frequent and this behaviour may have made them familiar with particular rock sites that they then started to use for other purposes, such as shelter.
The use of tools by the early hominids has been linked to a change of diet to include meat and other mammal products, like bone marrow. Tools, it is said, opened up the niche of the early hominids by allowing them access to foods that they could not have been able to process with their teeth. Some authors have gone as far as to make meat the central ingredient that made us human.301 am not convinced and will examine this contention in some detail in Chapter 4. We have seen here that the australopithecines ate a wide range of plants. Sediba has shown us how wide that choice really was. We have to assume, in the absence of direct evidence, that this extensive plant diet was complemented by related animal foods if present-day monkeys and apes living in similar situations are anything to go by. It does not take a huge mental leap to imagine an australopithecine picking up molluscs while wading in shallow water for aquatic plants, picking up a stick and digging into a termite mound, or stalking an unsuspecting lizard basking on a rock in the morning sun. That they would have stumbled across carcasses when venturing into open spaces has to be a given. It does not take much to imagine that the primate brain brought down from the forest canopy incited its owners to explore the content of these unusual finds and to figure out how to get some food from them. Did this ability extend to chasing live animals or to chasing dangerous predators away from their prize? We cannot be sure. I remain sceptical but I do accept that it would have been the next step, and that it was imminent.
4
The First Humans
1.8 MILLION TO 800 THOUSAND YEARS AGO
On 29 August 1985, the journal Nature reported the discovery of the skeleton of a boy who had died at the age of 12 around 1.6 million years ago. It was the most complete skeleton ever found of Homo erectus.1 The tall youth, with a stature estimated at 1.68 metres, became known worldwide as the Nariokotome Boy, after the name of the site in which he had been discovered. The site was on the south bank of the Nariokotome River in Kenya, close to where it drained into Lake Turkana on its western shore.
Homo sapiens erectus (as I am regarding them as a subspecies)—the first humans—came onto the scene around 1.7 million years ago, almost simultaneously in East Africa and south-east Asia, making its area of origin hard to determine.2 Some scientists prefer to separate the two populations, leaving the name erectus for the Asian population and classifying the African population as ergaster. In my earlier book, The Humans Who Went Extinct, I did not draw the distinction. I will stick to that preference in this book. In Africa, humans would have lived in similar geographical areas as Paranthropus and Australopithecus habilis/rudolfensis for around 300 thousand years but only humans survived after 1.4 million years ago.
The fauna recovered from the Nariokotome site, which included fish, crocodiles, turtles, elephants, and hippos, clearly indicated the presence of water where the Nariokotome Boy died, and had presumably lived. In the database for the australopithecines which was used in Chapter 3, seven African, including Nariokotome, sites were included from which I could tease out information on the habitat of the first humans. To my surprise no important habitat differences between these humans and their predecessors and contemporaries could be found.3 In fact, trees were always present in human sites from this period which put paid to any suggestion that this hominid had finally taken to the open, treeless plains of Africa. So, if humans lived in similar places to the australopithecines, what made them different enough to be able to persist among a suite of similar species without suffering the effects of competition? And if they did not have the edge over potential competitors—they seem to have lived alongside each other for close to half-a-million years—why were humans the only ones to survive after 1.4 million years ago? The answer, I will argue, is water in a world that was drying up.
The period between 1.8 and 1.6 million years ago saw the second of three major climatic downturns which were superimposed onto an overall climatic trend of cooling, drying, and increasing climate variability.4 The African climate became even drier and more variable than before and there were significant changes in the fauna. Homo sapiens appeared at this juncture. The effect of the climate changes on the land would have been significant and would have been expressed visibly by a landscape in which open spaces were becoming increasingly large and common at the expense of trees. But areas which until then had
remained covered by rainforest would also have given way now to open landscapes with trees. So what we would have observed would have been a loss of rainforest, perhaps little change in areas with trees and open spaces, and an increase of areas of open spaces in which trees were scarcer than before. I suggest that the areas with trees and open spaces may have remained similar because the losses in favour of open areas in the drier regions may have been offset by new areas which were once rainforest. Water sources would have become patchier still and seasonal effects would have become even more pronounced than before. Dry and wet seasons would have alternated.
This is the scenario in which we first find Homo sapiens. In The Humans Who Went Extinct I put forward the idea that the thrust of evolution was happening in marginal areas where populations were most stressed. In core areas, populations stuck to what they knew best and it worked—provided conditions did not change. Life was tougher on the margins and death and extinction were frequent. But when conditions changed in the direction of those that had been experienced on the margins, then these peripheral populations got their chance as the conditions in which they had managed to scrape a living spread. Humans would therefore have started off within marginal populations of Australopithecus habilis.
How do the newly arrived humans on the scene compare physically with habilis? This is an important question which might tell us something about their respective ways of life. A recent analysis of the skeleton of the Nariokotome Boy, for example, has revealed that it was not as tall as previously esti-mated.5 It now seems that the Nariokotome Boy had a growth spurt that did not reach the speeds of present-day humans and it ended sooner too, around 12.3 years of age. The earlier models that had been used to calculate growth rate, and from that its stature, were wrong. The Nariokotome boy was 163 centimetres (5’4“) and not 185 centimetres (6’1“) as had been thought for a long time. This new estimate fits in well with other estimates of the stature of humans from this period which are based on the lengths of fossil femurs and which fall in the 160-centimetre (5’3“, female) to 180-centimetre (5’11“, male) range. These estimates would still make humans considerably taller than habilis whose females stood at 125 centimetres (4’1“) and males at 157 centimetres (5’2“). Australopithecus and Paranthropus were even shorter. McHenry, who studied stature in early hominids,6 concluded that it was ‘not true that humans have been getting progressively taller throughout their evolutionary history. Some individuals were as tall as modern [meaning recent] humans 3 mya [million years ago], by 2 mya one individual stood about 173 cm, and by 1.7 mya a stature of 180 + cm was not uncommon.’
The Improbable Primate Page 5