For a primate, the transition from living in the tropical rainforest to living in a tropical seasonal forest would have been enhanced by having swamps, marshes, and other kinds of wetlands, which behaved as I have just described, in close proximity. This is because these wetlands are among the most highly productive environments on the planet, more so than tropical seasonal forest and only slightly less productive than the rainforest itself.20 Any primate faced with a shrinking rainforest and forced to live on the edge would have been better off with wetlands close by than it would have otherwise been if all it had around it was seasonal forest, woodland, and savannah.
If we return to the habitat descriptions that have been offered for Toumaï and Ramidus, we can find features in common that take us further in painting the picture of their home. All descriptions incorporate trees and the arguments fall around whether they were few and far between with plenty of grassy, open spaces (as in a wooded savannah) or whether they were, instead, tightly clumped (as in woodland or forest). There are so many possible permutations that we can make from the available descriptions that they tend to render the arguments futile, particularly as the spatial distribution of the habitats is unknown. You could easily have a home range of a troop of Ramidus that would have encompassed patches of forest, savannah, and wetland, and all could have formed part of the habitat of that troop with different amounts of time being spent in each. Then there are elements that are missing from the descriptions, most notably rocky outcrops.
The problem with habitat descriptions is that they tend to fall into the trap of allocating observations into boxes of predetermined habitat types. Much of the discussion between Cerling’s and White’s groups, for example, has had to do with how they have defined habitats. They have used UNESCO definitions21 which may make some sense in today’s world (although I even have my doubts about that) but which we really cannot use with the limited fossil data at our disposal. Perhaps then a better approach would be to define habitat from the perspective of the users, in this case the early hominids. We can do this quite easily by listing the elements of that habitat that we know were present at the sites. Those elements in the present example are seasonal shallow water, trees, and open (treeless) spaces (Fig. 3). These were the essentials, it would seem, of the early hominid real estate. I could describe this as a mosaic but I would be falling into the predefinition trap myself. A mosaic of habitats only exists in our minds when we predefine habitats. If we find several of the predefined habitats in an area then we conclude that the species in question lived in a mosaic. But if we just stick to established elements, the variety of elements is instead what the species needed to make a home, its habitat.
FIGURE 3. The human habitat: trees/open-spaces/water.
Trees seem to have been a critical component of the habitat of these early hominids and this is hardly surprising given a heritage of living in the forest canopy. What we do not know is whether or not they spent any of their time in the canopy, whether they fed on the trees or whether, instead, they simply used them as places to which to climb for safety. White’s study of the anatomy of Ramidus concluded that it walked on two feet22 but also spent time on the trees. Ramidus had not therefore committed to walking upright on the ground all the time. Such facultative bipedalism makes sense in a habitat in which trees and open spaces are interspersed, as seems to have been the case for Ramidus and Toumaï. Part of the time could be spent on the ground, in the open spaces or below trees, and movement on the ground may have been on two or four limbs. When on two limbs, the freed hands could have been used to collect food from shrubs or the lower branches of trees just as their ancestors would have done when walking on branches.23 Hence the habit and accompanying anatomy would have been another benefit of adaptations arising from a life spent handling fruit in the canopy.
The other component seems to be shallow water, ephemeral in nature. This is another first for our lineage and we do not know to what degree they exploited this element of their habitat. We have to guess but we can do so by seeing what other primates do in such situations. If the lineages of these primates are older than the early hominids, we can infer that the latter would probably have had the capability to perform behaviours that we see in these other primates. We can find many visual examples in photographs and film clips that are readily available on the Internet: beachcombing along the margins of shallow lakes and on the coast for crabs and other foods, bathing and swimming (including completely submerged) and wading in shallow water, on all fours but also on hind legs when in deeper water in search of aquatic plants and animals, are behaviours that we can observe among a diverse range of primates. These include several species of macaques (genus Macaca), baboons (Papio), proboscis monkeys (Nasalis), swamp monkeys (Allenopithecus), capuchins (Cebus), gorillas (Gorilla), chimpanzees and bonobos (Pan), orangutans (Pongo), and even some species of lemur (Hapalemur). It seems quite likely that the early hominids got close to water and may have behaved in similar fashion to some of the primates that I have just listed.
So what can we conclude about our earliest hominid ancestors? They seemed to have taken an important step from fruit-eating canopy dwellers towards omnivorous facultative bipeds. They spent time on the ground and time on the trees. The new diet probably reflected the new world of these early hominids but it probably took the form of a gradual change in the proportion of food types consumed, maybe upping insects and other invertebrates and lowering fruit and other plant matter. The precise allocation of different foods to the dietary repertoire would have depended on where each group lived so we would expect differences between groups and times of the year. The food search patterns would have been similar to those of their fruit-eating ancestors, seeking patches of plenty in an otherwise barren environment. Once again, the new search patterns were a consequence of forest canopy life, transferred to a new setting.
I imagine Ramidus and Toumaï behaving a little like the wild boar of Doñana, hanging around the shallow waters and taking everything that became available. This opportunism may have started a new way of exploiting the environment: by taking a wider range of foods than before, they may have been able to survive without having to roam across great distances. But if drought came then they would have had to move across greater distances in search of other water bodies, just like elephants do today. In doing so they may have found ways of using their newly found tastes for variety by exploiting the bits of ground between one water body and the next. We shall look at this in detail in Chapter 3.
What of the Aquatic Ape? Have we found any evidence for an obligate aquatic primate just at the time that the hypothesis predicts? None whatsoever. We should not confuse the evidence of association with shallow waters, which are also used by many non-naked quadruped primates, with proof of a fully aquatic existence. The fossils of the early hominids reveal nothing that even hints at such a way of life. But this chapter has taught us another lesson: that the dichotomy set up by supporters of the Aquatic Ape Hypothesis between an aquatic or a savannah lifestyle, polarizing the debate around which of these best explains the evidence, is false. Our earliest ancestors were not living on the open savannah, but that does not mean that they were fully aquatic either.
3
At the Lake’s Edge
4. 2 TO 1. 8 MILLION YEARS AGO
It was a hot September afternoon as we climbed to the top of the Rock of Gibraltar. With me were ecologists and ornithologists who had taken part in a conference on the evolution of bird migration. The climb was part of the post-conference field excursion. As we reached the highest, knife-edge ridge we could hear intense and incessant screeching. It was disturbing.
Once at the top we realized what was going on. A troop of Barbary macaques1 had been approaching a second group and was being challenged. One group was arriving from the slopes below and their objective was clear: a pond with fresh water that was controlled by the second troop. These animals were so engrossed in their personal war that we were able to walk among them as if
we were an inanimate part of the landscape. Females held the front line, infants screaming in fear as they clung on to their mothers’ backs. The males appeared more reticent, seemingly trying to hold the females back. Time and again the rogue group would launch an attack, and noise levels rose terrifyingly as individuals exchanged blows and bites. Blood was shed. I turned to one of my colleagues and said, ‘This is like watching our ancestors fighting over water in some remote African waterhole.’ That is how it felt. There was something primal about the scene and it was just too close for comfort. After several hours the rogue gang retreated, dehydrated, with the infants unable to take any milk from their stressed-out mothers. I don’t know how they fared in the days that followed.
September in Gibraltar is the height of the summer dry season. Typically, it does not rain at all between mid-June and October. The land is baked; there are few flowers and much of the ground vegetation is crisp and brown. Snails cluster on branches and have shut down all essential life-support systems as they try and bear the brunt of the drought. Hibernation in northern climates is a well-known phenomenon but the similar process of aestivation in southern lands, achieving a similar purpose, is not so familiar. It is hot, but it is water shortage that is the key limiting factor.
I was imagining an australopithecine2 war when I experienced this confrontation but was there any factual basis to my thoughts or were they pure romanticism? Perhaps we can try to answer this question by establishing where and how the australopithecines lived. In Chapter 2 we established several key components of the habitat of our earliest ancestors. One of them was water, but we found no evidence to support the idea that they had been fully aquatic. How important was water to the australopithecines that followed them, and had their habitat and habits changed at all?
To resolve this issue I looked at habitat descriptions that had been published in association with the various species that have been recovered from the fossil record.3 I separated the gracile australopithecines, usually placed in the genus Australopithecus, from the robust forms (genus Paranthropus) and the early forms ascribed by some authors to Homo (habilis and rudolfensis) and which I will refer to here as Homo-Australopithecus. Were there any significant differences in the habitats occupied by the different kinds of early hominids? Let’s start by taking a look at Australopithecus.
Australopithecus includes such famous fossils as Lucy and several species4 which occupied a wide geographical area of eastern, central, and southern Africa between 4.2 and 2.0 million years ago. For my analysis I have relied on two very useful summaries and additional individual descriptions.5 Eleven sites give us useful information about the places in which Australopithecus lived and, as we can imagine, not everyone is in agreement. To avoid the onerous task of having to adjudicate on the merits of the various descriptions, I have simply kept to the approach that I outlined in Chapter 2: of looking at elements of the environment that appear to turn up constantly in the descriptions. Of the 11 sites, nine (81.8 per cent) had water present in some form. The descriptions that indicate the proximity of water include riverside woodland and forest, wet grasslands,6 and lake margins.
All 11 sites give us clear signals that trees and open spaces were present. Rather than get involved in discussion of whether Australopithecus lived in forest, woodland, savannah, or grassland, or indeed of coming to the conclusion that they occupied habitat mosaics, what this approach does is tease out the main features of a typical Australopithecus habitat. Trees, like water, were essential components but so were open, treeless spaces. The proportion of each element probably varied significantly from place to place and between the different species but I do not think that we have enough information to fine-tune to this level of detail. What I do think we can conclude is that they lived neither in dense forest nor treeless plains.
Bushland, places dominated by shrubs, seems to have been frequented but not as much (six of the 11 sites) as places with trees and open spaces. Shrubs tend to produce dense thickets which may have been impenetrable and dangerous, places where predators could have lurked in ambush. The attraction of these places would have primarily been as sources of food, particularly fruit in season. But did Australopithecus eat fruit?
A battery of recent studies on dental microwear and stable isotopes7 in early hominids has provided information about what they ate, with a few surprises along the way. For Australopithecus we now have data for four species: A. afarensis, A. africanus, A. anamensis, and A. sediba.8 The first revelation was that, despite a morphology that had suggested that they consumed hard foods, dental microwear patterns showed the opposite: Australopithecus did not eat hard nuts, but they did not behave like modern leaf-eating primates either; tough leaves were not a part of their diet. One explanation that has been offered to explain this apparent conflict between a robust morphology and a relatively soft diet is that these hominids only went for hard foods in times of hardship. In other words they were fallback foods and they were taken at critical times only.9
Another important result was that the two East African species (anamensis and afarensis) showed little variation among individuals even though the specimens studied had been taken from areas that had been as far as 1,500 kilometres apart and were separated by as much as 1 million years.10 So they seemed to have been quite consistent with regard to the types of food that they ate and this varied little across their geographical range and altered little over time in spite of climatic and vegetation changes. Then came sediba.
When Amanda Henry and colleagues studied this species11 they were astonished because it was so different from the other species. One of the two sediba individuals that were studied had a microwear pattern in the teeth which indicated that it had eaten hard foods prior to death. It stood out from the other Australopithecus in this respect. A first in Amanda Henry’s study was the identification of microscopic particles of silica deposited in plants, known as phytoliths,12 in an early hominid. The recovered plant remains from the teeth provided direct evidence of the plants that sediba had been consuming. Sediba had been eating fruit, leaves, bark or wood (which contain protein and soluble sugars), shade- and water-loving C3 grasses and sedges. The sample was small but clearly indicated a varied plant diet. The absence of C4 phytoliths, which were common in the sediment from which sediba had been recovered, showed that sediba had avoided such plants just like modern savannah chimpanzees do. This result contrasted with the bigger picture for Australopithecus, which was of variable consumption of C4 grasses but at higher proportions than savannah chimpanzees or sediba. The regular consumption of C4 foods has been regarded as a fundamental trait, along with bipedalism, that permitted these early hominids to pioneer increasingly open and seasonal environments.13 Sediba was different.
So, the answer to the question of whether Australopithecus ate fruit, and hence entered bushland to obtain them, would depend on which species we were considering. Sediba clearly did, and the other species would probably have done so as well but were not as reliant on fruit as sediba would have been. So we would expect Australopithecus to have entered areas covered in bushes at times when fruit where available but it would not have been a feature of their regular haunts. The eating of sedges by sediba at least would indicate that it lived close to water and took aquatic plants. The other Australopithecus species also lived close to water and may well have eaten similar plants even if they were not as committed to them as sediba.
We are beginning to form an image of Australopithecus in its world but we need to answer one more question, did they walk upright? The answer to that question seems to be that they were indeed striding bipeds, much more committed to this method of locomotion than their predecessor Ardipithecus.14 The fossil tracks at the famous site of Laetoli give clear confirmation of bipedal walking even if the makers of the footprints, generally attributed to Australopithecus afarensis (Lucy), may not be known with complete certainty.15 But was Australopithecus also capable of moving freely in the trees, as Ardipithecus had done? This has been deba
ted for a considerable time and the anatomy of the foot raised doubts about how capable Australopithecus would have been as a tree climber. Sediba, once again, set the cat among the pigeons. Studies of the ankle, foot, wrist, and hand of this species suggested that it was indeed bipedal but also partly arboreal.16 It seems that not all Australopithecus species necessarily behaved in the same manner.
So what picture can we paint of Australopithecus between 4.2 and 2.0 million years ago? I see a group of sociable species that walk upright on the ground, usually close to trees which they rely on for cover, safety, and sometimes food. They probably sleep in the trees. Australopithecus are rarely far from water and they probably wade in search of sedges and other aquatic plants. It is unlikely that they ever venture right into the open but they are equally wary of dense vegetation, avoiding forest and cautiously venturing into bushy thickets when searching for ripe fruit. The choice of place in which to live is varied within the limits that I have set out—places with trees, open spaces, and water in varying amounts—as is their diet which may include C4 grasses, fruit, and harder foods, the latter at least in times of shortage. I will return to Australopithecus at the end of this chapter but first we need to have a look at the robust australopithecines and early Homo.
The Improbable Primate Page 4
