We may learn something about how and why they became successful by comparison with other species. Can we find a species that, like our ancestor, has departed significantly from the lifestyle of its lineage? Well, it may seem surprising that I should pick the giant panda of China as a good candidate but hear me out. The specialization of the giant panda in eating bamboo is well established and it seems to derive from less-specialized pandas of the Pliocene, providing a unique example of the adaptation of a meat-eating lineage for herbivory.7 So we have in the giant panda almost a mirror image of ourselves. We moved from a largely plant-eating line in the direction of meat-eating and the giant panda went the opposite way. You could say that we are inverted pandas. We both represent examples of species that have somehow managed to change their ways radically from the remainder of their relatives. Now, the giant panda succeeded in the specialized world of herbivores by focusing on bamboo, by becoming highly specialized in a trade that seems to have been largely neglected by other plant-eaters. But surely this cannot apply to humans? We are, after all, the epitome of the generalist. I take a different view which will become clear in the chapters that follow.
An adult female sits down to rest. She is tired and takes a bite from a ripe fruit while her restless children drink some water and carry on playing around her. Her partner steps away surreptitiously and tries to blend into the background. He is tired of all the start-stop movement involved in gathering. He has kept a safe distance as the female has jostled for position with other females trying to secure the best items. The highlight of his day was meeting a male friend who in the past has been with him and other males out on ‘banner-waving’ expeditions proclaiming territory and tribal identity. Soon the family party will be on the move once again as it prepares to head for the safety of its abode. Other females and juveniles meet up as they get closer to home, exchanging news and gossiping. There are males too and they slow down their pace to meet up with the other males, gossiping too about different kinds of things, all the time surveying the surroundings and keeping an eye on the youngsters and the females. Once in their core area the families split up to roost in their separate enclaves. Night is the time to sleep in the safety of your home.
Have I described a scene of a group of fruit-eating primates in a remote Miocene forest or does it sound like something closer to home? There is much in our story that is the result of evolutionary pressures, and much about the way we are and behave today, under the cloak of self-proclaimed civilized behaviour, that harks back to those very forest origins. Culture has cast a convincing veneer over our biology. But if we are able to shake it off we will find, underneath, an improbable primate, still behaving like an inverted panda.
2
And the World Changed Forever
7 TO 4. 32 MILLION YEARS AGO
Many fruit-eating monkeys and apes will eat insects or other sources of protein in order to supplement their diet.1 Dependence on a variety of foods would have started off as a way of balancing the fruit-based diet. Which foods to add and which to reject must have depended on what was readily available and on the willingness of the forager to take a chance at trying something new. In this risky world of opportunists some may well have fallen by the wayside when eating what they shouldn’t have and, no doubt, the bright sparks in the group would have soon seen the effects and avoided the danger. That would have been one of many advantages of living in a group. Experienced mothers, just like chimpanzees, would have taught their young what to eat, how to eat it, and what to avoid too. In this way cultures would have spread within populations, as happens among chimpanzees today,2 simply as individuals watched and learnt from one another. It could not have happened in solitary animals with small brains. Large brains and social life predisposed primates for culture at a very early stage. It is important that we appreciate that our remote ancestors of 7–6 million years ago,3 the starting point of this chapter, would have already held the potential for cultural development—the legacy of life in the forest canopy.
But by this time it seems that these remote ancestors were no longer living deep in the forest canopy. We are in a time of significant change and upheaval: deserts begin to appear and open, grassy environments dominate over large areas of tropical Africa where our ancestors are living.4 Nobody disputes that these dramatic climatic and environmental changes happened but there is considerable debate regarding the habitats that our ancestors actually occupied at this time. Knowing where our ancestors lived is as critical as knowing what they ate so it is important that we disentangle the debate. This revolves around whether our early ancestors lived in grasslands—‘the Savannah’—or instead remained within largely wooded habitats, never away from trees.5 A popular controversy also involves this period of our evolution although it is not usually entertained in academic circles.6 I am referring to the Aquatic Ape Hypothesis which was first put forward by Sir Alistair Hardy in 19607 and has been championed since then by Elaine Morgan in particular.8 So it is critical to our story that we take a look at the evidence and clarify the situation, including this long-standing dispute regarding our putative aquatic origins. To do so, I will rely on the available evidence, which concerns Toumaï (Sahelanthropus tchadensis) and Ramidus (Ardipithecus ramidus).9
I will start with Toumaï, which is the older of the two species. Its environment has been reconstructed using a combination of geological and fossil evidence.10 Fossils can be good indicators of environmental conditions when we compare them to their closest living relatives. In some cases we may be looking at species which are still around today and we assume that their requirements have remained largely unchanged in the interval between then and now. I don’t see this as a problem, especially in cases when anatomy is a good indicator of a species’ habits: for example, a stork has exceptionally long legs for wading in shallow water and most of the world’s storks live close to water, so finding a fossil stork strongly suggests that there was water somewhere close to where it died. When several species that are associated with shallow water are found fossilized in the same site the interpretation is strengthened—it is robust and gives us confidence.
Toumaï, at first hand, would not seem to be much help to us in clarifying matters. The geology of the deposits in which its fossils, along with other animals, were found is a kind of cemented sandstone which was very informative. It was a mix of windblown sand, from the nearby desert, and grains that had been reworked by Nature in a lake environment. These lake-deposit sand grains formed a matrix with mudstone and fossilized singlecell algae (diatomites) that would have lived in the ancient lake. The preserved ripple marks left by currents and waves in the lake along with the remains of sand bars gave an idea of the water movements that had taken place: these currents had not travelled in a single direction, as we would expect if a river had been flowing, but went in all directions. The conclusion was that the evidence indicated a place that had been flooded from time to time, draining at other times into the nearby desert environment. So the shoreline of the lake changed repeatedly in a battle with the ever-encroaching desert. This must have been an ever-changing and unpredictable landscape quite unlike anything within the forest canopy.
Putting this geological evidence together with the fossil fauna that was excavated alongside Toumaï’s bones, the scientists working at Toumaï’s site pieced together a varied world that existed between lake and desert. There were, undoubtedly, a variety of freshwater habitats judging from the richness of the aquatic and amphibious fauna. They found ten different types of freshwater fish belonging to seven distinct families, all but one having living representatives on Lake Chad today. One species—the tiger fish Hydrocynus—would have been a strict hunter of other fish that took its prey by sight in deep waters that were rich in oxygen. The water bodies must have been large as many of these predatory fish reached lengths of over 1 metre. This idea was supported by the presence of large numbers of fish-eating crocodiles that included a new species of gavial. Second, there would have been swamp v
egetation, clearly indicated by fish that would have lived in turbid, oxygen-deficient waters. One species—a knife fish Gymnarchus—would have used an electric sensory system to find its way around and hunt in murky waters.
Many fossils reflect life on the sandy banks at the lake edge. The animals that would have frequented the lake shores were anthracotheres, extinct relatives of hippos, and hippos themselves. Even complete skeletons of hippos have been found fossilized. There were otters, softshell turtles, and a python too. Remains of lianalike papilionoid plants indicate the likelihood of gallery forest that would have fringed the lake. Colobine monkeys would have lived here too. Further away from the shoreline there would have been wooded savannah, indicated by the abundant presence of elephants and their extinct relatives the gomphotheres, as well as now-extinct giraffes.11 Antelopes such as kob and their relatives indicate the presence of grassland where they would have grazed. The picture is completed by animals that probably moved between some of these habitats: land tortoises, monitor lizards, rodents, pigs, hyenas, and sabre-toothed cats.
This is quite a detailed picture of the location where Toumaï lived so why did I say, when I started this description, that it would probably not clarify matters regarding the environment exploited by our earliest ancestors? The reason is that we are unable to say whether Toumaï was aquatic, lived in the gallery forest with the monkeys, or ventured into the wooded savannah or even the grasslands. We know where many of the other species were likely to have lived because they left living relatives, because we have many fossils of extinct species, or because their anatomy is revealing. Not Toumaï, of whom we have a skull, a mandible, and not much else.
If Toumaï cannot resolve the question of the habitat which our earliest ancestors occupied, can Ramidus? Ramidus had been first described in 1994, from teeth and jaw fragments, but by the middle of the first decade of the 21st century the world awaited the publication of a much more complete set of specimens. In October 2009, Science dedicated an issue to a study of the remains of Ardipithecus ramidus, an unprecedented step for this journal. Eleven papers in all, published by a large international team, described this species and its habitat in great detail, based on 110 specimens, representing a minimum of 36 individuals recovered from 4.4-million-year-old levels in the Aramis region of Ethiopia. What was Ramidus like? A partial skeleton of a female allowed the researchers to estimate that she weighed 50 kilos and stood 1.2 metres tall. She had a brain the size of a chimpanzee’s. Males were probably similar in size as little evidence of sexual dimorphism was found in this species. Examination of the teeth suggested that Ramidus was more omnivorous than chimpanzees and probably fed on the ground as well as on the trees. The front and hind limbs, as well as the pelvis, tell us that Ramidus moved ably on trees supported on its palms and feet but it was not specialized for a clinging or climbing tree life. It could also walk on its hind limbs when on the ground but it seems to have been less committed to the terrestrial way of life than the later australopithecines.
This is helpful. We know that Ramidus, as probably Toumaï too, was eating a wide range of foods and had become less dependent on fruit-eating than chimpanzees. A broadening of the diet would have had an impact on Ramidus’ behaviour but what was its habitat? In spite of quite detailed information about the animals found in the same levels as Ramidus and also of the carbon isotope composition in teeth,12 controversy broke out regarding the interpretation of the evidence. On one side were the researchers that published the 11 papers, headed by palaeoanthro-pologist Tim White in Berkeley, and on the other was a group headed by geochemist Thure Cerling at the University of Utah. White is a leading palaeoanthropologist who has spent much of his career working in African sites while Cerling is the leading light in the study of the emergence of C4 grasses during the Miocene and Pliocene. So this was a debate between heavyweights and it centred on whether Ramidus lived in woodland (White’s group) or more open grassland habitats (Cerling’s group).
White’s group collected fossils of large mammals along a 9-kilometre-long transect within the 4.4-million-year-old horizon that contained the Ramidus fossils. They collected an impressive 4,000 specimens which represented 40 distinct species.13 Their interpretation of this fauna—dominated by leaf-browsers and fruit-eaters—was that it emitted a clear signal that the area where Ramidus and all these other species had lived had been dominated by woodland and not grassland. Only three primate species were recovered from the palaeontological beds and that included Ramidus which was the rarest of the three. In contrast, around a third of the large mammal remains came from a colobine monkey which lived in dense to open forest.14 White and colleagues claimed further support for the woodland setting from the other numerically-dominant large mammal of the assemblage—the spiny-horned antelope which was a leaf-browser that is thought to have lived in bushy to wooded habitats.15 In contrast, grazing antelopes were rare. The emergent picture of Ramidus’ habitat, according to White’s group, was of a dry woodland setting, with small patches of forest, far away from rivers.
Cerling’s team focused on the interpretation of soil and teeth-isotope analysis and claimed that the results indicated a very different picture from that suggested by White’s team. They weren’t convinced by the fauna either. The antelopes could have been inhabiting thickets close to rivers and they argued that the absence of duikers meant that there was no dense woodland or forest.16 Their conclusion: Ramidus lived in riverside forest or woodland in an otherwise open bushland and woody grassland.17 So, while White placed Ramidus in a dry and fairly open setting, Cerling included habitats which were more closed and near fresh water.
Rather than focus on the differences, I would like to establish what there is in common between these apparently disparate views. First, both interpretations recognize the presence of aquatic species, which indicates that there must have been a river or wetland nearby. White’s team places the river away from where Ramidus was living but does not discard the possibility, to my knowledge, that they were out of reach of water, especially when rivers overflowed during the wet season. The fossil-rich sediments were silts and clays that had been formed from materials deposited by rivers and lakes or on floodplains, and catfish, presumed to have been deposited during overbank flooding, were common. Hippos, crocodiles, freshwater turtles, and waterfowl completed the picture of shallow, seasonal, waters away from the main river.18 This portrait is not dissimilar to that which we found for the proximities of Toumaï’s territory.
I am very familiar with this world of seasonal flooding as I have spent many days of field research in the Doñana National Park in south-western Spain. Doñana is a huge area of marshland and dunes. It is hard to imagine how much life such an area can hold. The park had been a traditional hunting ground of kings who were obviously very aware of the rich game that lived within. For me the richest areas inside the park were always those in a part known as ‘La Vera’. This is where all the rainwater that has percolated into the sand dunes and down to the water table seeps back out onto the surface. On the dry side of La Vera are extensive areas of aromatic shrubs—lavender, rosemary, rock roses—and scattered cork oak trees. On the wet side lies impenetrable marshland. This inland coastline, which is after all what La Vera is, changes shape with the season and from one year to the next. It all depends on how much rain fell in the winter. Some years, the marsh can become an inland sea. This happened in the winter of 1995–6, when huge amounts of rain fell. Every animal in the marsh sought refuge on dry ground. Some became trapped on sandy islands and died, many drowned, and the remainder were tightly packed in La Vera.
In dry winters, such as 1994–5, almost no rain falls. The marsh remains parched, which is its typical aspect during the summer months. We could drive over vast areas where ducks had swum in previous winters. Now they were littered with the carcasses of dead animals that had been unable to find water to drink. The following winter, others would drown in the very same places. In these times of drought La Vera remained the best p
lace to find animals, as this was where they had the greatest chance of finding water, often by digging. The point I am making is that these areas that were subject to constant flux—winter flooding, a summer dry season, years of extremely high rainfall alongside years of almost no rain—attract many animals. They are a source of diversity and biomass.
We found a similar picture around shallow lakes and ponds that were scattered within the dunes themselves. They too had a shoreline that receded and expanded with rainfall and the response of animals was fine-tuned. In years when the water levels were not too high or too low some of the lakes were pink as thousands of flamingos poured in from distant lands to make the most of the optimum conditions. But other animals were not so fussy and they exploited the lake shores, and indeed La Vera, fully by being opportunistic. The wild boar comes to mind as the star of these amphibious lands. These intelligent and resourceful mammals were a permanent fixture and they left their mark as they dug deep into the mud for roots and invertebrates, transforming patches of ground in the process. But in this world of constant change the best tactic was to take everything that presented itself. One of the most amazing discoveries was that wild boar behaved like hyenas, seeking out the carcasses of dead deer and other large mammals, eating the intestines and the flesh and even taking the large bones away, cracking them open with their powerful jaws and extracting the marrow.19
So what is our take-home message so far? It is that areas subject to seasonal or between-year flooding may be in constant flux but they are also magnets for animals. When excessive rainfall floods low-lying areas, animals congregate on the marginal dry ground and when drought hits they gather around shrinking waterholes. Predators have plenty of scope in this world, and entrepreneurial species, like the wild boar, that are not too choosy about what they eat will find many and varied opportunities that will allow them to subsist in spite of changing conditions and without need to move very far. Our earliest cousins—Toumaï and Ramidus—probably lived in such a world or very close to it. So coming down to water, within the relative safety of trees at first, exposed these creatures to other sources of food that could be found and easily picked from the water’s edge. Quite by chance, as with so many elements of our story, this new behaviour brought them close to one of the world’s richest environments. It would have been a short step to get them into the water, wading, even swimming, after succulent morsels.
The Improbable Primate Page 3