Six million years ago, Kenya’s Mombasa beach. You and I forage in the leaf litter of the coastal forest. Every few seconds we pop insects into our mouths. We squat on our haunches, shuffling forward as we feed. You lift a rotting log with one hand and pry out juicy grubs with your other forefinger. I munch a mushroom, then strip a spiny ground herb to yield a mouthful of sweet white pith. Our arms are so long that we needn’t bend over to reach these titbits. Our spines are mostly erect above our buttocks. Our pelvises are somewhat basin-shaped to hold up the guts we are so busy filling, instead of being flanged like those of our cousins the chimpanzees, who are mere quadrupeds with bellies that hang down from more horizontal backbones. Our heads balance on top of our spines instead of jutting forward. We waddle over to a fig tree and climb it to gorge on ripe fruit. We climb easily, and we’ll sleep upstairs tonight in nests we weave of broken branches, but we soon clamber down again because we feel more at home on the ground where we don’t have to hang on. Sated, you roll over on your back on the forest floor while I pick through your fur.
I might already have protruding breasts. We spend so much of the day with trunks upright that I may signal my attractiveness from the front, not just from the rear. I am female but that does not necessarily imply you are male. Chimpanzees, bonobos and humans all have same-sexed kin, friends and sometimes same-sexed sex partners: our precursors were surely no different. If you do happen to be male, you swagger and display at rivals standing upright. We may also threaten predators by standing on our hind legs while we scream and throw things. Someday our great-great-grandchildren will stride out onto the savannah, because you and I have found better things to do with our hands than walk on them.
You and I are the eastern ground apes of Jonathan Kingdon’s Lowly Origin: suggested occupants of one of the last great blank spaces on the map of human evolution. Kingdon is a defiant eccentric in the world of anthropology. He is a wonderful artist: his illustrations in the Atlas of Evolution in Africa and in Island Africa are very fine. He is also an academic student of evolution. Lowly Origin is mainly aimed at those who already know enough to argue back. He describes all the fossils. He reconstructs the faces of our relatives and ancestors. He tells our history from the distant origin of primates through to the modern human ‘niche thief’ who lays claim to the resources of all other creatures. Throughout, he relates anatomy to ecology and geography: the specific place and the particular kind of forest or woodland where our ancestors lived, their journeys from Africa to Asia and back again.
At least, he tells his version of the story. He has left a number of his students at Oxford baffled about what to write in their exams: the orthodox views propounded by other tutors, or the Kingdon heresies? Or is it the Kingdon revelations? Some of his richly detailed account is far-fetched, and there are people who will authoritatively disagree with each step. Perhaps even the eastern ground apes will turn out to be wrong, if transitional fossils that fill the blank space are found in a quite different place and environment. However, any subsequent account will have to parry Kingdon’s version with equally explicit consideration of differing centres of speciation, and equally vivid pictures of ways of life.
I fleshed out Kingdon’s account of the eastern ground apes with likely modern parallels: gorillas’ plant-stripping, cebus monkeys’ prying into logs for insects, the predator-mobbing of chimpanzees, the female gelada baboon who sits upright and drags herself along on her bottom while wearing a sexually flushed ‘bleeding heart’ patch on her chest. Kingdon is interested in anatomy and ecology. The low-growing forests of East Africa’s coast would plausibly encourage apes to forage intensively for a rich food supply on the forest floor, which in turn could lead to the evolution of a trunk balanced upright on legs that did not need to walk very far. There isn’t a scrap of fossil to prove it, but this is a new idea for what our ancestors were up to during a largely fossil-free time.
The evolution of hominins (our relatives after we split from other surviving great apes) can be divided very roughly into three great periods, which lasted about two million years each: the blank space, the bipedal space and the brain space. Molecular evidence tells us that we diverged from the common chimpanzees and the bonobos about six to seven million years ago. There is a six or seven million-year-old fossil which left its jaws and teeth in Chad, and shares traits with gorillas, chimps and humans. If it wasn’t a common ancestor, it may have been on a short side branch not long afterwards. The next two million years are when chimpanzees became dedicated knuckle-walkers while humans stood up. There are very few fossils from five million years ago: Orrorin (the ‘dawn man’), who was bipedal in a slightly different way; and the Ethiopian Australopithecus kadabba, who consisted of nothing but teeth.
What were we doing back then that led us to abandon a perfectly good way of walking, with a leg at each corner, to totter along on only two? This is not like Darwin’s famous example of the eye. Each stage of the evolving eye was increasingly useful, from the first pigment spot which could sense the shadow of a predator swimming overhead. Our ancestors had first to unmake effective quadrupedal locomotion and go through a stage when they waddled inefficiently on bent knees. Why didn’t creatures like that just get eaten?
Were they on the savannah, standing up to look out for predators and prey over the tall grass? Did they hunt for meat, running after prey on the ground or in trees, but needing free hands to capture it? Did they carry things – wooden clubs, or handfuls of nuts – or help their babies to hang on? Did the savannah sun threaten to cook their brains, so they minimised its impact by showing it only their heads, not their backs? Were they, in the wildest theory, aquatic,standing up to keep their noses above water? Ian Tattersall, of the Natural History Museum in New York, has a simpler story: ancestral great apes were vertical-bodied clamberers in trees; when they came down they had an almost equal chance of becoming quadrupeds or bipeds.
The common thread in all of these ideas is that you cannot make sense of bipedal legs without a prior reason for an upright trunk and clever hands. If our ancestors moved from trees to open savannah as small-bodied quadrupeds, the evolutionary pressure, Kingdon says, would simply have made them faster quadrupeds, like the patas monkeys of present-day African grasslands. Patas are red-furred, long-limbed sprinters, the nearest primates get to antelopes. If we’d followed that route, you and I could still be trying to outrun cheetahs.
Kingdon’s answer is geography. Africa’s forests shrank or spread as the climate changed. During dry periods, the highlands to the east of the Great Rift Valley became a formidable barrier to forest animals, a parched savannah that blocked travel between central Africa and the coast. Woodland apes roamed throughout the region in wetter times, but then the chimpanzee ancestors retreated westwards to the tall rainforest jungles of central Africa, where they grew ever more arboreal. Meanwhile, the lower forests of the east coast persisted intact, but separate, over millions of years. There are no apes there today, but Kingdon claims the coastal forests as our own ancestral home.
This also makes sense of the next stage, the bipedal space. There was a huge proliferation of species in the period from four to two million years ago, all upright and bipedal, but with a brain to body ratio no larger than a modern chimpanzee’s. We used to picture hominin evolution as a ladder, with one rung leading directly to the next. We thought that our precursors, like ourselves, were generalists who occupied so many niches that each successive prehuman form filled all of Africa and tolerated no close competitors. Now it is clear that many species coexisted in an evolutionary bush of parallel stems. The mix of traits appearing in each form is so bewildering that one recently found fossil is called Australopithecus garhi – garhi means ‘surprise’ in the Afar language of Ethiopia.
Kingdon says that the explanation is ‘evolution by river basin’. The eastern ground apes’ coastal forest stretched from Ethiopia to South Africa. In dry epochs, its separated populations diverged genetically from each other, though all were committed to standing upright. Then, in wetter times, gallery forest spread up the rivers and tempted each form of ape to travel to higher ground. Kingdon sees four great highland basins, four different centres in which savannah-living bipeds emerged: Ethiopia, East Africa, the Zambezi basin and South Africa.
The fossil record begins again with the Ethiopian Australopithecine species group, which includes the famous Lucy. The oldest species found so far dates to 4.3 million years ago. Lucy-like Australopithecines followed the Wabi Shebelle and Juba rivers to leave their fossils by the headwaters of Awash in Ethiopia. (If this view is right, the Lucy fossils need a separate generic name, Praeanthropus.) Some Lucy-like descendants grew large, with heavy, grinding molars: this robust species spread away from its point of origin throughout Eastern and Southern Africa. South from Ethiopia is the East African centre, the home of the flat-faced forms that originally travelled inland along the Tana river. Further south lie Zambia and Zimbabwe, flanking the Zambezi. No fossils of three or four million years ago have been found there, but Kingdon argues that it probably held yet another lineage. Furthest south, the short, steep rivers of the South African coast leap down from the Drakensberg mountains. The ground apes that first followed them may have been nomadic: present-day animals trek or fly to the summer meadows of the mountains, and then retreat towards the warm coast in the freezing winters. The South African forms adapted somewhat differently, to become the classic Australopithecines discovered by Raymond Dart, who first realised that humanity’s original distinguishing trait is not a big brain but flat feet.
The emerging bipeds made their bases in belts of trees by the headwater streams and ranged out from there into the savannah. They had to lengthen their stride to cover ground. Kingdon thinks that there would have been little conflict between the different social bands, because their linear territories along streams shared such narrow borders. I think otherwise, because I study bands of ringtailed lemurs who live beside a river but move out daily onto open ground. Riverside forest, with its great trees and its safe spots on the bank to drink, is defensible and worth defending – much more so than the sparse savannah. Our early bipedal ancestors would have the same reasons as modern chimps for group solidarity, group proto-warfare, dawning social intelligence, and a dawning attempt to understand and analyse other animals’ behaviour.
Kingdon grew up in Africa, and knows that a primate who is smart about other species is much more likely to survive there. Those early bipeds followed elephant and hippo trails through the brush, watched for concealed leopards, sought out hidden baby antelopes as prey. Every student of human evolution puts him or herself into the picture – Kingdon more literally than most, since many of his illustrations are wry self-portraits. For him, the creatures that travelled up the rivers from the coast were precursors of man the naturalist.