Chapter 22 The human record

Sections

The walking tool users

The burdens of being bi-pedal

East Africa begins to break and rise

We are what we eat

Cutting loose from climate stress

Nine-month old babies placed in front of a mirror look intently at their own image, as they would at that of any other baby. Infants that young enjoy looking at babies’ faces, yet see their own image indifferently. A little older, they get a great kick from gesturing, smiling, touching themselves and recognising what they do in the mirror image. Surreptitiously daub the nose of a 15-month or older baby with rouge. Seeing themselves in the mirror, they show themselves to be fully self-aware, by gleefully touching their own red nose in surprise. Chimpanzee infants do much the same. Using this simple test shows that only the higher primates, the great apes, have this particular capacity for self-awareness. But there is a proviso. Chimps reared in isolation from others of the same species always treat their own image as a stranger. A concept of self seems to depend on living socially. Humans who are isolated from an early age through gross neglect also have little concept of their individuality, and find it almost impossible to communicate. We have the potential for self-awareness, but it only materialises in a social context.

Given half a chance, every adolescent and many an adult spends time in front of the mirror. As much as to seek blemishes, this is a time for wondering who we are and what we are for. We try to define ourselves, and a great deal of cod metaphysics stems from that. The problem with defining or placing limits on humanity, or indeed a single human, is that we continually change. Humans consciously, and repeatedly, set out to change their world and in doing so change themselves and their consciousness.

Writing this I am sorting through, passing on, and possibly even extending knowledge consciously gained from the world. I am communicating with the future by using a chip of silicon and about 100 years worth of a special kind of mathematical knowledge. The computer and its word processor are artefacts, extensions of my brain and body, as too the language and script. Although invented and made by people who I will never know, they are no different in essence from a sharp edge that anyone might make by breaking a hard, fine-grained rock. You or I would survive without the Pentium processor, a hard drive and MS Word, but even the most skilled gatherer-hunter living in Australia or the Kalahari Desert would perish in a few weeks without tools of some kind – a digging stick, cutting edge, hunting weapon, shelter and fire. Human jaws and teeth, sureness and fleetness of foot, and digestive systems now fall too short of those of our nearest biological relatives for us to survive au naturel.

Humans also make and use tools in a social context. Sure, many other animals have social graces. Some pick up objects and use them, even making complicated things such as nests. Scan the animal kingdom and it is possible to find nearly all the rudiments of human behaviour, even glimmers of consciousness and knowledge sharing. But outside of our own species all these attributes are never bundled together, and there is one that is truly unique. Humans take parts of the natural world, transform them and use them to intervene in their surroundings, including the creation of other kinds of tool. How to do this is passed on as part of a universal sharing of labour and its products, no matter what the social group or type of economy. Today that social sharing pervades an almost fully global economy without each of us being fully aware of it.

Consciously creating tools and using them socially produce entirely new conditions of life that rise above the ‘pure’ Darwinian concept of natural selection of the fittest individuals. Our individual fitness in a physiological sense is potentially augmented by the whole of human wealth, by a social culture and its history of experiences, consciously passed to future generations. Tens of thousands of individuals in the line leading to us survived to reproduce, not because they were necessarily fit in a purely Darwinian sense. Uniquely, they were cushioned from ‘nature, red in tooth and claw’ by their own conscious actions and those of other people, through tools and the social practices surrounding them. These unique conditions of life have provided opportunities for otherwise highly unlikely physiological developments of many kinds. Yet the changes have been limited by a physiological framework governed by genes and by ancestry. More fundamentally, many of the changes emerged while outward nature fluctuated to a degree and at a pace unprecedented since the glacial epoch of the Carboniferous and Permian, a quarter of a billion years earlier.

We are apes, albeit very strange ones, and members of the family Hominidae (hominids) within the Primate class that also includes lemurs, tarsiers and monkeys. Taxonomically, however, we are the only living member of the hominid Tribe known as Hominini, and all fossil members of that – those believed to be ancestral to humans – are known as hominins. Other apes rarely walk on their hind legs, chimpanzees (Pan troglodytes) being the most accomplished. As forest dwellers the great apes have hands and feet that are well-evolved for grasping, and an arboreal life demands excellent 3-D vision, hand-eye coordination and balance. That all four other great apes – common chimpanzees (Pan troglodytes), bonobos (Pan paniscus), gorillas and orangutans – share these characters, even though chimps, bonobos and gorillas spend much time on the ground, points strongly to a common arboreal ancestor. Human descent from a tree-living, ancestral ape has been a point of agreement among zoologists since Darwin. But which modern ape shares a common ancestor with us, and when did the divergence start?

While fossil evidence was the single available route towards an answer, the only option was to seek ‘missing links’, a difficult task as the hominid fossil record gets more fragmentary further and further back in time, and therefore more prone to misconceptions and doubts. Modern ideas about evolutionary descent see divergence as a bush of connections, with more lost causes than successful lineages. Most species and genera become extinct, only a few splitting for descendants to make it through time. As well as the whens and the hows, there are also the questions of where new species arise and the conditions under which they do so, which must wait awhile. For even a single fragment of an individual from one species to be preserved over hundreds of thousand or a few million years and then to be unearthed requires a palaeontologist to be extremely lucky as well as having a good idea about where to look. The fossil record of land-living animals is scanty as a result of scavenging, disarticulation and repeated erosion. So, fossil remnants of all species that were alive at the same time are even more unlikely to turn up. The laws of probability are strongly against us ever reconstructing the evolutionary bush fully. All that palaeoanthropologists can do is to link fragmentary finds by physical similarity, and attempt to put them in a time sequence. The study of human origins and evolution is a field that demands imagination, yet also one prone to wishful thinking and hubris; the factors that ensured the Piltdown forgery of a ‘missing link’ fooled scientists for almost half a century.

Truly reliable evidence of relatedness lies in the genetic material of living organisms, but comparative molecular biology does help rationalise the fossil record. The first ‘rough draft’ of a single human genome was announced in 2000, at a cost of 3 billion US dollars and 15 years research. Only since 2010 has it become possible to fully reconstruct DNA recovered from fossilised remains older than a few hundred years. At the time of writing such palaeogenetic studies had penetrated as far back as 700 ka, for a fossil horse and 430 ka for a hominin bone. Partial analysis of DNA, targeted for instance at small sections of the double helix most useful in assessing close relatedness by DNA profiling, or at sections in mitochondria or individual chromosomes, shows up regional differences between living human populations and degrees of relatedness between different living species. These approaches have revolutionised understanding of the last stages of human evolution, and are covered in the next chapter.

Comparing genetic material between living primates brought some surprises. In the human population 99.5 % of DNA is shared by all people who have been analysed. When the focus is on functional genes there is only 0.1% difference between any two individuals, often even less. We differ in DNA sequences from chimpanzees and bonobos by about 1.2%; gorillas are about 1.6% more different; orang-utans 3.1% and all of the great apes differ from rhesus monkeys by ~7%. Chimpanzees and bonobos are our closest living relatives. Surprisingly, the difference is less than that between species of the warbler family of birds, almost indistinguishable except by their songs. Were it not for profound differences in behaviour and physiology, we could be regarded as the third species of chimpanzee. Because genetic differences, and eventually speciation arise by random mutation, the degree of genetic difference amounts to a ‘molecular clock’. With a great many assumptions, that approach can roughly date the main branchings in the evolutionary bush whose end products live today. The key to this dating is the rate at which mutations take place, which can be estimated by counting the number of genetic differences that have accumulated between generations. Unsurprisingly, this is fraught with problems, partly because mutation-triggering factors, such as radiation, mutagenic chemicals, errors during cell division etcetera probably vary with time and from place to place.  Applying mutation rates in chimps and humans – both estimated at about one mutation per 2 billion base pairs per year – to their DNA differences, with a few arcane correction factors, now places the split at between 7 and 10 Ma ago. Whether or not this is in the right ‘ball-park’ can only be tested by the radiometric ages of fossils at or near the split. It is worth noting that since 2000 three different human-ape mutation rates have been proposed, giving a range from 4.5 to 13 Ma for the age of the chimp-bonobo-human last common ancestor.

To whet your appetite for the later account of genomes of humans who separated since 1 Ma ago, much the same approach has been applied to the DNA of lice from us and living apes. The human head louse (Pediculus humanus) is genetically close to that which infests chimpanzees (Pediculus schaeffi), both of which shared an ancestor at roughly the estimated time of the split between humans and chimps. More alarmingly, the pubic louse or ‘trouser shrimp’ (Phthirus pubis), which is caught neither from toilet seats nor by leaping from host to host, is related to the gorilla louse (Pthirus gorillae). The molecular date for that exchange is around 3.3 Ma, before the genus Homo entered the scene. Moreover, to choose pubic hair cover rather than join with head lice suggests that our bipedal hominin ancestor was already mostly naked at that time. By such minutiae paleoanthropology is enriched …

The walking tool users

Finding an object that is plainly a tool in some sedimentary stratum shouts to us, ‘Consciousness!’ It does not matter that its maker may have brachiated through the trees, had a brain the size of a grapefruit, or the face and table manners of a baboon. In fact, we do not need to have a clue about the maker; in the case of the earliest tools their makers remain unknown. Whomever, they had created a potentially powerful ‘buffer’ between them and the rest of the natural world. For tools to crop up again and again through the later geological record signifies also that their makers passed on their skills. Tools demand dexterity to make them, and that requires hands and nimble fingers freed from an habitual four-legged gait. They are the human calling card.

Simple cutting edges on broken pebbles of flinty rock occur in 3.3 Ma old river sediments exposed by the shores of modern Lake Turkana in Kenya. Sceptics might justifiably claim that such broken bits are found by any river, were it not for four plain facts. None of them is of rock that will not take a razor-sharp edge. Each fits snugly in the hand, mostly the right hand. The edges are not single breaks but serrated by smaller fracture surfaces. Together with the tools are stone flakes (debitage) from their production. Until this find, tools dated at 2.6 Ma at Gona in the Afar Depression of Ethiopia were thought to be the first. The Gona tools have clearly been shaped deliberately and include robust ‘choppers’ and hammer stones, and more delicate, sharp-edges flakes. Some resemble scrapers and borers, hinting at the use of skins for clothing or as carrier bags. That remains of the makers are yet to be definitely linked with either set of tools is hardly surprising. Unless deliberately buried – a recent habit for humans – corpses of land dwellers attract scavengers that scamper off with the bits to gnaw them, or they rot and fall apart to be smashed by flood water. But stone tools are almost indestructible.

The first tools to be discovered, by members of the famous Leakey family, in close association with what then (1960) were the oldest known human-like remains are from a stratum in the Olduvai Gorge of Tanzania. The sediment, laid down by a river flowing through a semi-arid land into a lake, is about 2 Ma old. The tools (Figure 22.4a) are the same as those from Gona, but are termed Oldowan because they were described first from this locality. The fossils are mainly of teeth – being small and hard they survive intact – together with bits of jaw and cranium, and remarkably 20 fragments of a left hand. By 1986 three individuals had been unearthed at the site, one providing arm and leg bones. Teeth are commonly used to assess relatedness among hominin fossils, in the frequent absence of much else. A complete cranium of about the same age, found in 1973 at Koobi Fora, Kenya – thought to be a related species, Homo rudolfensis – proved to have closely similar teeth to the Olduvai specimens. Together the four sets of specimens have helped to define H. habilis (’handy man’) as a flat-faced (by comparison with other contemporary hominins) individual with a brain size a bit less than half that of modern humans. How the spinal chord enters the skull base through the foramen magnum enables habitually upright or four-legged gait to be assessed, in this case showing H. habilis to be bipedal. Limb proportions – relatively long arms – hint at chimp-like climbing abilities, although the hand bones show signs of a more human-like precision grip. Mystery still surrounds the genus Homo in its earliest manifestations, largely because finds are few and generally fragmentary. But, much in the manner of buses, after a long wait the Leakey family excavated three further individuals at Koobi Fora in 2010. However, they are regarded as three separate Homo species who cohabited northern Kenya between 2.0 and 1.8 Ma. Now, that is pretty odd, as a threefold morphological distinction ought to reflect three lifestyles sufficiently different to support the species over several hundred thousand years. Palaeoanthropologists remain silent on that seeming paradox.

Large chewing teeth but much smaller canines than any living ape except us, together with microscopic wear patterns, suggest that H. habilis had no especially favoured vegetable diet. However cut marks on associated animal bones indicate some consumption of meat. But meaty limb bones of food animals are rare at the site. Coupled with signs of cut marks superimposed on grooves made by the gnawing of large predators, this rules out any notion of big-game hunting. The first known tool users apparently ate meat scavenged from carcasses left by big cats. Perhaps, like modern hunter gatherers – the Hadza –  on the Tanzanian plains, they drove large predators from their prey by sheer chutzpah. Possessing sharp tools they could hack lumps from a carcass and then beat a hasty retreat. But it’s probably more likely that they cut through tough skin and dried meat on abandoned kills.  Animal bones found in abundance with the tools are in several concentrations at the site, which may indicate repeated use of a communal eating place. Homo habilis were ingenious, but not the complete masters of their milieu.

Homo habilis was neither the earliest tool maker nor the first hominin to acquire and satiate a taste for meat, merely the first to be found. The tools from Lake Turkana, Kenya were made at about the same time as clear cut marks on bones of large prey animals from Dikika in Ethiopia’s Afar Depression. Dated at around 3.3 to 3.4 Ma, well before the time of H. habilis, this evidence points towards those earliest hominins bing worthy of the soubriquet ‘Homo’. But who might they have been? Australopithecus afarensis, made famous by Donald Johanson’s discovery of the almost complete skeleton of ‘Lucy’ in the Afar Depression is clearly a candidate. (Remarkably, analysis of 3-D CT scans of her bones by John Kappelman and colleagues at the University of Texas, Austin, revealed many unhealed fractures in her legs, arms, torso and cranium, which indicate that she fell out of a tree to her death. That the skeleton was so complete suggests rapid burial of her undecayed body in stream sediments before it was subject to the attention of scavengers.) She was excavated was from same sedimentary stratum, some distance away at Dikika, that also contained the butchered bones. Fossils of Au. afarensis also occur in sediments from Chad to South Africa, suggesting that they were the most successful members of their genus.  Yet without direct evidence on the issue of tool use we remain in the dark. The fact that Au. afarensis is the only hominin known from this middle Pliocene age range does not rule out other species occupying the same landscape. At 1.9 Ma there is evidence for six co-existing species in Africa and, through much of the last 100 thousand years, Eurasia had 5. So, it seems that only modern times saw diversity reduced to a single member of the anatomical Tribe Hominini. Chances are that a yet-to-be-discovered, more Homo-like creature made and used the first known tools.

The australopithecines extend back to 4 Ma in the fossil record, all having upright bipedal habits, brains roughly a third the size of ours, a height between 1.2 to 1.4 m and a slender or ‘gracile’ build. An important characteristic shown by statistical analysis of the abundant Au. afarensis fossils is sexual dimorphism. Males were as much as 50% larger than females, as are those of all living apes except gibbons, whose sexes are almost identical, and modern humans in which males are rarely more than 15% larger. Australopithecines had a similar body shape to chimpanzees and bonobos, but with flatter faces and much smaller canine teeth. In common with chimps, gorillas and orangutans, all had pear-shaped bodies and large bellies. In the mid-1970s Mary Leakey excavated a track of footprints preserved in volcanic ash at Laetoli in Tanzania (Figure 22.1), which bears witness to two adult Au. afarensis and a juvenile trudging through an area laid waste by a volcanic eruption, the footprints of one adult overlaid by those of another. The Laetoli prints, together with a near-complete Au. afarensis foot found in South Africa, have a gap separating the big toe from the rest, which shows that Au. afarensis had the branch-gripping ability of a chimpanzee’s hind foot. They probably walked and climbed equally well but not perfectly, perhaps explaining why Lucy fell to her death from a tree. One of the later (~2 Ma) species, Au. sediba, left us a near-complete, surprisingly human-like hand, raising the controversial possibility that it just might be a direct evolutionary link from australopithecines to humans. The widely favoured view is that it cohabited Africa with H. habilis who was descended from an unknown ancestor.

F22_1

Figure 22.1 Trackway at Laeotoli, Tanzania showing footprints of three individuals – probably Australopithecus afarensis – preserved in a 3.6-Ma-old volcanic ash deposit. The larger tracks show evidence of an adult walking in the footsteps of another, whereas the smaller tracks are probably those of a juvenile. Accompanying the hominin tracks are those of a small feline predator and an extinct horse.

Early researchers into southern African hominins, divided australopithecines into two groups: those with light-boned (‘gracile’) skulls, now the genus Australopithecus; and a group with heavy-boned skulls (‘robust’), now separated as Paranthropus. Analysis of the wear patterns on australopithecine teeth and carbon isotopes from their dental plaque suggest a diet of soft, nutritious leaves, bark and roots and maybe some meat; similar to that of modern chimpanzees. The paranthropoids had formidable heads, with massive flat teeth set in large square jaws. Bony crests atop their skulls anchored powerful jaw muscles, similar to those in modern gorillas. However, they stood not much higher than australopithecines, but had slightly larger brains. Their tooth wear shows a diet of fibrous plants, seeds and nuts, carbon isotope analyses of their tooth enamel pointing to a high proportion of grasses and sedges.  Like gorillas, such exclusive vegetarians would have had large guts and a girth to match, so that they could digest a low-grade diet. Paranthropoids were more successful than the lightly built australopithecines, from which they are thought to have descended, and outlived them by around a million years. Four species spanned the period from 2.5 Ma to possibly as late as 1.0 Ma ago, and coexisted with a succession of Homo species. Once considered a sideline to human evolution, in 2008 the paranthropoids sprang a major surprise in the form of a collection of heavily worn bone tools. The wear patterns suggest that they used large long bones to dig, perhaps for roots or maybe for termites. Moreover, a site in the Sterkfontein Caves of South Africa, in which undated paranthroid remains had been found, yielded evidence of the controlled use of fire about 1.5 Ma ago, but no direct evidence of who lit it.

Although it is not yet possible to associate the earliest tool production and use (~3.3 Ma) directly with any hominin fossils, evidence suggests that both activities had already arisen more than a million years before the appearance of fossils (H. habilis) assigned to our genus. Australopithecus afarensis that inhabited southern and East Africa at that time definitely walked upright and is the most likely known candidate for first making tools. Yet the trail for bipedalism goes back a good deal further in time. Over 20 individuals of Au. anamensis have turned up in sediments from between 4.0 to 4.5 Ma around Lake Turkana, Kenya and one from Ethiopia’s Afar depression, thanks to work by Maeve Leakey and colleagues. Anatomically they are sufficiently like Au. afarensis for some researchers to consider them to be directly ancestral. The australopithecine lineage runs cold with this species. Yet no more than 10 km from the site in Afar a 0.2 Ma older fossil of Ardepithecus ramidus emerged. Remains of ten individuals were assigned by Yohannes HaileSelassie, Gen Suwa and Tim White to the same genus (Ar. kadabba) in 5 more Afar sites for which radiometric dating suggested a time span from 5.5 to 5.7 Ma (Upper Miocene). Based on its skull, pelvis and leg bone architecture Ardepithecus, though more primitive than the australopithecines, was also bipedal. The trail of upright gait continues in earlier Miocene sediments, through the discovery by Brigitte Senut and Martin Pickford of Ororrin tugenensis (5.7 to 6.1 Ma) in Kenya and of Sahelanthropus tchadensis (~7 Ma) in Chad by Alain Beauvilain. Both are deemed to have been bipedal, from the upper joint of the femur and position of the foramen magnum respectively. Sahelanthropus tchadensis may be the closest we shall get to a common ancestor of hominins and chimpanzees on account of its antiquity and its 350 cm3 brain size, with one important proviso: like all the other hominins it has much smaller canine teeth then living chimpanzees, an issue to which we will return.

Assessing the early stages of human evolution from a common ancestor shared with chimpanzees is hampered by an almost complete lack of ape-fossils from the critical Miocene to more recent times. That may be a reflection of the ready and generous funding for exploration of human origins compared with that for paleontological expeditions seeking less newsworthy fossils. But equally it might reflect the fact that sediments deposited by the rain-forest habitats that other apes inhabit today are less likely to preserve bones. Moreover sediments of the humid tropics are rare in the geological record of those areas frequented by palaeoanthropologists. Without some fossils demonstrating the evolution of living chimpanzees from the chimp-human common ancestor, resolving whether that ancestor more resembled chimps or hominins is impossible. Modern chimpanzees and bonobos, at some stage in their separate evolution from hominins, may have undergone specialisation that conferred their present phenotypes and traits.

We return to the evolutionary story of Homo itself later in this chapter and in the next, considering both bones and, for its later stages, genetic information derived from some of them and from living humans. First let’s examine some factors that may have influenced the evolutionary leap from diminutive, small-brained, foraging and scavenging australopithecines to species of Homo with the brains, physique and wherewithal to colonise and eventually to dominate the rest of the natural world

The burdens of being bi-pedal

In order to pass from generation to generation, any major evolutionary step must increase an organism’s fitness in an environmental context. Yet such steps sometimes allow further positive potentialities to emerge, such as bipedalism’s freeing of the hands to evolve in a way that was increasingly less related to locomotion. But the outcomes can include developments that trend in a very different direction. Switching from habitually using four legs to get around, even if that was the knuckle-walking and/or brachiating used by other living apes, for locomotion to depend on hind limbs alone is a drastic change. The limb bones and their articulation and the basic matter of balance would have had to change during that evolutionary shift, as would the ability to make a fast get-away from large carnivores.

F22_2Figure 22.2 Reconstructions, skulls and pelves of various hominins compared with chimpanzees. Each set is at a common scale

One of the crucial changes would have centred on the pelvis, the fulcrum of hind-limb articulation. That part of the skeleton had evolved in quadrupeds since the first vertebrates colonised the land in the late Devonian almost 400 Ma ago. Becoming bipedal was an evolutionarily step fraught with potential problems centred on the pelvis. In the absence of fossilised pelvic fragments, telling male and female apart is difficult. A female pelvis encloses the birth canal, which provides the most reliable female-male distinction. Because the cranium is the largest inflexible part of a newborn, comparing that pelvic opening in a hominin female fossil with her species’ adult body mass and head size suggests the pace at which foetuses developed in both stature and brain capacity. Walking upright involves transforming the pelvis to a major load distributor supporting the entire upper body including all the internal organs. In early hominins it became more robust and changed to a more bowl-like shape than the narrow, tall pelvis of knuckle-walking apes (Figure 22.2). Matching an adequate birth canal with bipedalism poses special problems for females. Walking would become increasingly difficult as the pelvis widened to allow delivery, thereby posing a limit on the birth canal’s diameter and thus the size of a newborn’s head.

Unlike all other primates, modern human mothers experience hours of pain when giving birth. The foetus must turn through 90° to emerge head-first and then shoulder by shoulder. Not surprisingly, because of their wider pelvis women have a distinctive gait compared with men, which is redolent with evolutionary meaning and has become a new, secondary sexual characteristic. The modern female pelvis is close to the limit posed by our basic architecture – if they had to give birth to babies with ape-like proportions scaled-up to human adult dimensions, they would be doomed by the pelvis to walking on all fours. Because brain capacity in relation to overall body mass in human adults is much larger than that in other primates, modern human infants must emerge with proportionally smaller heads. They are born at an earlier stage of foetal development than other primates, remain dependent on adults for longer – 10 to 13 years compared with the 2 to 3 for chimps – and undergo more rapid brain- and therefore cranial growth before reaching reproductive age. Evolving an upright gait presented a major problem for reproduction, only partly solved by this neotony or early birth. It also demanded protracted infant care, perhaps the underpinning of long-term female-male bonding among humans, and in producing a unique ethos of social sharing of labour and produce so that infants might survive. These social developments must have outweighed the risks to infant survival of an otherwise grossly ‘premature’ birth, and increasing neotony was probably a central feature of human evolution.

Before shifting focus to the environment in which the early hominins evolved, two issues need to be highlighted. Firstly, not a single hominin species since 7 Ma ago possessed the fearsome canine teeth shared by the other great apes with almost all other primates, including fossils such as the early Miocene (24 Ma) Proconsul, widely regarded as an ancestral ape. Since no primates, other than humans and occasionally chimpanzees, eat meat other than insects – their diet is dominated by vegetable matter – why the large, sharp canines? The answer among other living primates is twofold; for defence and, in the case of males, for social dominance of other males and females. Interestingly, among the few late Miocene ape fossils is Oreopithecus dated between 9 and 7 Ma. This ape lacked large canines, had hands superficially like those of many australopithecines and had a pelvis and thighbones that have been suggested as evidence for an upright gait. The problem is that Oreopithecus was found in Italy. The second oddity is that hominin sexual dimorphism suddenly decreased with the emergence of the early species of Homo. Did that reflect a revolutionary change in social and sexual relations – a shift from fierce competition for sex and male polygamy to pair bonding and greater group cooperation? At any rate that is where things stand, albeit imperfectly, with modern humans.

East Africa begins to break and rise

The blood-red laterite soils of Eritrea (Introduction; Chapter 8) formed over a lengthy period before 30 Ma ago. They are not unique, and relics occur throughout Africa. That such soils of a similar age also blanket Australia and crop out in India and elsewhere in the tropics points to some kind of global climate control over their formation. In the Yemen the pre-30 Ma laterites interfinger with marine sediments, so we can be pretty sure that they formed on a low-lying continental mass. Today, the laterites of East Africa rise from near sea-level to as high as 3 kilometres. A once horizontal stratum has been bulged up. Sitting on top of them are flood basalts formed by a superplume, which reached the base of the north-east African lithosphere about 30-40 Ma ago. They now rise to as high as 4.6 km to form plateaus famous for the long-distance runners of Kenya, Ethiopia and Eritrea. The basalt outpourings lasted just a few million years, but sediments only returned 13 Ma ago and are restricted to the Red Sea lowlands. The lowest are conglomerates full of basalt boulders and those of the ancient crystalline rocks buried beneath the laterites. Those sediments show the onset of the bulging, but not its climax. Uplift accelerated when the bulge was sufficient for gravity to take a hand. A tectonic split ripped through East Africa and extended the crust to form the great East African Rift system. As a result the already hot asthenosphere was unloaded. Partial melting added to extension, thereby helping to drive apart the flanks of what is now the Red Sea by slow sea-floor spreading. This also set the continental volcanoes of the Great Rift into renewed magmatic action. The bulk of extension and uplift of the rift flanks began at the start of the Pliocene, about 5 Ma ago.

From a nearly featureless low dome Africa acquired a respectable topography, for the first time in perhaps 300 Ma. That transformation is what makes East Africa the most exciting place on Earth for the whole of humanity. The geologically sudden upheaval lies at the roots of why we are capable of awe and curiosity. The laterites, and similar soils interleaved with the later flood basalts show tropical Africa with rain forest from Atlantic to Indian oceans before about 13 to 18 Ma ago. There was nothing to stop the flow of moist south-westerly trade winds to water those forests. The rising flanks of the East African Rift threw up a barrier to air flow, so that the Indian Ocean and its strongly seasonal monsoon climate became the dominant influence on East Africa (Figure 22.3). But this climate is complicated by superimposed effects of irregular variations in elevation; the Ethiopian Highlands and the roughly north-south chain of actively volcanic mountains up to 5 km high along the Rift. Each plateau and mountain forms a cool and even frigid climatic ‘island’ in the tropics. They encourage rainfall to feed semi-permanent rivers flowing across the intervening lowlands, and to fill a necklace of lakes along the Rift itself. Each upland mass has zoned vegetation from cold tundra at the top, through mist forests, to steppes, savannah, scrub and grasslands in the surrounding low ground. Tectonic forces have transformed a uniform blanket of rain forest to a mosaic of ecosystems more diverse than anywhere else. Rain forest had existed for perhaps 150 Ma, and its western remnant in the Congo Basin, like that of Amazonia, forms one of the greatest repositories of plant, insect and small-vertebrate life on the planet. East Africa hosts far more large land-vertebrate species than anywhere else, and that reflects its wealth of young ecosystems.

F22_3Figure 22.3 Simulation of the vegetation cover of tropical Africa, based on models of air flow and moisture transport: (a) with the present topography; (b) without the East African Rift System. Dark green – tree cover exceeds grassland; light green – grassland exceeds tree cover; orange – arid and semi-arid. After Sepulchre et al. (2006).

Stable and widespread ecosystems contain large populations of most species that they host. Genetic mutations that do not decrease fitness dissolve in a large gene pool in succeeding populations. Species change slowly. In small populations viable mutations become more common in the gene pool through genetic drift, and show in the phenotype (body plan and function) more quickly. Speciation can accelerate. In Africa, the formation of the Great Rift and its ecological mosaic split formerly widespread, large populations into many fragments, isolated in climatic and vegetation ‘islands’. The gene pools of organisms in the ‘islands’ lost contact with those of the uniform rain forest, through isolation of forest habitat by the spread of seasonally dry plains covered with grasses, shrubs and isolated savannah trees.

For arboreal creatures such as primates, digestible foodstuffs of the forest became separated by wide tracts of the inedible. Into the plains spread large numbers of rapidly evolving animals adapted to that habitat – browsers and grazers together with predators, various scavengers and opportunists that survived at the fringes of the main trophic pyramid. Miocene pigs, elephants and rhinos show sudden dental changes and become more suited to grasses than to leaves. These new inhabitants presented competition and threats to the original fauna. For animals adapted to moving through forest canopy, wide tracts of open ground presented a formidable barrier to movement between the ‘islands’. Any primates attempting to migrate across more open ground would have faced starvation, dehydration or being eaten. Possibilities for the more rapid accumulation of genetic mutations in small, isolated communities were accompanied by new selection pressures and new opportunities for changed habits. Individuals able to exploit resources beyond the forests and to move safely there would increase their fitness by such diversification. For many types of animal the fragmentation of ecosystems prepared the ground for an almost universal adaptive radiation, set in motion by a continuously changing landscape.

We are what we eat

Forest apes and monkeys already possessed some useful traits for plains life. Primate diets are rarely highly specialised and include fruits, nuts, leaves, grubs, small animals and eggs. Scrub and savannah offer varied menus, but food items are more widely scattered than in forests, and their abundance fluctuates between dry and wet seasons. So the same diet would not always be available. Arboreal life favours 3-D colour vision, excellent balance and agility, and a tendency to move in bands of up to 50 individuals. All four limbs in primates are adapted to grasping, and any visitor to a wildlife park will have been struck by monkeys’ human-like hands as they tear off the car radio aerial. Primates followed two evolutionary paths to life in more open habitats. Monkeys, having tails, are not well-equipped for upright walking. Various species of baboon adopted quadrupedal life with powerful and aggressive males maintaining guard on the smaller females and offspring. Apes have no tails and a succession of changes in form lead to a permanent bipedal gait. Moving on two legs is energetically more efficient than using all four and freed hands can wield staves of wood or hurl stones, still commonly used by anxious chimps. More important, they can carry food and thereby provide a greater measure of security than having to gorge at a food source or protect it from competitors.

About half a million years after the East AfricanRift began to form we see the products of such a body re-organisation in the upright Ardepithecus ramidus and then early australopithecines from the Kenyan-Ethiopian border area. Both are ape-like, except for being bipedal, with no particular increase in the relative proportion of brain to body sizes. The show no sign of human-like developments in their teeth, except for one; they do not have the menacing canines common to most higher primates. With Au. afarensis and the later Au. africanus, teeth had changed to incorporate thick enamel and low, blunt cusps. They show wear patterns that indicates omnivorous dining, perhaps including some meat. Their jaws are less elongate than those of the earlier forms and closer to our own almost semi-circular arrangement. Interestingly, the pelvises of female Au. afarensis are too small to cope with foetal heads half the size of adult ones (the proportions in modern chimpanzees). Neotonous birth seems likely even at this early stage of hominin evolution. The tracks of two afarensis adults and an infant in the Laetoli volcanic ash could indicate the basic human family unit.

The more robust paranthropoids ate grasses, sedges and perhaps roots and termites for which they developed digging sticks. There is little possibility that primate digestion could have coped with grass eating alone, for that requires ruminant digestive systems that evolved in grazers to suit this quite recent foodstuff. The emergence of grasses and their peculiar C4 metabolic pathway is linked to the decreasing CO2 content of the atmosphere during the mid- to late-Palaeogene and Neogene. It presented a dietary opening followed by large ungulates, which separated from the primates more than 50 Ma ago. For primates efficiently to exploit the energy and nutritional potential of grassland required their intervention in the main food chain involving grazing herds and large predators. Unaided, a one metre tall australopithecine would not only have been incapable of killing an antelope or horse, but could never have bitten through its leathery hide. It would also have been a toothsome snack for early large predators, defended only by being part of a band, perhaps carrying sticks and being able to climb. Even to come upon abandoned carcasses, dried by the sun would present little opportunity for sustenance.

Australopithecines lacking sharp tools either ate fresh meat exposed by the teeth of predators, or none at all. Early human ancestors must have evolved as bystanding observers of the main East African drama; opportunistic omnivores, little different in diet from others, such as porcupines or pigs. However, opportunists must be capable of recognising and remembering a wide diversity of foodstuffs, seasonal in nature and variable in their location. The dry season presents large problems, because the only nutritious vegetable foods suitable for primate digestion are either buried roots and bulbs, or fruits on different tree and shrub species. Foraging offers no clear-cut advantages and demands continual movement. Primates carry little in the way of adaptations to this lifestyle, such as the powerful snouts and tusks of pigs. Nor do primates have a keen sense of smell, and rely mainly on vision. They are not as agile as small predators. Physically they are more or less defenceless against large predators. They do however, have the wit to observe, mimic and remember, as the famous potato-eating macaque monkeys of Hokkaido in Japan demonstrate. That is a speciality worth having; watch pigs foraging and you will find a ready source of food, when they depart – and in hard times pigs will have demonstrated the art of digging.

Lowly as this picture of our origins might seem to be, to survive demanded an encyclopedic memory for every potentially nourishing part of the surrounding environments. That in itself constitutes a rudimentary form of culture. To have intervened habitually in the dominant food chain required two developments; that it became essential for survival at a time of great stress and that, somehow, large-animal flesh was rendered edible to the australopithecine dentition. The last demands cutting tools; the great leap to the fringe of humanity. But what novel stress could there have been in an environment successfully occupied by upright apes for at least four million years? Sediment cores from the floor of the Arabian Sea off East Africa show an increase in grass pollens over those of trees, beginning around 7 Ma ago. Terrestrial sediments suggest a late-Miocene dominance by mixed woodlands and savannas, grasslands expanding through the Pliocene. This is supported by carbon isotope data from fossil soils in East Africa, which show that since 6 Ma tree cover rarely rose above 40% in the central area of hominin evolution. The first clear sign of aridity appears at about 3.0 Ma as windblown grains in sediment cores from the Arabian Sea

Continental ice sheets began to form in the northern hemisphere at ~2.5 Ma. This marked the beginning of the enhanced, astronomically forced climate fluctuation of the Pleistocene. Studies in Africa of pollen and lake-levels show that rainfall declined during each northern ice advance. Grassland and open savannah increasingly dominated low tropical latitudes, and deserts spread at higher latitudes. Although expansion of grassland favours herbivores and their large predators, and probably encouraged the emergence at this time of the paranthropoids, it would have held few advantages for australopithecines. Indeed it would have further limited the availability of food for such foraging species and their competitors. One evolutionary option was to eat the meat of herbivores, thereby getting around the poor digestion of C4 grasses by higher primates. The first appearance of tools and butchered bones at 3.3 Ma surely marks a response to increasingly hard times by one group, probably among the australopithecines. Paradoxically, it may have been the wet season that posed the greatest hardships. That is when inedible grass grows quickly, but when fruits, grains and tubers are at a premium.

So how did tools come to be invented?  That sharp edges were somehow discovered by trial and error is absurd. Imagine starving australopithecines coming upon abandoned kills, desperately poking at the hide and meat with every conceivable object. Finally, after hundreds of generations, in an Arthur C. Clarkian cognitive dawn, one succeeds in administering a cut with a sharp stone!  Slow wit of that kind in hard times spells doom. Imagine instead the experience of walking barefoot across a landscape strewn with all manner of broken rock. The concept of sharpness for an upright creature with fleshy feet would be regular experience. Applying sharpness of broken rock to ripe-smelling carrion, would dawn on the dimmest of hungry beings with the hands to exploit this natural phenomenon. The positive selection pressure attending the use of such rich protein sources in an otherwise inedible environment would have been immense. More individuals would survive to reproduce. Remember too that our ancestors were apes. To creatures that habitually hurl rocks and use them for pounding, the leap to producing cutting edges themselves, and thereby serviceable tools, would not be spectacular in itself. Its consequences would have been revolutionary in many different ways. Primarily it removed by far the greatest problem for opportunistic foragers; that of finding the next meal.

Boy Scouts and Girl Guides learn that a cutting edge opens wide cultural horizons. Making wooden and bone tools, and the use of skins needs sharp instruments. Digging for roots would be easier, and both the means of production and the fruits of labour could be carried around in skin bags. Long after the first known stone tools and signs of butchery, at 2.0 Ma we see the outcome of this ‘buffer against nature’ – fossils with basic human form associated with the most durable of these ‘buffers’. Homo habilis had half our cranial capacity. Though heavily boned, particularly around the brows, their skulls are considerably lighter than those of australopithecines. So far as we can judge from scanty remains, the rest of their frame had much the same proportions as our own, with slightly denser and stronger long bones but retaining a smaller, australopithecine-like stature; about 1.35 m at most. Much of succeeding human physiological evolution focused on the skull and an increase in brain capacity and on stature. Here there are two important considerations.

Lightness of both cranium and jaws are the main characteristics of our skulls. Both are prone to being easily damaged by impacts – surely a feature that carries disadvantages in a purely evolutionary sense. But what if heavy-boned skulls were a hindrance to growth of the brain?  Foraging and tool making demand and encourage brain power. With such a lifestyle, increase in brain size carries a positive advantage in terms of the individual’s survival to reproduction. The access to a high-protein, scavenged diet provided by meat-cutting tools removed the advantages of having heavy, multipurpose jaws. Cup your upper head between both hands and go through the motions of chewing. What can you feel?

In the region of the temples you will feel the muscles of the lower jaw working. They are quite small, and attach to almost imperceptible ridges on the cranium. Lower down are attachment sites on the cheek bones. For heavy chewers, such as the paranthropoids, to a lesser extent australopithecines and even less so for early humans, these attachments were more robust than are yours. They had to be, because the mechanics of a muzzle-like face with large lower jaws demand large muscles to drive their leverage. Declining muzzle and expanding cranium go hand in hand, as the reduced need for chewing power renders heavy cheek- and brow ridges redundant. Humans succeeding H. habilis became flatter in the face. Curiously, the recession of the arch of lower teeth exceeded that of the upper jaw, and this eventually results in the chin. The only primates with chins are anatomically modern humans. One avenue for the evolution of increased brain capacity was therefore anatomical redundancy due to a changed diet. Changing environment, different habits and a new diet would not only affect chewing apparatus and the bones that support it. More easily grasped than the gradual change in skull morphology is a tendency for a change in our predecessors’ guts. We can surmise this from comparing the bowels of living humans with those of apes.

Primates evolved powerful digestive systems to cope with vegetable foods that are low in nutrition in proportion to bulk. Take a look at a gorilla, particularly a large male, and your immediate thought is, ‘My goodness, what a belly!’ Though there are sights almost equally as worrying in any public bar, stadium or shopping mall, a physically fit human is diminutive in that department. A high-protein, high-energy diet made the original primate gut largely superfluous, and so the human gut is the only energy-demanding part that is strikingly small relative to body size, compared with those of other mammals. Relative belly proportion has its reflection in overall body shape. The torsos of apes and australopithecines are pear shaped, getting wider downwards. We, on the other hand are built with a barrel-like chest and a narrow waist, and we see such lithe frames in all humans from H. habilis onward.

You can easily imagine the opportunities presented by this transformation in shape, even if it would not take our earliest ancestors to the front page of Vogue or Esquire magazines. There is more room for lungs, thereby increasing stamina and, with a decreased bulk, greater agility and speed. Interestingly, the pelvis of all Homo species suggests large buttocks; now, what might they signify? Try this, walk a few paces clutching your buttocks; apart from looking ridiculous you will not feel them doing much. Break into a trot and, there you are; the gluteus maximus muscles are working (although you will look even more stupid). When reasonably fit, humans up to a ripe old age can keep up a steady trot for hours, even on a hot day if they are naked to allow sweat to evaporate and cool them down. Australopithecine pelvises indicate diminutive buttocks. One of the greatest hunting weapons humans have is the ability to run for hours. In the tropics few of our prey can shed enough heat to last the distance. They collapse from heat exhaustion long before we would.

Large lungs with sophisticated muscle control also open up the route to speech. But by far the most important new avenue stemmed from the 2.0 Ma model gut and a high-protein diet that decreased the energy demand of digestion. Our brains are, proportionate to size, five time larger than those in other mammals. Big brains are not only expensive to use and maintain, but building one is the most energy- and protein-intensive part of child development. To bear and then to breast feed an infant require a women’s nutritional needs to rocket during pregnancy and early motherhood. The tripling in size of the human brain in the period up to puberty places a similar load on children’s energy and protein intake. Even the brain of the most dedicated couch-potato consumes more than 20% of their energy intake, yet makes up only 2% of their mass. Decreased energy needs of shrinking bowels, plus a more concentrated food input, freed more for the brain. Of course, not all meat-eaters are clever, but primates had to be smart in the first place. Given that starting point, all the environmental changes and the physical characteristics and habits that stemmed from them, it is not surprising that human evolution focused predominantly on the head and what lay between the ears. Freed of the ape’s belly, it opened up the physical requirements for becoming predators – speed and the endurance that allow any fit human eventually to outrun even a horse. The new diet and ways of getting it also reduced the time during daylight hours needed to get enough to eat; giving spare time to observe and to reflect.

Modern human craniums are light. By comparison with apes and extinct humans our skulls are more like those of juveniles than of adults. During maturation our faces change much less than do those of apes. We fail to become fully robust. Unlike those of all other primates, the skulls of human infants are almost disarticulated, plastic and capable of growth to accommodate the trebling in size of the developing brain. The other physiological factor contributing to growth of the human brain therefore stemmed from neotonous birth, a direct legacy of upright posture and hinted at in 3.4 Ma Au. afarensis. But why grow a bigger brain in the first place?  That presupposes a need and an advantage set against the rest of the world.

Cutting loose from climate stress

The first tools and signs of meat eating appear 300 ka  before the onset of dryness at about 3 Ma, which coincides with the split between australopithecines and paranthropoids. The first debatable signs of human anatomy in Kenya followed at about 2.4 Ma. Up to 700 ka ago climate cycled more or less every 40 ka, or once every 2 to 3 thousand generations but shows no clear correlation with the record of changes among hominins. Maybe that pace of events was too fast for human anatomical evolution to respond directly, or perhaps the emerging tool-using and meat-eating culture and a resulting growth in population and gene pool made it unnecessary for its practitioners to undergo such change in order to survive. As Figure 21.1 shows, the range of global climatic cycles up to 1.3 Ma was not as dramatic as it became later. Yet by 1.9 Ma the australopithecines had disappeared, whereas the tool-using paranthropoids continued with little change for another million years. From about 1.9 Ma onwards the Oldowan tool kit turns up from South Africa to Egypt, though rarely with fossils. But a newcomer made its first appearance in Kenya, but has also been found in South Africa, Tanzania, Ethiopia and Chad.

Apart from their skulls, these beings are barely distinguishable from modern humans. Even a near complete male early teenager (the Kenyan ‘Nariokotome Boy’; Figure 22.2) was 160 cm tall and may have reached 185 cm and 70 kg had he lived to maturity, i.e. in the upper quartile of modern male stature. His arms, ribs, legs and spine have fully upright human proportions. The main difference from us is his low, sloping forehead with prominent brow ridges, more protruding jaws and the absence of a chin. His brain size was 880 cm3, about half that of an adult modern human but may have reached a greater size had he reached maturity. The major significance of those like the Nariokotome Boy was that they added a new item to their tool kit; in fact their appearance in Kenya around 1.8 Ma accompanies the first example of this implement.  It is a flat, pear-shaped, deftly worked object with cutting edges along both edges of its sharp end (Figure 22.4b).

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Figure 22.4  Early stone tools: (a) Oldowan pebble tools; (b) Acheulean biface axes and flake tools.

Fitting comfortably in the hand, the biface or Acheulean hand axe (after the village of St Acheul in France, where examples were first found in very much younger strata) had been struck from the core of large pieces of suitable rock. The waste flakes are razor sharp, and found many other uses as makeshift knives, scrapers and borers. Whoever made the first such tool must have visualised it within the raw material, in the manner of a modern sculptor. Even to us it is an object to be admired; it takes a highly skilled knapper to make one in a day. We can imagine a whole range of uses for the axe, a true multi-purpose tool of great value to its possessor. Some bear the wear marks of cutting wood, so one of its uses was to make other tools. However, it could hardly have been a hunting weapon, without its owner leaping onto the backs of fleeing prey – unless it was hurled. Some biface axes are as aerodynamic as a discus, so potentially it would be a formidable projectile weapon. Yet strangely, it is not uncommon to find pristine, unused examples. So another aspect of its manufacture may have been for ‘showing off’; a display of fitness aimed perhaps at potential mates. Whatever, this dramatic technical development along with the body of a distance runner has earned the species its name, Homo ergaster (‘Working Man’ or ‘Action Man’).

It took until 1.4 Ma for Acheulean culture to be added to the Oldowan tool kit throughout Africa. This delay may reflect the great intrinsic value of the biface axe; few would be discarded and most finds can be assumed to have been mislaid. The proud possessor of a magnificent hand axe may also have casually used easily made Oldowan-like tools as disposable items. To this day nomads in Afar, Ethiopia often use knapped volcanic glass to skin and butcher livestock, despite possessing iron weapons. For another 1.5 million years the Acheulean culture thrived and diffused to Europe, Palestine, the Indian sub-continent, and very late to east Asia. It remained the central part of the tool kit for successive human species including early members of our own

In 1891 Eugene Dubois, bent on a quest for the ‘missing link’, discovered a single skull cap at Trinil in Java. Its heavy brow ridges, yet large cranial capacity provided the first evidence of brainy beings earlier than modern humans to engage popular interest. Dubois called his discovery Pithecanthropus erectus, the ‘Erect Apeman’, also known as ‘Java Man’. Within 20 years similar remains were found in the Zhoukoudian cave near Beijing – ‘Peking Man’, and there is now a wealth of such bones from China, Indonesia, India and Sri Lanka. Reclassified as Homo erectus, the species’ similarities with H. ergaster are so striking that some authorities use the names interchangeably; e.g. ‘African’ and ‘Asian H. erectus’. But there is one difference, no biface hand axes have been found in association with the earliest Asian fossils, merely the more primitive Oldowan tools. Dating the finds from Asia initially proved difficult, but with improved methods it now emerges that H. erectus was present in Asia from 1.7 Ma to as late as 27 ka; they coexisted with anatomically modern humans for over 20 thousand years. Dubois’s Trinil site is notable for another aspect of H. erectus. Recent examination of his fossil collection from there yielded freshwater bivalve shells that bear engraved geometric patterns (Figure 22.5), dated between 430 and 540 ka; the earliest example of deliberate adornment, or art.

F22_5Figure 22.5 Pattern etched on a shell from Trinil Java. From Joordens et al. (2015)

Hominins left Africa long before anatomically modern humans evolved. Migration of H. erectus over 10000 km in at most 200 ka implies an average of only 50 m per year. Their exodus was no purposeful expedition, but a steady, almost random diffusion. Yet it took the first African wanderers through forty degrees of latitude with all the changes in climate, terrain and ecosystems which that entails. Such an unprecedented population shift of Africans from their hot tropical origins to a latitude as high as 35°N, a bleak place even in today’s winters, could only have been accomplished with the ‘buffer against nature’ made possible by the crude tools that they carried and a culture more advanced than any that preceded them in Africa. What route did the first migrating Africans take, when did that journey begin and who made it?

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Figure 22.6 Five skulls excavated at Dmanisi in Georgia. Credit M.S. Ponce de Leon & C.P.E. Zollikofer, University of Zurich, Switzerland

In 1991 archaeologists working at the Georgian site of Dmanisi at 41°N, which had been an important town near the western end of the Silk Road, found human remains, but they lay beneath a stratum in which several extinct mammals had been found.  As work progressed to deeper levels, head bones emerged.  They seemed exceedingly primitive, and associated with equally archaic tools; not the elegant biface stone axes of Homo erectus and later, anatomically modern humans , but from the Oldowan culture first directly associated with the earliest Homo habilis in Tanzania. When the stratigraphic level was dated at 1.9 to 1.8 Ma that in itself caused a stir, as H. ergaster had only just appeared in Kenya by then. Who the migrants were was at first a mystery. The finds are anatomically rich, with fossils of at least 5 individuals, both male and female, including 5 well-preserved skulls. One skull most resembles the oldest African H. habilis, from Ethiopia, dated at 2.3 Ma. With a braincase of 546 cm3 it is on the small side of H. habilis and in the range of late australopithecines. Others resemble, but are smaller than H. ergaster. If found in widely separated localities most anatomists would assign the five skulls to entirely different species (Figure 22.6). Ruling out the highly unlikely, chance association of several human species far from their Africa origins, they may represent a population showing the result of extreme genetic drift in a long-isolated, small band. One in particular clearly had lost most of its teeth and its jaw bones had atrophied yet had survived, which makes a strong case for his/her having been cared for. At this point we break the human story at the stage of first intercontinental  migration, returning in the next chapter.

Biface axes mark a leap in human consciousness to abstraction of the potential usefulness that may hidden within natural materials. But beyond that breakthrough an entirely new aspect to culture must have revolutionised human ability to use the rest of the natural world beyond the tropics. The earliest evidence for controlled use of fire comes from a 1.7 Ma occurrence of ashes in cave-floor sediments associated with H. erectus, stone artifacts and animal bones at Lantian in Shaanxi Province, Central China. At least three sites in Kenya and others in Ethiopia, South Africa, Israel and Java also associate H. erectus with fire. A marked reduction in the size of molar teeth proportional to body mass in H. erectus and later Homo, compared with those in earlier hominins, has been used to suggest that the emergence of H. erectus stemmed from the control of fire and cooking; i.e. around 1.9 Ma. It has been shown experimentally that the time needed to gather and eat sufficient food to survive is reduced by about 40% after cooking was invented, because of its tenderising effect and release of more nutrition. That step would have further encouraged a reduction in gut size and growth of the brain. It would also free time for other productive activities and reflection on the world. Despite their ‘primitive’ appearance to us, early H. erectus had a basic subsistence strategy little different from that of our own direct ancestors a mere 100 thousand years ago.

The first out-of-Africa exodus is a good point at which to summarise who begat whom and when among the well-populated Tribe Hominini in its earlier history, insofar as anatomical evidence alone is capable of doing that (Figure 22.6). For later times a far more powerful means is available. DNA analysis has become possible for fossils up to 700 ka ago, albeit for a horse, and inferences can be made from human genomes from as far back as 400 ka.

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Figure 22.7 Early history of hominin evolution and evidence for climate change in East Africa. Based on a diagram at the handprint.com website

If you enjoyed reading this chapter online and want to learn more, Stepping Stones is now available to download as an eBook

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