by Paul Jordan
From Apes to Hominids
We have seen that the European evidence for early man peters out at somewhere around 1 mya. The trail leads back beyond that time to Africa as the cradle of humanity. There were non-human primates in Europe back from some 3 mya but they neither evolved into early forms of man nor persisted as part of the European fauna.
Homo sapiens sapiens is the only extant subspecies of the species Homo sapiens which in its time has embraced (as far, at least, as many anthropologists have believed) another subspecies we call neanderthalensis and another rather varied and loosely defined one that people are in the habit of calling ‘archaic’ or early Homo sapiens, without gracing it with a proper piece of Linnaean nomenclature (because, frankly, it is too loosely defined). The genus Homo has seen a number of species before sapiens: heidelbergensis, erectus, ergaster, habilis. Within a given species, individuals look more or less alike (though with a wide range of superficially different appearances, on occasions) and can all interbreed. The different species within a genus will have significantly diverged in appearance and interbreeding between species will be very rare, though sometimes possible. Speciation occurs when small populations become relatively isolated from their erstwhile fellow species members and are subject to pressures of natural selection (or to genetic drift sometimes) which can bring about very rapid evolutionary change. Because speciation is an affair of initially small numbers and rapid change, fossils that record periods of transition between species are almost impossible to find. The paucity (and inevitably somewhat random nature) of the fossil record of human evolution is the result of this state of affairs, exacerbated by vagaries of vastly differing chances of survival and discovery in different locations today.
The genus Homo belongs to the family Hominidae, which it is convenient to know more colloquially as the hominids. The Australopithecines of Africa were hominids, too. The hominid family belongs to the superfamily Hominoidea, which groups the hominids with the pongids (great apes and their ancestors) and hylobatids (gibbons and theirs). The Hominoidea belong to the Primate order of mammals, which also includes both the monkeys and such unmonkey-like animals as the bushbaby and the tree shrew (and included some even more unlikely relatives of ours in the distant past). It is interesting to note that all the living non-human primates are tropical or subtropical animals. Old World monkeys, apes and humans (on grounds of teeth, ear and eye bone structures and nasal form) belong more together than even Old World monkeys do with New World monkeys. Apes and humans, in turn, are more alike than apes are to Old World monkeys, in terms of trunk shape, shoulder structure (permitting rotation of arms around the shoulder joint) and absence of tails. The pongid apes are closer to humanity than are the gibbons, and the chimpanzees and gorillas are closer still than the orang-utans. Gorillas, in fact, share nearly 98 per cent of their genetic material with humans, and chimps share over 98 per cent, though the two apes have it distributed over 48 chromosomes and humans have 46. This situation is a pointer to the fact that it is in areas of control gene structures that human beings most crucially differ from the apes (the control genes orchestrate the sequence and duration of genetically guided developments as each new individual grows up from the fertilized egg). It is the control gene function that makes a chimp so different from a human being despite the near totally shared genetic material in common. And it is evolution by natural selection that has brought about the divergent evolutionary courses of apes and men since the time, reckoned to be only some six or seven million years ago, when the hominid and pongid lines separated. The differences that distinguish human beings from the apes and all the other primates have evolved because they are useful to survival in certain ecological circumstances; the walking posture of bipedalism, regular toolmaking, the unique features of human social arrangements, the progressive enlargement of the brain, the use of language, the quality of mind that produces art and even irrational belief, all these specifically human traits and more have been evolved because they have so far benefited the survival and reproduction of the human line.
When we review the trends and processes of human evolution and try to reconstruct not just the physical remains of our remote ancestors but also their likely social habits and states of mind, we have in the world today just the two opposite ends of the scale to look at for glimmers of enlightenment. At our own end, as it were, we can consider the ways of life of still existing or only recently vanished foraging folk (non-farming hunter-gatherers) like the inhabitants of Tierra del Fuego who might be said to have taken the simple life to extremes or the coastal Indians of north-west America who lived very well without any technology that went beyond that of Crô-Magnon Man. Much can be learned about the common human patterns of life that go with foraging from people like these but, like us descendants of the first farmers and city-dwellers, they too have long histories and have had plenty of time to sophisticate themselves in all sorts of ways that were not open to, say, Homo heidelbergensis. What we can, minimally, conclude to be the core traits of life for Homo sapiens sapiens were set out when we asked, in Chapter 7, whether the Neanderthalers truly shared any significant number of behavioural ways in common with modern foraging peoples. Those traits include home life with provisioning of wives and children by males, meat sharing with family and relatives, more or less monogamous but decidedly regular mating (with marriages outside the immediate group), kinship systems and alliances in a wider social network. One of the most interesting questions of human evolution asks when these traits were assembled into the fully human pattern. Teasing some hints of an answer (or answers) to that question out of the exiguous archaeological remains that have come down to us is one of the chief pleasures of the study.
At the other end of the scale of evidence available to us today when we want to think about the evolution of the human way of life is the world of the apes, in particular the chimpanzees who most resemble us not only genetically but also behaviourally, though the resemblance could not be called close. The chimpanzees, too, have enjoyed as many years of evolution as we have since their ancestors diverged from ours and they have, necessarily, adapted to a particular survival niche in the world, which is quite unlike our own; thus what they can tell us about the likely lifestyle of our own remote ape-like ancestors must be limited, but it is suggestive, if only for the contrast it offers to humanity as we know it.
Before we explore the life of the chimpanzees, it is worth looking into the general character of the primates, to see what inheritance we and the chimps already share from tens of millions of years ago. The primates in general retain some primitive mammalian features like clavicles (collar-bones) and a five-fold pattern of digits at the ends of the limbs. Their digits are highly mobile and prehensile, with opposable big toes as well as thumbs (though this feature has been lost in human beings). Mostly primates have flat nails on their digits instead of claws, and tactile pads on their fingers and toes, whose sensitivity chimes in with the dexterity of their feet or hands. Locomotion is largely powered by the hind limbs. The sense of smell is relatively well developed but not dedicated to any specialized extent; colour vision on the other hand is exceptionally good with the blessing, thanks to the forward positioning of the eyes in the skull, of stereoscopic imaging in depth, probably evolved to facilitate the negotiation of a world of tangled branches in dimly lit forests on the look-out for insects and fruit to eat. The all-important eyes of the primates are protected by bone around and (in the higher primates) behind them. Primate brains are often larger in relation to total body size than those of other animals (strikingly so in human beings) and it may well be that the processing of complex visual information from the two convergent eyes played an important part in forcing brain expansion in the course of primate evolution. Primate brains also exhibit a unique pattern of folds on the surface under the skull. To go with their sound but unspectacular olfactory performance, primates usually have short muzzles with reduction of the numbers of incisor and premolar teeth vis-à-vis their mammal
ian ancestry and unspecialized molars of relatively simple and primitive cusp pattern. The lower primates, like the lemurs, show less development of the distinctively primate traits and put more emphasis on the sense of smell and on night vision, which probably witnesses to the nocturnal nature of our remote primate ancestry of about 65 mya.
The very earliest primates (or potential primates) we can discern in the fossil record of the world were mouse-sized creatures like Purgatorius that lived on insects and the somewhat larger Plesiadapis that probably ate leaves and fruit. By about 55 mya the first recognizably primate forms, resembling today’s lemurs and tarsiers, were flourishing in North America – Notharctus being the best-known fossil of this sort. These were the first of the primate line to possess brains larger in relation to body size than the general run of animals of their time; they must have had a use for those enlarged brains and that use most likely related to their cleverness in spotting and securing high quality food in the shape of ripe fruits and new leaves against the visual background of a noisy clutter of branches and foliage. High quality food could sustain the energetic nervous activity of their clever brains without requiring the elaborate and sizeable gut needed to extract nutrition from poorer food. Thus was initiated a promising evolutionary feedback involving good food, braininess and reduction of gut that has played an important part in human evolution.
Primate ‘society’ (the grouping of primates in relationships between themselves) shows considerable variety, but most monkeys and apes live in social groups in a way that is unusual, if not of course unknown, among mammals. Society brings liabilities in the shape of greater chances of disagreement and competition and greater risk of disease spread, but also benefits in the form of mutual support through cooperation and shared defence against enemies. Because primate females, like the mammals they are, carry, give birth to, suckle, nurse and feed their young, society among the primates is based on the females who perform these steady duties – male primate roles range from mere impregnation without further responsibilities to constant attendance, care and protection. Physical differences between the sexes among the primates are emphasized or downplayed in relation to the social arrangements of particular creatures. Gorilla males, for example, are large beasts in relation to their females because they live in social circumstances where there is heavy competition between males to secure harems; the monogamous gibbons who live largely as couples outside a wider society do not show much sexual dimorphism. Social relationships, whether between sexual partners and rivals or relatives and allies within the group, are an important part of primate life; monkeys, let alone apes, show a keen awareness of close kinship, for example. The demands of social life probably played as big a part in the evolution of quick-wittedness and cleverness among the higher primates as did stereoscopic vision and the rewards for securing a high quality diet. Though we must use anthropomorphizing words when we talk about it, it is clear that monkeys and apes in social groups prosper by their mental capacity for buttering one another up and deceiving each other, for cooperation and selfishness as it suits, for forming alliances and breaking them in season, for second-guessing the intentions of their fellows and dissembling where their own are concerned. All this requires great brain power to run experimental models of behaviour and consequences in the ‘mind’, with a good back-up of memory; indeed, all this is very likely the reason for the evolution of the mind and that elusive entity we call consciousness. In many sorts of animals, it is the social species that are the brightest and biggest-brained – parrots, dolphins, wolves, and apes all exemplify this – but the application of their cleverness can be very restricted to their social world. Vervet monkeys, for example, are finely focused on their social interactions, but relatively dense about everything else including dangers from their enemies that look obvious to us. It is the great trick of modern humanity to have extended our social acuity to so many other areas of experience: a topic to which we shall return in due course.
The lineage that runs from the first primates to the common ancestor of the great apes and humanity includes, at about 35 mya, the oldest known monkey-like creature called Aegyptopithecus, from the Faiyum lake deposits of Egypt. Between 35 and 31 mya many genera of related monkey-like forms flourished in the Faiyum region. Their small eye sockets indicate that these were no longer nocturnal primates and their teeth suggest that some of them, at least, were in fact closer to apes and men than to monkeys. Though they had larger brains per body size than the lower primates before them, their eyes were not so perfectly forward facing as those of later higher primates and their muzzles were longer. They were about the size of a modern gibbon, but they lacked the very long arms of tree-swinging apes of today.
It is during a gap in the fossil record of about ten million years after the last known Faiyum specimens that genetic studies suggest the split between monkeys and apes was consolidated. By 23 mya East Africa was home to many genera of hominoids in its environment of tropical forests and woodlands. There were few lower primates left in this region and very few monkeys, who were probably thriving in some less forested area like North Africa. Proconsul is the best known of the early apes of East Africa, representing a definite progress over Aegyptopithecus towards the apes of today: muzzles were shorter, brains large both relative to body size and absolutely, with shoulders and elbows suggesting the development of the suspensory, tree-swinging habits seen in modern apes. Some at least of the earlier monkey-like forms appear to have had tails – these later hominoids of East Africa probably did not. They enjoyed a long run of evolutionary success, down till at least 14 mya and possibly millions of years later. Some of them show dental traits that point towards the living apes and men (with indications that the diet could include hard nuts and tubers as well as soft fruit) but the long arms of today’s apes (and of our earliest ancestors, too) were not yet in evidence.
After about 15 mya, to go with the onset of colder conditions in the northern latitudes, the tropical and subtropical world grew drier and the increasing aridity of the East African environment put selective pressure on its primate inhabitants which saw a decline in the fortunes of the apes in that part of the world (except for one aberrationally specialized sort of ape) and a corresponding rise in those of the monkeys. The apes of Africa began their retreat into the tropical forests where they hang on today, though relatives of theirs were able, thanks to Africa’s final closure through continental drift with Europe and Asia, to spread out into the woodlands of southern Europe and Asia as far as south-west China. The orang-utans are thought to have differentiated from this generalized ape line at about 12 mya. Some relative or descendant of Proconsul gave rise to the line of the African pongids and the hominids between about 10 and 7 mya – remains from the Samburu Hills in Kenya show a creature with rather gorilla-like molars (though very thickly enamelled) that belongs roughly to this evolutionary phase. Genetic studies suggest a date of something like 6 mya for the divergence between the chimpanzees and the hominid line, though other lines of evidence might add a million years or so to that estimate. Unfortunately, direct fossil evidence for the period when the ancestors of chimpanzees, gorillas and men parted company is lacking. In terms of genetics and way of life, the chimpanzees (and, perhaps, the pygmy chimps called bonobos, in particular) are our closest living non-human relatives and can, provided that we always remember that they and ourselves have enjoyed six million years or more of separate evolution in very different circumstances, shed some light on the likely constitution of our very remotest ancestors, who lurk just behind the veil, as it were, of hominid fossil availability at about 4.5 mya.
