The origin of humans

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Why was man created on the last day? So that he can be told, when pride possesses him: God created the gnat before thee.

The Talmud

The universe may have a purpose, but nothing we know suggests that, if so, this purpose has any similarity to ours.

Bertrand Russell

Why should the final chapter of this book be about the origin of humans? The argument could be that this falls in correct chronological order: we have looked at the origin of life, sex, skeletons, land life, dinosaurs, and then humans come next. That's all very well, but we could equally look at the origin of sparrows or cats or sweet potatoes. It is virtually impossible not to focus on human origins, because we are human. Therein lies a danger. Humans are not the pinnacle of evolution. Everything that has gone before was not a prelude to the appearance of human beings who arrived to a great fanfare.

But humans are special: no other species on Earth, to our knowledge, writes books, or even reflects on the history of its own species. Wise philosophers through the ages have warned us to be humble. But humility is not what this is about. The key point is that evolution is not teleological, or 'goal driven'. There can be no pathway laid out into the future for evolution: species come and go, buffeted by the vicissitudes of history.

In the Late Triassic, a naturalist would have had no reason to suppose that dinosaurs would become large and diverse, and dominate terrestrial ecosystems for more than 160 million years, and that mammals would remain small and humble denizens of the night. Equally, when the dinosaurs were cleared from the surface of the Earth by the KT mass extinction, crocodilians, birds, or mammals all stood a reasonable chance of becoming top predators. In South America, which was isolated from other parts of the world, certain crocodilians became rather terrestrialized, and took on the role of predators. In South America too, but also in North America and Europe, giant birds with massive 1-metre long bone-cracking beaks preyed on the ancestors of horses and cats.

At the beginning of the new world, during the Palaeocene Epoch o of the Tertiary Period of the Cenozoic Era (see p. 18), the closest 1'

o ancestors of humans were weedy little squirrel-like animals u scuttling nervously along the tree branches. No sign there of latent 1 genius or incipient domination of the Earth.

The first primates

An intriguing fossil was reported in 1965 as 'the oldest primate'. The specimen was named Purgatorius, after the locality Purgatory Hill in Montana near where it was found, and the report caused a sensation. Here was our distant ancestor, living side by side with Tyrannosaurus rex and peering, perhaps knowingly, from behind a branch. Sadly, this report has subsequently been discounted -the fossil was just an isolated tooth, but the identity of the tooth is not so much in question as the age of the rocks in which it was found. The tooth was in a channel that had been cut down into the latest Cretaceous from the overlying Palaeocene, and so post-dated the dinosaurs. Since 1965, no other convincing find of a Cretaceous primate has been reported, and the image of monkeys and dinosaurs living together cannot be confirmed - even though molecular evidence strongly points in that direction. More of that anon.

The primates are one of the eighteen orders of modern placental mammals, named from the Latin primus, 'first'. As primates, it was our privilege to call ourselves members of the 'first' order -this privilege extended to the Church as well, in which bishops and archbishops are termed primates. There was a time when books entitled 'The sex life of primates' could not be sold safely in England. All primates share a number of features that give them agility in the trees (mobile shoulder joint, grasping hands and feet, sensitive finger pads), a larger than average brain, good binocular vision, and enhanced parental care (one baby at a time, long time in the womb, long period of parental care, delayed sexual maturity, long lifespan).

o o t Purgatorius is a plesiadapiform, a group of squirrel-like animals o

| that may have climbed trees. They had long tails, grasping hands,

1= and fed on fruit and leaves.

The plesiadapiforms lived among a diverse array of unusual mammals in the 10 million years following the KT extinction. Mammals had of course originated much earlier, in the Late Triassic (see p. 136), and they had diversified substantially during the Jurassic and Cretaceous, but most of the Mesozoic groups either died out during the Mesozoic or soon after. The three modern orders originated in the Jurassic and Cretaceous as well -the monotremes, marsupials, and placentals.

Monotremes today are restricted to Australia and New Guinea, being represented by the platypus and the echidnas. These mammals are unique in still laying eggs, as the cynodont ancestors of mammals presumably did. The young hatch out as tiny helpless creatures, and feed on their mother's milk until they are large enough to live independently.

Marsupials, such as the modern kangaroos, koalas, and wombats of Australia and the opossums of the Americas, are reproductive intermediates. They do not lay eggs and produce live young, but these young are tiny and underdeveloped, and they complete their gestation in the mother's pouch.

Placental mammals are the most diverse of the three living groups.

They produce young that are retained in the mother's womb much longer than is the case in marsupials, and they are nourished by blood passed through the placenta. Placental mammals are remarkable for their diversity of sizes, from tiny shrews and bats weighing a few grams, to the African elephant, weighing up to

5 tonnes, and the blue whale, weighing perhaps 100 tonnes

(although no one has ever weighed a large whale, nor does anyone know quite how this could be achieved). Their ecological and geographic range is vast too, from desert-living rodents to polar h bears, and from bats to whales. o i o

Mammals, morphology, and molecules f a s

Mammalogists have struggled for two or more centuries to understand the relationships of the major groups of living placentals - are cattle related to horses, bats to monkeys, whales to seals? Some morphological evidence was found to show that, for example, rabbits and rodents are closest sisters, elephants are closely related to the enigmatic African hyraxes and the aquatic sirenians, but many other supposed relationships were hotly disputed. Ironically, the more effort palaeontologists applied to this question, the less certain were their conclusions.

Molecular phylogeny reconstruction methods seem to have cut through the knotty problem. The story began in 1997, when Mark Springer of the University of California Riverside and colleagues discovered the Afrotheria, a clade (a clade is a group in an evolutionary tree that originated from one ancestor and includes all descendants of that ancestor) consisting of African animals, linking the elephants (Proboscidea), hyraxes, and sirenians with the aardvarks (Tubulidentata), tenrecs, and golden moles. The last three groups had all been assigned various positions in the classification of mammals, but their genes show they shared a common ancestor with the elephant-hyrax-sirenian group.

After 1997, everything else seemed to fall into place. The South American placentals, the edentates, formed a second major group, the Xenarthra. And the remaining mammalian orders formed a third major clade, the Boreoeutheria ('northern mammals'), split into Laurasiatheria (insectivores, bats, artiodactyls, whales, perissodactyls, carnivores) and Euarchontoglires (primates, rodents, rabbits). So, in the course of two or three years, several independent teams of molecular biologists solved one of the outstanding puzzles in the tree of life.

5 But why had this proved to be such a phylogenetic puzzle? Some o t suggest that the major splits among placental mammals happened

| very rapidly, and there was no time for shared morphological

¡= characters to become fixed. But the morphologists are fired up to find such characters: if the Afrotheria really is a clade, then there must be some obscure anatomical feature shared among them all! Early suggestions included the prehensile snout (elephants, tenrecs) or testicondy (retention of the testicles within the abdominal cavity). But none of these really applies to all afrotheres. In 2007, a possible shared morphological character was at last identified: afrotheres all have additional vertebrae in the lower back.

The other element of the debate had been the timing of all these splits. Early molecular analyses, around 1995, yielded dates for the deep divergences of placental mammals at 120 to 100 million years ago, well within the Early Cretaceous. Although placental mammals of that age are known, and these include the spectacular little Eomaia from China, these early fossil forms do not belong to any of the modern orders or superorders. The molecular dates were a challenge to palaeontologists, because the oldest fossils belonging to modern placental orders came from after the KT mass extinction, as in the case of the plesiadapiforms like Purgatorius.

Many palaeontologists, including myself, argued that the molecular dates for placental mammal origins must be too ancient, and perhaps for similar reasons of mis-calibration as the initial ancient dates for metazoan radiation in the Precambrian (see pp. 67-8). Indeed, some of the dates were revised upwards, but only into the 100- to 90-million-year range. Additional solace came from the zalambdalestids and zhelestids, fossils found in the mid Cretaceous, perhaps 90 million years ago, of Uzbekistan. These fossils were assigned to basal positions among the Boreoeutheria, and so seemed to fill substantial gaps in the fossil record. h

3"

Peace appeared to be about to break out, until a bombshell struck 1'

o in 2007. In a thorough re-examination of the zalambdalestids and u the zhelestids, as well as other Cretaceous placental fossils, John 1 Wible from the Carnegie Museum in Pittsburgh, and colleagues, showed that they all fell outside the clade of modern placentals. So, the gap was restored: molecular data suggest clearly that there had been a considerable amount of placental evolution in the Late Cretaceous, with the basal split into South American, African, and northern clades, and their subsequent division into the major orders, including primates. The oldest fossils are unequivocally Palaeocene and Eocene in age, so there is at least a gap of 25 to 30 million years where fossils are seemingly absent.

It has been easy for supporters of the ancient molecular dates to say that the Late Cretaceous fossil record is deficient. It is true that mammal fossils are rare, but the point is that several dozen species of mammals are known from a number of Late Cretaceous localities around the world and yet, despite Herculean efforts to assign these to modern orders and superorders, such efforts have been rejected by Wible and colleagues. If primitive placentals are being found, and sometimes as quite complete fossils, where are the missing modern forms? This debate is likely to rumble on for a while.

The monkey-rabbits

One of the surprises of the molecular phylogenies was that primates were allied with rodents and rabbits. The molecular studies confirmed that primates are members of Archonta, the clade comprising also Scandentia and Dermoptera. The Scandentia are the tree shrews, a tiny order of some nineteen species of tree-climbers from south-east Asia. The Dermoptera, or flying squirrels, are only two species, both of which have a skin membrane between their arms and legs, down each side of the body, and they can glide from tree to tree. The three orders within

5 Archonta are all characterized by some shared features of the ear o t region of the skull, as well as by 'the possession of a pendulous

| penis suspended by a reduced sheath between the genital pouch

Another clade that had long been recognized by the morphologists was the Glires, consisting of rodents and rabbits. The Order Rodentia is by far the largest placental order, comprising 2,000 species, about 40 per cent of all mammals. Rodents are fiendishly adaptable, and rats and mice have proved highly successful in human environments. The group includes also the cavies of South America, some of them quite large, as well as squirrels, beavers, and porcupines. Rabbit and hares, order Lagomorpha, share with rodents their constantly growing incisor teeth, a key factor in the success of both groups.

The molecular evidence confirmed the reality of the clades Archonta and Glires, and that both were close relatives within a larger clade, termed, with true inventiveness, but without regard for our dentures, Euarchontoglires. The Euarchontoglires are a major subdivision of the northern superorder of mammals, the Boreoeutheria, and so we must look to the northern hemisphere for the origin of those groups. Indeed, the oldest fossil primates and rodents, for example, come from the Palaeocene of North America and Europe.

Basal primates

Living primates are sometimes divided into prosimians, monkeys, and apes. These are convenient enough terms, although the 'prosimians' include quite a ragbag of forms that are neither monkeys nor apes, such as lemurs, lorises, and tarsiers.

There are over fifty living species of lemurs, which include the lemurs, indrises, and the aye-aye, all of them restricted to the island of Madagascar. Lemurs have long bushy tails, often striped h black and white, and they are nocturnal, feeding on insects, small o vertebrates, and fruit. The indrises include the woolly lemur, 1'

o which is nocturnal and lives in trees, while the indri and the sifaka h

are diurnal animals that live in troops on the ground, and rarely 1 move about bipedally by leaping along the ground. The aye-aye (Daubentonia) is a cat-sized nocturnal animal that probes for insects in tree bark with its slender elongated fingers.

Close relatives are the lorisiforms, thirty-two species of lorises and galagos (bushbaby), known from Africa and southern Asia. Fossil lemurs were known, until recently, only from Madagascar, but an earlier possible relative has been found recently in Pakistan, and the oldest possible loris fossil comes from the Eocene of Egypt.

Tarsiers, two species from the Philippines and the Indonesian islands, are tiny animals with huge goggly eyes that live furtively in the trees, and feed on insects, snakes, and birds. A diversity of ancestors of the tarsiers, as well as the entirely extinct omomyids and adapids, were significant tree-living animals in the Eocene of

North America and Europe, but also, later, in Africa and Asia. What other mammals lived at that time?

Tiny horses and giant rhinos

Most people have images of the evolution of horses, and some of the other familiar mammal groups. The first 10 million years of the Cenozoic saw a great deal of experimentation among mammals. The eighteen modern orders diversified, as well as a number of groups that have since become extinct. So it seems there was a kind of sorting out of the major mammalian lineages in the Eocene, which spanned from 56 to 34 million years ago.

Eocene horses, such as Hyracotherium, were indeed tiny, no larger than a terrier. Hyracotherium was a secretive woodland-dweller, adapted to scuttle through the tropical forests of Europe and 5 North America, feeding on succulent leaves from the trees. The o t ancestors of cattle and of the flesh-eating carnivores such as lions o

| and bears were also smallish woodland-dwellers.

Then a major habitat change occurred in the Oligocene, some 34 to 23 million years ago. Climates had been becoming slowly cooler since the end of the Mesozoic, and this cooling caused the climates in the centres of the continents to become arid. The lack of moisture meant that the lush forests died back, and grasslands spread more and more. Grasses had originated in the Cretaceous, but they did not become a dominant group in world ecosystems until the Oligocene. The secretive, camouflaged, forest mammals were squeezed into smaller and smaller patches, and many of them went extinct. Others ventured out onto the new savannas, and set off on a new evolutionary course.

The plant-eaters, such as the ancestors of cattle and horses, evolved to be larger. They had had four or five toes on each leg, and these reduced to three, and then to two in cattle and one in horses. The toe reduction was part of a process of leg lengthening and adaptation to fast running. On the open plains, camouflage was no longer a useful means of escape from predation, but height and speed were advantageous. As horses and cattle evolved to be larger, their teeth changed too. Leaves are relatively soft, but grass is hard because it contains small grains of silica. Horses and cattle evolved deeply rooted, ever-growing teeth with complex ridges of enamel and dentine on top to assist in grinding their food.

Some plant-eaters became huge. Among the afrotheres, elephants in Africa evolved from the size of pigs or small hippos to their modern size, and their trunks descended so they could continue to reach the ground. Rhinos, relatives of the horses, evolved into a diversity of forms, from medium-sized to very large. The largest of all, Indricotherium, was 5 metres tall, and looked like a cross between a buffalo and a giraffe. h e o

As their prey increased in size and speed, the predators had to 1'

o adapt too. Bears stayed largely in the woods, and continued to u hunt woodland creatures, but also diversified their diets to include I fruit, honey, and fish. Dogs never became very large, but they adopted new social structures, hunting much larger prey in packs, and relying on their endurance and their intelligence, to harry their prey to death. Some cats, such as lions and tigers, became large, and used their stealth to be able to creep up on their prey unseen, and then make a mad dash at the last minute.

Other mammal groups adapted in their own ways. Bats and whales were committed to life in the air and in the oceans respectively. By emerging at night, bats had found a new set of niches that birds did not occupy. Whales, descendants of land-living creatures, rediscovered the role of giant marine predators, vacated at the KT mass extinction by plesiosaurs and mosasaurs. The mammals of South America and Australia evolved rather independently from those of the Old World: Australia became a land of marsupials, including giant kangaroos and wombats, and South America had its own unique groups that mimicked horses, cattle, and rhinos.

Africa remained an island for most of the Cenozoic, but land bridges were formed across the Arabian Peninsula from time to time so that elephants and primates were able to pass across into Asia. Into this land of expanding savannas and diminishing forests came the first monkeys and apes.

Monkeys

After the Eocene, the omomyids and adapids became extinct, and modern 'prosimians' survived in some obscurity in Madagascar and south-east Asia. But a major new clade, the Anthropoidea, or monkeys, had arisen and was growing in importance. Monkeys

5 differ from their precursors in having rounded, instead of the o t slit-like, nostrils, large canine teeth, and premolars and molars, o

| the cheek teeth, modified for crushing plant material.

The origin of anthropoids is hotly debated: the traditional view is that the clade originated in Africa, while a new proposal is that they arose in Asia. The oldest African anthropoid appears to be Algeripithecus from the Middle Eocene of Algeria, based on isolated molars. More complete materials of anthropoids are known from the late Eocene of Egypt, and some of these show remarkable sexual dimorphism (physical differences between the sexes), as in many modern monkeys, where the males were twice the size of the females. This suggests that there was already a pronounced social structure, with males perhaps fighting their rivals for control of substantial harems of females. The Asiatic primates include several forms from the Eocene of China and Thailand: some may be non-monkeys, perhaps adapids, but others appear to be true anthropoids, and so further work is required to establish which came first, the monkeys from Africa or from Asia.

Monkeys today are divided into catarrhines, the Old World monkeys, and platyrrhines, the New World monkeys. These two groups seem to have diverged back in the Eocene or Oligocene, when the first platyrrhines somehow floated or swam across from Africa to South America. The two groups may be distinguished by their nose shapes: catarrhines ('narrow noses') have narrow noses with the nostrils placed below the nose, while platyrrhines ('broad noses') have broad noses and forward-facing nostrils. Platyrrhines also have prehensile tails, so if you see a monkey hanging by its tail, it's from South America.

New World monkeys include capuchins, tamarins, marmosets, and howler and spider monkeys. The Old World monkeys are more diverse, with smaller, tree-dwelling colobus monkeys, and larger ground-dwellers, such as baboons and mandrills, well known from Africa, but including also the Barbary ape of h

Gibraltar. These ground-dwellers live in large troops, the males o are often much larger than the females, and they have often 1'

o reduced or lost their tails. It's no wonder perhaps to realize that a u specialized group of Old World monkeys became the apes. 1

Apes

The apes arose from the Old World monkeys before the end ofthe Oligocene and the group radiated in Africa in the Miocene. Indeed, Africa in the Miocene, 23 to 5 million years ago, has sometimes been described as the continent of the apes. A typical early form is Proconsul, which was named in 1933 on the basis of some jaws and teeth from Kenya. The name refers to a chimp called Consul who then lived at London Zoo, and entertained visitors with his bicycle riding and pipe smoking. Since the 1930s, much of the skeleton of several specimens of Proconsul has been found, and these show that this earliest of apes had a monkey-like body, and it probably ran along branches, and fed on fruit.

Proconsul was clearly an ape, and not a monkey, because it had no tail, its brain was relatively large, and it had broad molar teeth with a particular arrangement of the cusps, just as in the modern apes and in humans. Early ape evolution happened in Africa, but there were several migrations out of Africa between 25 and 10 million years ago. One migration passed through the Middle East to Europe, and Late Miocene apes are known from Hungary to Spain. Other migrations passed eastwards into the Indian subcontinent and south-east Asia.

The Asiatic ape diaspora included the ancestors of modern gibbons and of orang-utans, which diverged from the African ape line about 25 and 20 million years ago respectively. The gibbons have an obscure fossil history, but they specialized in brachiation, swinging through the trees from arm to arm. The orang-utan also evolved in south-east Asia, and specialized in tree life. Orangs

5 evolved through a number of fossil forms called collectively the o t ramamorphs. The oldest examples come from Africa, but later

| ramamorphs, such as Ramapithecus and Sivapithecus, are known

¡= from Turkey, Pakistan, India, and China. Sivapithecus was rather like the modern orang-utan, with heavy jaws and broad cheek teeth covered with thick enamel, all of which suggest a diet of tough vegetation. The most extraordinary ramamorph was Gigantopithecus, which was ten times the size of Sivapithecus, and adult males might have reached heights of 2.5 metres and weights of 270 kg. This huge animal stalked the forests of south-east Asia from 5 to 1 million years ago, and many regard it as the source of stories of yetis in central Asia, and Big Foot in North America.

The gorillas and chimpanzees continued to evolve in Africa. Although both groups are able to walk on the ground using their own peculiar mode of locomotion, knuckle walking, they prefer to remain deep in the forests, and move slowly about in the trees, feeding on fruit and leaves. It has always been clear that gorillas and chimpanzees are the closest relatives of humans, but the fossil record is not helpful in providing evidence. However, as is well known, DNA evidence suggests that humans share most of their genome with the African apes, and our similarity to chimpanzees is greatest. The current best estimates from molecular clocks and from fossils are that gorillas diverged first, about 10 million years ago, and the ancestors of humans and chimps separated about 6-8 million years ago.

What is a human?

Early palaeoanthropologists followed the understandable prejudices of their time and assumed that the first humans must have distinguished themselves from their ape relatives by possession of a large brain. There was logic in this: among modern humans, our large brain could be said to be our defining characteristic. Whereas the brain volume of modern humans is 1,200-1,400 cubic centimetres (cc), gorillas have a 500 cc brain h volume, and chimps a meagre 350 cc. o i 1

Brain volume gives a rough measure of intelligence, but only when u considered in proportion to body mass. (Note that the blue whale | has the largest brain volume of all, some 9,000 cc, but we would perhaps be loath to say that whales are eight times as intelligent as humans.) What matters is the encephalization quotient (EQ), the ratio of brain volume to body mass. The EQ for a whale is 1.8, higher than that for a horse (EQ = 0.9) or a cow (EQ = 0.5). Apes of course have rather high encephalization quotients, and the values are, in ascending order: gorilla (EQ = 1.6), chimpanzee (EQ = 2.3), and human (EQ = 7.5).

The other key human characteristic is bipedalism, walking on our hind legs. Dinosaurs and birds evolved bipedalism independently, and some lizards, monkeys, and apes can dash about on their hindlimbs for short spells. Among mammals in general, and primates in particular, humans are the only accomplished bipeds. Standing and walking fully upright led to many profound anatomical changes in our skeletons: the foot became flat and ceased to be able to grasp things, the ankle and knee became rather simplified hinge-like joints, and the hip joint modified enormously so the thigh bone fits into the hip socket with an inturned head. The pelvis has become bowl-like to support the guts, and the backbone is held more or less vertically, and it is S-shaped to accommodate the new pressures exerted by gravity. Quadrupedal mammals, including gorillas and chimps, have a long pelvis and a massive rib cage to hold the guts.

All the other peculiarly human characteristics stem from these two features. The large brain permitted or enabled language, social groups, extended care of children, adaptability to challenging environments, and technology. Bipedalism freed the hands for gathering food, tool-making, pot-making, scratching, and writing.

It seemed clear that humans acquired their large brains first, and

5 then bipedalism. Early fossil discoveries in the nineteenth century, o t such as Neanderthal man from Germany and Java man, Homo

| erectus from Java, did not help much because palaeontologists

¡= were unsure of their relative ages.

The key support for the 'brain-first' theory came in 1912 when a remarkable skull was found in southern England, at the village of Piltdown. Here was an early human with a large brain. When the first important finds were reported from Africa in the 1920s, their significance was not realized, and it was only when Piltdown man was shown to be a fake in the 1950s that the true story emerged.

The skeletons of early hominids from Africa showed that bipedalism had arisen by 4 to 6 million years ago, and yet the increase in brain size came much later, perhaps 2 to 1 million years ago. Perhaps the first humans were forced to become bipeds as the central African forests diminished in size and the grasslands expanded between 10 and 5 million years ago. Modern chimps and gorillas are restricted to the great Congo forests in the west, whereas the first human fossils are known from a broad crescent over East Africa from South Africa, up through Kenya, Tanzania, and Ethiopia, to Chad in the middle of the Sahara Desert.

Sacré bleu! Les fossiles humains les plus vieux - ou non?

Until 2000, the oldest human fossils had been reported from rocks dated in the range 4 to 2 million years. Then, in 2001 and 2002, two rival French teams reported much older human fossils, each about 6 million years old. Both finds proved controversial, and there has been much name-calling and squabbling over the respective finds.

First was the report by Brigitte Senut and her team from Paris. In 2001 they reported the new hominid Orrorin tugenensis based on teeth, jaw fragments, and limb bones from Kenya. Senut and her h colleagues argued that the teeth were rather ape-like, and that the o arm bones suggested Orrorin could brachiate like an ape. 1'

However, the femur showed that Orrorin stood upright, and so u this was a true early human. 1

The second discovery was by Michel Brunet and his team from Poitiers who reported Sahelanthropus from Chad in 2002. Sahelanthropus is based on a rather complete skull, some fragmentary lower jaws, and teeth. The Sahelanthropus skull indicates a brain volume of 320-80 cc, similar to a modern chimpanzee, but the teeth are more human-like, with small canines. The position of the foramen magnum, the hole through which the spinal cord passes out of the brain, is disputed: Brunet claims it is located beneath the skull, which would indicate that Sahelanthropus stood upright.

The australopithecines

The oldest substantial hominid skeletons, Praeanthropus afarensis, come from rocks dated at about 3.2 million years ago, and these show clear anatomical evidence for advanced bipedalism, but still an ape-sized brain. The famous skeleton of a female P. afarensis from Ethiopia, called Lucy by its discoverer Don Johanson in the 1970s, has a rather modern human pelvis and hindlimb. The pelvis is short and horizontal, rather than long and vertical as in apes, the thighbone slopes in towards the knee, and the toes can no longer be used for grasping. Lucy's brain, however, is small, only 415 cc for a height of 1 to 1.2 metres, and this yields an encephalization quotient not much different from a chimpanzee.

The human genus Australopithecus continued to evolve in Africa from about 3 to 1.4 million years ago, giving rise to further small species, including A. africanus, the species Raymond Dart first found in 1924. These australopithecines show advances over Praeanthropus afarensis in the flattening of the face and the small 5 canine teeth. They also show some specializations that place them o t off the line to modern humans. For example, the cheek teeth are | more massive than in A. africanus or modern humans, and they ¡= are covered with layers of thick enamel, adaptations to a diet of tough plant food.

The robust australopithecines, sometimes called Paranthropus, reached heights of 1.75 metres, but their brain capacities did not exceed 550 cc, still a rather ape-like measure. They had broad faces, huge molar teeth, and a heavy sagittal crest over the top of the skull, a feature also seen in large male gorillas. These are all adaptations for powerful chewing of tough plant food. Even the sagittal crest supports this interpretation since it marks the upper limit of jaw muscles that were much larger than in A. africanus or in Homo. The robust australopithecines may have fed on tough roots and tubers, while the gracile A. africanus perhaps specialized in soft fruits and leaves in the wooded areas.

The first members of our genus, Homo, appeared in Africa about this time, so we have the extraordinary concept of several human species living side by side. All modern humans, Homo sapiens, are one species - not for reasons of political correctness, but based on biological evidence. Generally, members of a species all look rather similar, but some mammalian species show considerable variation in form. The key test of species uniqueness is that members of a species can all interbreed and produce viable offspring, the so-called biological species concept. This is why we know that all domestic dogs, even through they may be as wildly different as a Chihuahua and a Great Dane, are members of one species. Likewise, all modern humans can interbreed and produce perfectly healthy children.

Modern humans, the genus Homo

The leap to modern human brain sizes only came with the origin of a new human genus, Homo. The first species, Homo habilis, h lived in Africa from 2.4 to 1.5 million years ago, and had a brain o capacity of 630-700 cc in a body only 1.3 metres tall. H. habilis 1'

o may have used tools. The first fossils of H. habilis were found in u 1960 by the famous palaeoanthropologist Louis Leakey. His wife 1 Mary Leakey had found the human tracks in volcanic ash, as well as numerous other fossils from Africa. Their son Richard Leakey found the most complete skeleton of a similar form by the banks of Lake Rudolph (now Lake Turkana), and these have been named

H. rudolfensis, a species closely related to H. habilis.

So far, human evolution had been happening only in Africa. But the next species, Homo erectus, escaped from Africa. The oldest examples are indeed known from Africa in rocks dated at about

I.9 million years ago, and similar dates have been suggested for H. erectus specimens from Georgia and from China. H. erectus had a brain size of 830-1,100 cc in a body up to 1.6 metres tall.

One of the richest sites for H. erectus is the Zhoukoudian Cave near Beijing in China, the source of over forty individuals of 'Peking Man'. They were found in cave deposits dating from 0.6 to

0.2 million years ago, associated with evidence for the use of fire, the use of a semi-permanent home base, and tribal life of some sort. Homo erectus sites elsewhere show that these peoples manufactured advanced tools and weapons, and that they foraged and hunted in a cooperative way. H. erectus in Africa perhaps made the Acheulian tools, which show significant control in their execution with continuous cutting edges all round.

Truly modern humans, Homo sapiens, may have arisen as much as 400,000 years ago, and certainly by 150,000 years ago, in Africa, having evolved from H. erectus. It seems that all modern humans arose from a single African ancestor, and that the H. erectus stocks in Asia and Europe died out. H. sapiens spread to the Middle East and Europe by 90,000 years ago.

The European story is particularly well known, and it includes a

5 phase, from 90,000 to 30,000 years ago, when Neanderthal man o t occupied much of Europe from Russia to Spain and from Turkey

| to southern England. Neanderthals had large brains (average,

¡= 1,400 cc), heavy brow ridges, and stocky, powerful bodies. They were a race of H. sapiens adapted to living in the continuous icy cold of the last ice ages, and had an advanced culture that included communal hunting, the preparation and wearing of sewn animal-skin clothes, and religious beliefs. Some paleoanthropologists see the Neanderthals as distinct enough to be given their own species, H. neanderthalensis.

The Neanderthals disappeared as the ice withdrew to the north, and more modern humans advanced across Europe from the Middle East. This new wave of colonization coincided with the spread of Homo sapiens over the rest of the world, crossing Asia to Australasia before 40,000 years ago, and reaching the Americas 11,500 years ago, if not earlier, by crossing from Siberia to Alaska. These fully modern humans, with brain sizes averaging 1,360 cc, brought more refined tools than those of the Neanderthals, art in the form of cave paintings and carvings, and religion. The nomadic way of life began to give way to settlements and agriculture about 10,000 years ago.

The history of life has not ended. We are viewing the story from a particular timeline, and the story would have been different had this book been written by a plesiadapiform or a dinosaur. It is hard to avoid the classic narrative form in such an account. The earliest story tellers realized you must have a hero, who goes on a quest, faces untold challenges, and eventually succeeds in reaching his goal. Perhaps books about the history of life look like such a narrative, with a series of ever-more complex organisms emerging from the primeval slime, shaking off their competitors en route, and conquering the environment to emerge triumphant and in control of the Earth. h

3"

The record of human evolution seems to show an ever-quickening 1'

o pace of change. Major innovations have occurred in succession: u bipedalism (10-5 Myr), enlarged brain (3-2 Myr), stone tools 1

(2.5 Myr), wide geographic distribution (2-1.5 Myr), fire (1.5 Myr), art (35,000 yr), agriculture and the beginning of global population increase (10,000 yr). The rate of population increase was about 0.1 per cent per annum at that time, rising to 0.3 per cent per annum in the eighteenth century, and about 2.0 per cent per annum today. In other words, the total global human population will more than double during the lifetime of any individual born today. In numerical terms at least, Homo sapiens has been spectacularly successful.

Evidence that the history of life is not a classic fictional narrative, however, is threefold.

First, evolution is not teleological. It is a fallacy to compare the evolution of life to ajourney. Humans plan their journeys and have a goal in mind. Evolution cannot work that way. Evolution works for the moment, selecting mercilessly which sibling survives, and which is thrown from the nest. The detailed criteria that worked in favour of sibling A last year might work against that sibling this year. A change in rainfall patterns, the death of a particular tree, a chance visit to the nest by a snake, or a new virus could change everything. Then, it may be entirely different the year after. Natural selection and fitness are relative, not absolute.

Second, evolution has not stopped. Evolution continues today as it always has; species arise and species become extinct. Human beings are affecting the Earth and the remainder of life in a more profound way than any species before. There is no evidence that when Homo sapiens has gone, everything will fall to pieces; probably quite the reverse in fact.

Third, cockroaches are the pinnacle of evolution - to other 5 cockroaches. We might like to regard ourselves as the most o t successful species on Earth because we occupy so much of the | Earth's surface, and control so many million square kilometres of ¡= farmland. But there are probably more cockroaches than humans. And, taken further, there are certainly more bacteria and other microbes than humans. We can define ourselves as the most successful species on Earth by careful choice of the terms by which that decision is made. Doubtless a sapient cockroach would write a different book.

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