Millions of years,not ten thousand

Six million years ago,a few bands of apes in the shrinking African forest were running out of luck...then some of them began to walk upright,and their descendants took over the world.This is the story of an extraordinary ape.

Ann and Patrick Fullick

HUMANS could justifiably be called the most successful of all living organisms. We have colonised every continent, and our technology, cultures, communication skills and use of world resources make us appear very different from the rest of the animal world. Yet we have a remarkable biological similarity to our nearest living relatives, the chimpanzees.
No one knows for certain when our lines diverged - the evidence is hard to find and difficult to interpret, so models of human evolution are constantly disputed and altered (see Box 2). In 1859, Charles Darwin's book On the Origin of Species made the idea of ape ancestry a compelling scientific hypothesis. Since then, many hundreds of fossils and many tens of thousands of stone tools have been found to prove that Darwin's theory applies to humans. This prehistoric evidence - as well as comparisons between living humans and other animals, and the study of human culture - tell us more each day about the path we trod between the African forest and modern life.

Humans are mammals, just like horses, mice and dolphins. More precisely, we are primates, a group of mammals that first appeared more than 60 million years ago and which includes the lemurs, monkeys and apes. Primates are especially good at surviving in the trees. Climbing and running through branches requires dexterity, good eyesight and excellent judgment - a fall from a tree can be fatal, and food trees and insects must be discovered, remembered, gathered and caught. Primates are well endowed with brains. They can manipulate food items with their hands. They generally give birth to only one offspring at a time, and consequently have good maternal care, which ensures a good chance that at least a couple of the low number of offspring survive to reproductive age. The primates are long - lived by comparison with other mammals of similar size. Chimpanzees regularly reach their forties in captivity, while the much bigger horse is old in its twenties.

The animal in us all Life in the trees

PRIMATES are often social animals and communicate by a number of facial expressions and barks or grunts. Those that live in social groups often have strict hierarchies of social status. In some species females are dominant, in others the males are obeyed. Males which defend groups against predators are often larger than the females, and have large canine teeth - the pointed teeth third from the midline. But the primate way of life is also relatively free from predators, and the rich diet from the trees has given primates the spare time to interact with each other. Many hours of the day are spent grooming, making friends and engaging in social intrigues.

From 25 million years ago until about 5 million years ago there were many species of apes. This was the age of the apes. The climate in much of Africa and Asia was warm and humid, and forests grew throughout the semitropical and tropical parts of the Old World.
But by about 7 million years ago, the climate began to change. Temperatures dropped, the forests began to recede, and the apes became fewer and isolated. Savannah grasslands and scrub became more common, and led to an increase in the number of monkeys.

The African apes, the gorilla and chimpanzee, are our closest relatives. Their skeletons and ours have every bone in common, and the genetic distance between humans and chimps, measured by similarities in proteins, is less than 1 per cent.
That small difference disguises big changes. While we can recognise our ape ancestry in our blood, bodies, hands, expressions and behaviour, prehistory must try to explain how, when and why human beings began to walk upright and have a skeleton that is well adapted for this style of locomotion. Many adaptations are needed for this, the foot is at a right angle to the leg and has a heel, the pelvis is rounded to carry the weight of the abdominal organs, the spine is curved into an S-shape to carry the weight of the head and arms, and it is the base of the skull that joins to the spine, not the back of the skull as with most four-legged animals.

Other differences that must be explained are why we are (at least most of us) relatively hairless. Humans have opposable thumbs and can grip objects with precision against the first and second fingers. Our faces are flattened, unlike the apes, that have protruding jaws. The spurt of development seen during puberty in young human beings is not seen in other primates-so perhaps stroppy adolescents are a purely human phenomenon.
Last but certainly not least, we have a greatly expanded brain which enables us to develop and use tools of quite amazing complexity. The size and structure of the human brain also makes possible the construction and use of language, not merely to communicate simple biological drives such as sexual receptiveness, hunger and aggression but also complex ideas, plans and philosophies. Human culture and the use of rituals to mark the various stages of life are unique in the animal world.

1: Interpreting the evidence
FOSSILS are scarce, and even the tiniest fragment of bone most be subjected to detailed examination. A tossil jaw and teeth can give an idea of the lower face.
The shape, size and wear on the teeth, including microscopic analysis, can indicate the type of diet eaten. A pelvis or limb hone can give evidence of weight and height, and indicate posture and the efficiency of opright walking. Ridges and bumps on bones are often places where muscles are attached, giving an indication ot the build of our ancestors, while marks on the inside of skulls can even give an idea of the structure of the brain.

The "missing link" species between apes and humans has yet to be found. But the likely time to look would be about 6 million years ago, in Africa. Our closest relatives still live in Africa, and fossil hominids there date back at least 2.5 million years earlier than elsewhere.
Chimps and the other apes do not walk upright except occasionally. So it is probable that our common ancestor also lived on all fours. Some time between 6 million years ago and about 4.5 million years ago, the age of the oldest biped yet found, descendants of the common ancestor began to walk upright.

Upright walking is usually accepted as the first real step made by the "hominids". This group includes all living people today, all their ancestors back to the time of upright walking, and several species of fossil bipeds who later died out.
Most of the early hominids belonged to the genus Austrolopithecus, or "southern ape". The earliest is older than 4 million years, and over the next 3 million years a number of different species of australopithecines evolved, some of which were almost certainly our ancestors, while others became extinct.

Common ancestors Close relatives

EVEN the earliest australopithecines could walk upright, though there are some features of leg bones from the oldest specimens which have led some anatomists to suggest that they could still scamper up a tree for food or for protection.
One of the best known of all early hominid finds is a 3-million-year-old australopithecine called "Lucy", who lived more than a million years after the first known upright walker, but in almost exactly the same spot. She got her name because her discoverers, led by Donald Johanson, now at the Institute for Human Origins, were listening to the famous Beatles tune Lucy in the Sky with Diamonds at the time. She was discovered in 1974 at Afar Hadar in Ethiopia at the northern end of the East African rift valley. Lucy was about 1.07 metres tall and probably weighed only 30 kilograms-less than half the weight of a typical human female.

It is clear from Lucy's pelvis that she was an accomplished walker, just like us. Her brain, however, was no larger than a chimpanzee's. So being brainy was not the crucial factor that first distinguished us from the apes, it was upright walking. But what advantage did this give our ancestors over their four-legged contemporaries?
Peter Wheeler of the University of Liverpool has suggested that bipedalism evolved as an adaptation for searching for food in hot, equatorial latitudes. Most African mammals find it very hard to keep cool in the heat of the day because they expose a large surface area to the overhead sun. They overcome this by resting in the shade during the day and only hunting early in the morning and in the evening. The brain in particular is difficult to keep cool, and the larger the brain, the hotter it becomes.

2: Controversial theories of human evolution
FOSSILS and comparative physiology are open to widely differing interpretations. For example, more than 20 years ago, writer Elaine Morgan developed an earlier theory of the marine biologist Alistair Hardy. The so-called aquatic ape theory argues that upright walking, hair loss, the development of fatty tissue under the skin, sweating, and several other curious distinctions between humans and apes are adaptations to a shoreline life. Morgan's theories, which were originally part of an analysis of the evolution of women, remain a fascinating and bitterly disputed controversy.
In March 1997, Simon Easteal and Genevieve Herbert from the Australian National University in Canberra resurrected an earlier idea of the science journalists John Gribbin and Jeremy Cherfas that the African apes are actually descendants of the australopithecines, which were bipedal. Easteal and Herbert say that the genetic distance between humans and the African apes is so small that their divergence cannot be greater than 4 million years ago. If this is true, then chimps and gorillas would probably be descended from upright walkers, the australopithecines. Perhaps the chimps are descended from the slender forms, and gorillas from the robust species.
The recent discovery of a surprisingly modern looking ape from 22.6 million years ago in Africa, called Mororopithecus bishopi, has prompted some palaeontologists to speculate that a date of 6 million years ago for the divergence between apes and the human lineage may be too recent. In fact, the presently accepted date is based largely on biochemical evidence. In the 1960s, when fossil evidence was the only clue, the split was thought to be at least 10 million years ago, and possibly 15 million years ago.

Many mammals have complex chambers with moist linings in the nose and a heat exchange system to keep the blood cool as they pant to speed up evaporation. This was not an option for early hominids as they did not have a muzzle in which to house a cooling system. However, an upright posture would solve many of the problems, especially combined with a reduction in body hair. Upright walking means that less of the body surface is exposed directly to the sun at midday, while heat can be lost faster and any breezes are more likely to cause evaporation of sweat and so cool the body down. Retaining hair on the top of the head and perhaps the shoulders acts as a shield for the areas directly exposed to the sun.

An improved ability to control body temperature would mean that our ancestors could forage around midday, when there was less competition and fewer predators nearby. If this is correct, hair loss probably occurred relatively early in evolution, and is linked to bipedalism.
It was a very successful lifestyle. Unlike the other apes, the australopithecines could venture into the savannah from the wooded riverbank areas which were probably their preferred habitat. They spread as far south as South Africa, and they developed over the next 2 million years into a number of different types. Some gave rise to humans, but long after humans began to make tools, there were still australopithecines wandering the African savannah. Some species, the robust australopithecines, developed huge grinding teeth, and muscles for chewing so large that they had to be attached to crests of bone running front to back on their skulls as well as wide, flared cheek bones.

It seems unlikely that australopithecines were tool users. Some bones found in caves in South Africa have worn ends with microscopic markings similar to those produced in experiments by archaeologists. But these were probably made by early Homo species. Iris generally thought that the australopithecines never made the leap to the systematic production of tools as an essential method for obtaining food.
Around 2.3 million years ago, the earliest Homo species evolved, presumably from a slender australopithecine. H. habilis was the first stone toolmaker, and underwent the first of two major evolutionary spurts of growth in brain size.

The earliest stone tools take an expert eye to find. To most of us, they would look like any other naturally chipped stone. But microscopic analysis, many years of experimental work by archaeologists who have learnt how to knap stone tools, and detailed analysis of the way in which bones and tools are scattered around archaeological sites, show conclusively that they were deliberately fashioned.
The most common shapes of this very early culture, known as the Oldowan, include: "spheroids", which as their name suggests are spherical river pebbles which microscopic analysis reveals were used for hammering other stones or bones; choppers, which are stones with a few flakes knocked off at one end to give a sharp cutting edge; and the flakes themselves, which also had sharp edges.

Breaking bones Making tools

WHILE upright walking distinguished the hominids from their forebears, stone tool making is the hallmark of the genus Homo.
Johanson believes that early stone tools were used to extract bone marrow from scavenged carcasses abandoned by Africa's big predators, the lions, hyenas and leopards. This food cannot be used by other animals, and would also have provided the rich nourishment needed to sustain a relatively large brain.

Lucy: over 3 million years ago, she walked upright, stood about 1 metre tall, had an ape-like brain and possibly slept in the trees. She may even have been one of our ancestors.

Up to three early toolmaking species lived in Africa around this time, and one of these, known as H. ergaster, might be an early form of Homo erectus, the next major player in the story of human evolution.
The earliest fossils of this type are 1.8 million years old. H. erectus, or "upright human", was tall, though with a flattened head, but would probably not stand out too much in a crowd today. Their brains were larger again than their predecessors, at about 900 cubic centimetres for adults. Although the modern human brain averages around 1350 cubic centimetres, the H. erectus brain is the first to fall within the normal range for modern people.

The species H. erectus was a great traveller, and appears to be the first of our recent ancestors to venture beyond Africa. Fossils of this species, some of great age, have been found as far away from Africa as China and Java. But archaeologists often depict the stone-tool culture of H. erectus as one of monotonous regularity. Fairly soon after its first appearance,H. erectus began making Acheulian hand axes -and carried on making them for a million years. This multipurpose pear-shaped tool, named after a French village where the axes were found, had two cutting edges running to a point at one end and a rounded grip at the other end. The maker took a suitably shaped stone and, in a carefully planned sequence, knocked small flakes off it until it was the required shape. This was a significant advance over the Oldowan tools, many of which were probably discarded soon after being made. But the Acheulian hand axe required considerable skill to make and was almost certainly carried around, and occasionally resharpened, until it became unserviceable.

The similarity of these hand axes from sites throughout Africa, Europe and Central Asia, and the obvious intention of the makers to have very symmetrical shapes, suggests that the knapping skills would have needed more than simple mimicry to learn. Perhaps, then, these tools are evidence of the first lessons.
Does this mean that H. erectus could talk? The balance of opinion is that if there were the beginnings of language, they did not amount to much. Ann MacLarnon of the Roehampton Institute in Britain has studied the spinal column of H. erectus , and concludes that it probably had none of the fine control of the diaphragm that we need to propel air into the larynx in complex bursts as we speak. An absence of language, or at least its simplicity and lack of use in technical matters, might explain the very slow rate of change in the stone tool technology.

The pelvis of H. erectus is so small that the newborn brain would have been small, and the baby very dependent. But brain growth in early postnatal life must have been rapid, a characteristic which has been preserved in modern humans.
For about 500 000 years, African H. erectus shared the savannah with robust australopithecines, before they died out. But the rapid spread of people into Asia and Europe may have already begun. Some controversial dates from Java suggest H. erectus was there soon after it appears in Africa. But H. erectus had probably begun to migrate around million years ago.

The Flashing Blade
Blade technology: developed by Homo sapiens, this required two stages, preparing the core, and striking off the blades.

About 500 000 years ago, a new type of stone technique appears, called the "prepared core" technique. First, a cylindrical core was made by knocking flakes off the original lump of stone, much as the Acheuhan hand axe was formed. But then, longer, thinner blades were struck off the core, giving a variety of tools which were sharper and generally better fitted for the tasks they had to perform.
Though preparation of the stone took longer, the new method produced a dramatic improvement in the quality and quantity of the final tools. Archaeologists have uncovered ancient stone workplaces, known as knapping floors, where the tools were produced, and have been able to work out how they were made by fitting discarded flakes back together again.

A second burst of growth in brain size was taking place too, and fossils associated with the new stone technique are generally called archaic H. sapiens. The larger brain was probably an evolutionary advantage for social reasons as well as for technology. The ability to memorise social relationships and cooperate in foraging and hunting are important mental skills.
But the brain is energetically expensive. In modern humans it is only 2 per cent of an adult's body mass but demands about 20 per cent of the body's total energy requirement. Leslie Aiello of University College London and Wheeler have suggested that this high-energy uptake of the brain must have been compensated for by a reduction in energy demand elsewhere. Modern humans have smaller and less energy-demanding digestive tracts than other primates, made possible by a shift away from vegetable foods, and the use of cooking, which breaks down food prior to digestion and destroys toxins.

In China, by 500 000 years ago,H. erectus appears to have used fire in caves. Some limited use of fire may extend back much further, to one of the earliest Homo species. But the evidence is inconclusive. Perhaps the earlier Homo species could occasionally use fire taken from naturally caused bush fires to keep predators away at night. And maybe the use of fire for heating caves had been learned by H. erectus. But the earliest evidence of a hearth, surrounded by stones, comes from Brittany around 430 000 years ago.
Europe went through a series of very cold spells around this time, and the control of fire was probably essential for survival there. One group of humans, the Neanderthals, who are found from 150 000 to 35 000 years ago, became physically specialised for such conditions. They had large bodies, and even bigger brains than modern humans. They used a slightly more advanced prepared core technique, called the Mousterian. Their bones show that they were very muscular, died young and suffered regular injuries. There is some evidence that they cared for the sick, and may have buried their dead, though apparently without much ceremony.

While the Neanderthals were adapting to the cold in Europe, genuinely modern - looking humans were evolving in warmer climates. By 100 000 years ago, in the Middle East and the southernmost tip of Africa, some fossils show the domed crania of modern humans.
Around 50 000 years ago, these modern humans underwent what archaeologists have called a "cultural revolution". Unlike the Neanderthals, modern humans developed a more advanced blade technology and created compound tools, made of several interlocking parts of different materials. They constructed huts out of mammoth tusks, stones or wood. They produced cave paintings, buried their dead with grave ornaments, traded in shells, carved figurines, and made music.

The Neanderthal sites show none of this new cultural activity. But the two species lived side by side in Europe and the Middle East for many thousands of years. The superior technique and culture of the modern humans probably gave them an economic edge over the Neanderthals, who eventually died out.

Artistic inclinations Cultural revolution

THERE is some doubt as to whether modern speech and language arose at the time of the cultural revolution, and may even partly explain it, or whether language had developed earlier. Some development of language would certainly help to explain the cultural revolution.
These cultural humans probably came from Africa, and over the next 40 000 years they populated every part of the world, reaching the Americas some time after about 14 000 years ago and surviving in the most extreme environments.

Populations grew rapidly, and began to deplete traditional resources. Several game species became extinct, which was perhaps the spur for humans to take up herding and planting.
Agriculture appears to have emerged independently in both the Near East and Middle America, some time after 10 000 years ago. Humans increasingly abandoned their ancient nomadic way of life, settled, built long-lasting homes and cities and started to record their actions. This is where the prehistory of our extraordinary species is said to end, and its history begins.

Are we still evolving? There are not many physical differences between us and our ancestors of 50 000 years ago, and natural selection may not be as severe now thanks to modern hygiene and medicine. But selection is taking place, through differences in rates of population growth around the world and within populations.
If we look back over our prehistory, it tells us something of what made us human. In general we are clearly more dextrous, imaginative, talkative, cooperative, long - lived, hygienic, parental, adventurous and safety-conscious than our ape-like ancestors, who were similarly better at these things than the mouselike ancestral mammals. If we can improve at these characteristics, perhaps we can become even more human still.

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Further Reading

From Lucy to Language by Donald Johanson and Blake Edgar (Weidenfeld and Nicolson, 1996), has full-sized photos of major fossils, with text.
Human Evolution by Roger Lewin (Blackwell, 1989) is a general reader.

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Ann and Patrick Fullick are science educators and writers. All graphics are by Nigel Hawtin.





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