SCIENCE / Flesh on the bones of early man

LAST month's announcement of the discovery of Australopithecus ramidus, which some have called the 'missing link', was an intriguing clue to understanding the evolution of humans from our ape-like ancestors. More details of the complex story of human pre-history are emerging from other archaeological finds in the cradle of humanity - Africa. The aim eventually is to address the big questions in evolution: how, when and why did humans arise?

Since Darwin's day it has been assumed that there is a direct line of ascent from ape to early human to Homo sapiens, modern man. Homo sapiens is the direct descendant of Homo erectus, a hominid that lived almost two million years ago. His immediate ancestor was Homo habilis, the first known species in the Homo family and arguably the first tool user. Certain things are known about Homo erectus, far less about the far older Homo habilis. From scant material, anthropologists have to piece together the complex puzzle of human evolution. Where do they begin?

Brain capacity is one indicator of where the transitions from ape to human may have occurred. A significant aspect of recent human development is that infants today are born virtually helpless and experience prolonged childhood. As every parent knows, children later go through an adolescent growth spurt, during which they put on inches at an alarming rate. Humans are unique in this respect.

The helplessness of newborn human infants is, however, less the result of a cultural adaptation than a biological necessity. Human infants come into the world too early, a consequence of our large brain and the engin-eering constraints of the human pelvis.

Biologists have recently come to understand that brain size influences more than just intelligence. It affects a number of what are known as life-history factors, such as the age of weaning, the age at which sexual maturity is reached, gestation length and longevity. In species with large brains, these factors tend to be stretched out: infants are weaned later, gestation is longer, and individuals live longer.

A simple calculation, based on comparisons with other primates, reveals that gestation length in Homo sapiens, whose average adult brain capacity is 1,350 cubic centimetres, should be 21 months rather than nine. Human infants therefore have a year's growth to catch up on when born, hence their helplessness.

From an evolutionary point of view, we can say that in principle humans departed from the apelike growth pattern when the adult brain exceeded 770cc. Beyond this figure, brain size had to more than double from birth; thus began the pattern of helplessness in infants, who came into the world 'too early'.

Homo habilis, with an adult brain size of about 800cc, appears to be right on the cusp between the ape growth pattern and that of the human being, while the brain of early Homo erectus (some 900cc) pushes the species significantly in the human direction.

This, remember, is an argument 'in principle'; it assumes the birth canal in Homo erectus was the same size as it is in modern humans. In fact, we were able to get a clearer idea of how human Homo erectus had become in this respect from measurements of the pelvis of the skeletal remains known as the Turkana Boy, which I and others found west of Lake Turkana in northern Kenya in 1984.

By measuring the size of Turkana Boy's pelvic opening, we obtained a good estimate of the size of his mother's birth canal. (In humans, the pelvic opening is similar in size in males and females.) Alan Walker, my long-time friend and colleague, reconstructed the boy's pelvis from bones that had been separ-ate when we unearthed them. He measured the pelvic opening, found it was smaller than in Homo sapiens, and calculated that the new-borns of Homo erectus had brains of about 275cc - considerably smaller than the brain size of modern human newborns.

The implications are very clear. Homo erectus infants were born with brains one-third the adult size, as modern humans are, and must have come into the world in a helpless state, as modern humans do. We can infer that the intense parental care of infants which is part of the human social milieu had already begun to develop in Homo erectus, one million seven hundred thousand years ago.

We cannot do similar calculations for Homo habilis, the immediate ancestor of erectus, because we have yet to discover a single habilis pelvis. But if habilis babies were born with erectus-size brains, they too would have been born 'too early', but not by as much; would have been helpless at birth, but not for as long; and would have required a human-like social environment, but to a lesser degree.

It therefore seems that Homo had moved in a human direction from the very beginning. Similarly, the australopithecine species had ape-size brains, and so would have followed an apelike pattern of early development. Australopithecines, or 'southern apes', lived between four and one million years ago and are much older than the Homo group of hominids to which modern humans belong. It is thought that early Homo species evolved from Australopithecine ancestors.

An extended period of helplessness in infancy, a period during which intensive parental care was required, was characteristic of early Homo species. But what of the remainder of childhood? When did this become prolonged, enabling practical and cultural skills to be absorbed, followed by an adolescent growth spurt?

The prolongation of childhood in modern humans is achieved through a rate of physical growth that is reduced compared with apes. As a result, humans reach various growth 'landmarks,' such as tooth eruption, later than apes. For instance, the first permanent molar appears in human children at about the age of six, compared with three in apes; the second molar erupts between 11 and 12 in humans and at seven in apes; the third molar shows up at 18 to 20 in humans and nine in apes. In order to answer the question of when childhood became prolonged in human prehistory, we needed a way of looking at fossil jaws and determining when the molars erupted.

The Turkana Boy died just as his second molar was beginning to show. If Homo erectus followed the slower, human pattern of childhood development, this would mean the boy died when he was 11 years old. If, however, the species had an ape-like growth trajectory, he would have been seven.

In the late 1980s, Holly Smith, at the University of Michigan, developed a way of deducing life-history patterns in fossil humans by correlating brain size and the age of eruption of the first molar. As a baseline, Smith amassed data for humans and apes; she then looked at a range of human fossils to determine how they compared. Three life-history patterns emerged: a modern human grade, in which first-molar eruption occurs at six and life span is 66; an ape grade, with first-molar eruption at just over three and a lifespan of 40 years; and an intermediate grade.

Later Homo erectus - that is individuals who lived after 800,000 years ago - fit the human grade, as did Neanderthals. All the much older and more ape-like australopithecine species, however, slotted into the ape grade. Early Homo erectus, like Turkana Boy, was intermediate: the boy's first molar would have erupted when he was a little more than four years old; had he not met an early death, he could have expected to live about 52 years.

Smith's work showed that the australo-pithecines' pattern of growth was not like that of modern humans; it was apelike. She further showed that early Homo erectus was intermediate between modern human and ape in its growth; we now conclude that the Turkana Boy was about nine when he died and not 11.

The ability to infer biology from fossils through research in life-history factors and tooth development is enormously important to anthropology; it allows us metaphorically to put flesh on the bones. For instance, we can say that the Turkana Boy would have been weaned just before his fourth birthday and, had he lived, would have become sexually mature at about 14.

His mother probably had her first baby at 13, after a nine-month pregnancy, and thereafter would have been pregnant every three to four years. These patterns tell us that by the time of early Homo erectus, human ancestors had already moved in the direction of modern human biology while the australo-pithecines remained in their ape grade.

SO MUCH for the biology of Homo erectus. What about its behaviour and social organis-ation? We can glean something about this from the tools these early hominids made. A crucial question, for instance, is whether they were hunters, requiring a sophisticated social structure, or merely scavengers.

In the late 1970s, my friend and colleague Glynn Isaac addressed this contentious question by embarking on a three-year excavation in Kenya, on the shore of Lake Turkana, at a place known as Site 50. When a small group of Homo erectus visited this spot 1.5 million years ago, they chose a sandy, tree-shaded bank on a curve in a seasonal river. In the riverbed was a plentiful supply of lava cobbles from which to make flake tools. 'For some short period of time, these protohumans used this riverbank, and they left behind them what at first sight looks like an unpromising jumble of bones and stones,' Isaac wrote.

Some of the bones at Site 50 bore the tell-tale marks of ancient butchery - the cut marks left by the sharp edges of stone flakes - and some bones had been broken open with a stone hammer, presumably to give access to the nut-ritious marrow inside. Isaac could therefore reasonably conclude that the association between stones and bones was meaningful, not some accident of nature.

The bone pieces - parts of giraffe, hippo-potamus, antelope and a bit of catfish cartilage - were not strewn randomly over the ancient site but were concentrated in the north-west corner. Significantly, stone tools were collected here, too, and the picture one got was of an area of activity: individuals making tools and using them to but-cher pieces of carcass that they or others had brought to the site. The earliest tools were small flakes, produced by striking one stone against another. They measured about one inch long and were surprisingly sharp. Though simple in appearance, they were put to a variety of uses, as we know from miscroscopic analysis carried out by Lawrence Keeley of the Unversity of Illinois, and Nicholas Toth of Indiana University. They found marks on the flakes indicating that some had been used to cut meat, some to cut wood, and others to cut soft plant material, like grass.

When we find a scattering of flakes like this, we have to be inventive to imagine the complexity of life that took place there long ago because the relics themselves are sparse. Gone is the meat, the wood, and the grass. We can imagine a simple riverbank campsite, where a human family group butchered meat in the shade of a structure made from saplings and thatched with reeds, even though all we see today are the stone flakes.

The flakes designed to cut through meat were highly effective implements, capable of cutting through all but the toughest of hides to expose the flesh inside. Whether they were hunters or scavengers, the humans who made and used these meat-cutting flakes thereby made available to themselves a new energy source - animal protein, which, in turn, enabled them to extend their foraging range.

Access to meat also improved their chances of successfully producing offspring. The reproductive process is an expensive business, and the expansion of the diet to include meat would have made it more secure. Make no mistake: the discovery of a way to produce consistently sharp stone flakes capable of cutting hides was a major breakthrough in human prehistory.

I cannot say that the results of the project at Site 50 confirm the hypothesis that Homo erectus lived as hunter-gatherers, moving every few days from one temporary home base to another. But there is sufficient evidence to dispense with the notion that early Homo was little advanced beyond the chimpanzee grade of social, cognitive and technological competence.

Glynn Isaac is more cautious still. 'My guess now is that, in various ways, the behaviour system was less human than I originally envisaged,' he notes. 'It is my strong suspicion that if we had these hominids alive today, we would put them in zoos, not in academies.'

Perhaps not in academies, I agree - but my belief is that the behaviour system of a group of Homo erectus would be far more complex and human-like than any behaviour we see in zoos today. We would recognise these hominids for what they are - our ancestors.

WHAT does all this teach us about the ascent of humanity? Were the Homo erectus meat-eaters at Site 50 the earliest toolmakers, or were primitive implements also used by earlier Australopithecus species? Toolmaking may be another key factor that marks the transition from ape to human.

Chimpanzees are adept tool users. They use sticks to harvest termites, leaves as sponges, stones to crack nuts. But so far no chimpanzee in the wild has been seen to make a stone-flake tool. Despite their appearance, these are surprisingly difficult to make. Stone tools are simple in structure, but considerable skill is required to make their edges consistently sharp.

I have seen naive people try to make 'Stone Age' tools by bashing two rocks together. That is not how it was done. To work efficiently, the stone knapper has to choose a rock of the correct shape, bearing the correct angle at which to strike; the striking motion itself requires great practice. Nicholas Toth spent years perfecting techniques for making stone tools, and has a good appreciation of the skills of the first toolmakers. 'There's no question that the earliest toolmakers possessed a mental capacity beyond that of apes,' he says. 'Toolmaking requires a co-ordination of significant motor and cognitive skills.'

Mary Leakey, my mother, spent many years at Olduvai Gorge studying this earliest of technologies, and in so doing established early African archaeology, which is nowadays termed the Oldowan industry, after Olduvai Gorge.

Toth suspects the earliest toolmakers didn't have the specific shapes of individual tools in minds when making them. More likely, the various shapes were determined by the original shape of the raw material. This technology, which lasted until about 1.6 million years ago, was essentially opportunistic in nature.

What led to its downfall was the appearance in Africa of a new form of assemblage which archaeologists call the Acheulean. For the first time in human prehistory, there is evi-dence that the toolmakers had a mental template of what they wanted to produce, that they were intentionally imposing a shape on the raw material they used. The find that suggests this is the so-called hand axe, a teardrop-shaped implement that must have required considerable skill and patience to make. It took Toth and others several months to acquire the skill to produce hand axes of the quality found in the archaeological record of that time.

The appearance of the hand axe coincides with the emergence of Homo erectus, the puta-tive descendent of Homo habilis and ancestor of Homo sapiens. Although species of australopithecines existed at this time as well, it is reasonable to conclude that Homo erectus individuals, endowed as they were with significantly larger brains than Homo habilis, were the makers of the hand axe.

Does this suggest the earliest toolmakers were also Homo rather than Australopithecus species? The prehistoric record is not clear- cut. We know from the prehistoric record that one million years ago only Homo species exis-ted, and we also know they made stone tools. Until there is good reason to suppose otherwise, it seems cautiously wise to conclude that only Homo made tools earlier in prehistory. The australopithecine species and Homo clearly had different specific adaptations, and it is likely that meat-eating by Homo was an impor-tant part of that difference. Stone tool-making would have been an important part of a meat-eater's abilities; a plant eater could do without these tools.

In his studies of tools from archaeological sites in Kenya, and in his experimental toolmaking exercises, Toth made yet another fascinating and important discovery: the earliest toolmakers were predominantly right-handed, just as modern humans are. Although individual apes are preferentially right-or left-handed, there is no population preference; modern humans are unique in this respect. Toth's discovery gives us an important evolutionary insight: two million years ago, the brain of Homo was already becoming human.

Richard Leakey. Taken from 'The Origin of Humankind', published in the 'Science Masters' series by Weidenfeld and Nicolson at pounds 9.99