These were the australopithecines, and for the next five million years they would be the world’s dominant hominid species. (Australis from the Latin for “southern” and has no connection in this context to Australia.) Australopithecines came in several varieties, some slender and gracile, like Raymond Dart’s Taung child, others more sturdy and robust, but all were capable of walking upright. Some of these species existed for well over a million years, others for a more modest few hundred thousand, but it is worth bearing in mind that even the least successful had histories many times longer than we have yet achieved.

The most famous hominid remains in the world are those of a 3.18-million-year-old australopithecine found at Hadar in Ethiopia in 1974 by a team led by Donald Johanson. Formally known as A.L. (for “Afar Locality”) 288–1, the skeleton became more familiarly known as Lucy, after the Beatles song “Lucy in the Sky with Diamonds.” Johanson has never doubted her importance. “She is our earliest ancestor, the missing link between ape and human,” he has said.

Lucy was tiny—just three and a half feet tall. She could walk, though how well is a matter of some dispute. She was evidently a good climber, too. Much else is unknown. Her skull was almost entirely missing, so little could be said with confidence about her brain size, though skull fragments suggested it was small. Most books describe Lucy’s skeleton as being 40 percent complete, though some put it closer to half, and one produced by the American Museum of Natural History describes Lucy as two-thirds complete. The BBC television seriesApe Man actually called it “a complete skeleton,” even while showing that it was anything but.

A human body has 206 bones, but many of these are repeated. If you have the left femur from a specimen, you don’t need the right to know its dimensions. Strip out all the redundant bones, and the total you are left with is 120—what is called a half skeleton. Even by this fairly accommodating standard, and even counting the slightest fragment as a full bone, Lucy constituted only 28 percent of a half skeleton (and only about 20 percent of a full one).

InThe Wisdom of the Bones , Alan Walker recounts how he once asked Johanson how he had come up with a figure of 40 percent. Johanson breezily replied that he had discounted the 106 bones of the hands and feet—more than half the body’s total, and a fairly important half, too, one would have thought, since Lucy’s principal defining attribute was the use of those hands and feet to deal with a changing world. At all events, rather less is known about Lucy than is generally supposed. It isn’t even actually known that she was a female. Her sex is merely presumed from her diminutive size.

Two years after Lucy’s discovery, at Laetoli in Tanzania Mary Leakey found footprints left by two individuals from—it is thought—the same family of hominids. The prints had been made when two australopithecines had walked through muddy ash following a volcanic eruption. The ash had later hardened, preserving the impressions of their feet for a distance of over twenty-three meters.

The American Museum of Natural History in New York has an absorbing diorama that records the moment of their passing. It depicts life-sized re-creations of a male and a female walking side by side across the ancient African plain. They are hairy and chimplike in dimensions, but have a bearing and gait that suggest humanness. The most striking feature of the display is that the male holds his left arm protectively around the female’s shoulder. It is a tender and affecting gesture, suggestive of close bonding.

The tableau is done with such conviction that it is easy to overlook the consideration that virtually everything above the footprints is imaginary. Almost every external aspect of the two figures—degree of hairiness, facial appendages (whether they had human noses or chimp noses), expressions, skin color, size and shape of the female’s breasts—is necessarily suppositional. We can’t even say that they were a couple. The female figure may in fact have been a child. Nor can we be certain that they were australopithecines. They are assumed to be australopithecines because there are no other known candidates.

I had been told that they were posed like that because during the building of the diorama the female figure kept toppling over, but Ian Tattersall insists with a laugh that the story is untrue. “Obviously we don’t know whether the male had his arm around the female or not, but we do know from the stride measurements that they were walking side by side and close together—close enough to be touching. It was quite an exposed area, so they were probably feeling vulnerable. That’s why we tried to give them slightly worried expressions.”

I asked him if he was troubled about the amount of license that was taken in reconstructing the figures. “It’s always a problem in making re-creations,” he agreed readily enough. “You wouldn’t believe how much discussion can go into deciding details like whether Neandertals had eyebrows or not. It was just the same for the Laetoli figures. We simply can’t know the details of what they looked like, but wecan convey their size and posture and make some reasonable assumptions about their probable appearance. If I had it to do again, I think I might have made them just slightly more apelike and less human. These creatures weren’t humans. They were bipedal apes.”

Until very recently it was assumed that we were descended from Lucy and the Laetoli creatures, but now many authorities aren’t so sure. Although certain physical features (the teeth, for instance) suggest a possible link between us, other parts of the australopithecine anatomy are more troubling. In their bookExtinct Humans , Tattersall and Schwartz point out that the upper portion of the human femur is very like that of the apes but not of the australopithecines; so if Lucy is in a direct line between apes and modern humans, it means we must have adopted an australopithecine femur for a million years or so, then gone back to an ape femur when we moved on to the next phase of our development. They believe, in fact, that not only was Lucy not our ancestor, she wasn’t even much of a walker.

“Lucy and her kind did not locomote in anything like the modern human fashion,” insists Tattersall. “Only when these hominids had to travel between arboreal habitats would they find themselves walking bipedally, ‘forced’ to do so by their own anatomies.” Johanson doesn’t accept this. “Lucy’s hips and the muscular arrangement of her pelvis,” he has written, “would have made it as hard for her to climb trees as it is for modern humans.”

Matters grew murkier still in 2001 and 2002 when four exceptional new specimens were found. One, discovered by Meave Leakey of the famous fossil-hunting family at Lake Turkana in Kenya and calledKenyanthropus platyops (“Kenyan flat-face”), is from about the same time as Lucy and raises the possibility that it was our ancestor and Lucy was an unsuccessful side branch. Also found in 2001 wereArdipithecus ramidus kadabba , dated at between 5.2 million and 5.8 million years old, andOrrorin tugenensis , thought to be 6 million years old, making it the oldest hominid yet found—but only for a brief while. In the summer of 2002 a French team working in the Djurab Desert of Chad (an area that had never before yielded ancient bones) found a hominid almost 7 million years old, which they labeledSahelanthropus tchadensis . (Some critics believe that it was not human, but an early ape and therefore should be calledSahelpithecus .) All these were early creatures and quite primitive but they walked upright, and they were doing so far earlier than previously thought.

Bipedalism is a demanding and risky strategy. It means refashioning the pelvis into a full load-bearing instrument. To preserve the required strength, the birth canal must be comparatively narrow. This has two very significant immediate consequences and one longer-term one. First, it means a lot of pain for any birthing mother and a greatly increased danger of fatality to mother and baby both. Moreover to get the baby’s head through such a tight space it must be born while its brain is still small—and while the baby, therefore, is still helpless. This means long-term infant care, which in turn implies solid male–female bonding.

All this is problematic enough when you are the intellectual master of the planet, but when you are a small, vulnerable australopithecine, with a brain about the size of an orange,48the risk must have been enormous.

So why did Lucy and her kind come down from the trees and out of the forests? Probably they had no choice. The slow rise of the Isthmus of Panama had cut the flow of waters from the Pacific into the Atlantic, diverting warming currents away from the Arctic and leading to the onset of an exceedingly sharp ice age in northern latitudes. In Africa, this would have produced seasonal drying and cooling, gradually turning jungle into savanna. “It was not so much that Lucy and her like left the forests,” John Gribbin has written, “but that the forests left them.”

But stepping out onto the open savanna also clearly left the early hominids much more exposed. An upright hominid could see better, but could also be seen better. Even now as a species, we are almost preposterously vulnerable in the wild. Nearly every large animal you can care to name is stronger, faster, and toothier than us. Faced with attack, modern humans have only two advantages. We have a good brain, with which we can devise strategies, and we have hands with which we can fling or brandish hurtful objects. We are the only creature that can harm at a distance. We can thus afford to be physically vulnerable.

All the elements would appear to have been in place for the rapid evolution of a potent brain, and yet that seems not to have happened. For over three million years, Lucy and her fellow australopithecines scarcely changed at all. Their brain didn’t grow and there is no sign that they used even the simplest tools. What is stranger still is that we now know that for about a million years they lived alongside other early hominids who did use tools, yet the australopithecines never took advantage of this useful technology that was all around them.

At one point between three and two million years ago, it appears there may have been as many as six hominid types coexisting in Africa. Only one, however, was fated to last:Homo , which emerged from the mists beginning about two million years ago. No one knows quite what the relationship was between australopithecines andHomo, but what is known is that they coexisted for something over a million years before all the australopithecines, robust and gracile alike, vanished mysteriously, and possibly abruptly, over a million years ago. No one knows why they disappeared. “Perhaps,” suggests Matt Ridley, “we ate them.”

Conventionally, theHomo line begins withHomo habilis , a creature about whom we know almost nothing, and concludes with us,Homo sapiens (literally “man the thinker”). In between, and depending on which opinions you value, there have been half a dozen otherHomo species:Homo ergaster, Homo neanderthalensis, Homo rudolfensis, Homo heidelbergensis, Homo erectus , andHomo antecessor .

Homo habilis(“handy man”) was named by Louis Leakey and colleagues in 1964 and was so called because it was the first hominid to use tools, albeit very simple ones. It was a fairly primitive creature, much more chimpanzee than human, but its brain was about 50 percent larger than that of Lucy in gross terms and not much less large proportionally, so it was the Einstein of its day. No persuasive reason has ever been adduced for why hominid brains suddenly began to grow two million years ago. For a long time it was assumed that big brains and upright walking were directly related—that the movement out of the forests necessitated cunning new strategies that fed off of or promoted braininess—so it was something of a surprise, after the repeated discoveries of so many bipedal dullards, to realize that there was no apparent connection between them at all.

“There is simply no compelling reason we know of to explain why human brains got large,” says Tattersall. Huge brains are demanding organs: they make up only 2 percent of the body’s mass, but devour 20 percent of its energy. They are also comparatively picky in what they use as fuel. If you never ate another morsel of fat, your brain would not complain because it won’t touch the stuff. It wants glucose instead, and lots of it, even if it means short-changing other organs. As Guy Brown notes: “The body is in constant danger of being depleted by a greedy brain, but cannot afford to let the brain go hungry as that would rapidly lead to death.” A big brain needs more food and more food means increased risk.

Tattersall thinks the rise of a big brain may simply have been an evolutionary accident. He believes with Stephen Jay Gould that if you replayed the tape of life—even if you ran it back only a relatively short way to the dawn of hominids—the chances are “quite unlikely” that modern humans or anything like them would be here now.

“One of the hardest ideas for humans to accept,” he says, “is that we are not the culmination of anything. There is nothing inevitable about our being here. It is part of our vanity as humans that we tend to think of evolution as a process that, in effect, was programmed to produce us. Even anthropologists tended to think this way right up until the 1970s.” Indeed, as recently as 1991, in the popular textbookThe Stages of Evolution , C. Loring Brace stuck doggedly to the linear concept, acknowledging just one evolutionary dead end, the robust australopithecines. Everything else represented a straightforward progression—each species of hominid carrying the baton of development so far, then handing it on to a younger, fresher runner. Now, however, it seems certain that many of these early forms followed side trails that didn’t come to anything.

Luckily for us, one did—a group of tool users, which seemed to arise from out of nowhere and overlapped with the shadowy and much disputedHomo habilis . This isHomo erectus , the species discovered by Eugène Dubois in Java in 1891. Depending on which sources you consult, it existed from about 1.8 million years ago to possibly as recently as twenty thousand or so years ago.

According to theJava Man authors,Homo erectus is the dividing line: everything that came before him was apelike in character; everything that came after was humanlike.Homo erectus was the first to hunt, the first to use fire, the first to fashion complex tools, the first to leave evidence of campsites, the first to look after the weak and frail. Compared with all that had gone before,Homo erectus was extremely human in form as well as behavior, its members long-limbed and lean, very strong (much stronger than modern humans), and with the drive and intelligence to spread successfully over huge areas. To other hominids,Homo erectus must have seemed terrifyingly powerful, fleet, and gifted.

Erectuswas “the velociraptor of its day,” according to Alan Walker of Penn State University and one of the world’s leading authorities. If you were to look one in the eyes, it might appear superficially to be human, but “you wouldn’t connect. You’d be prey.” According to Walker, it had the body of an adult human but the brain of a baby.

Althougherectus had been known about for almost a century it was known only from scattered fragments—not enough to come even close to making one full skeleton. So it wasn’t until an extraordinary discovery in Africa in the 1980s that its importance—or, at the very least, possible importance—as a precursor species for modern humans was fully appreciated. The remote valley of Lake Turkana (formerly Lake Rudolf) in Kenya is now one of the world’s most productive sites for early human remains, but for a very long time no one had thought to look there. It was only because Richard Leakey was on a flight that was diverted over the valley that he realized it might be more promising than had been thought. A team was dispatched to investigate, but at first found nothing. Then late one afternoon Kamoya Kimeu, Leakey’s most renowned fossil hunter, found a small piece of hominid brow on a hill well away from the lake. Such a site was unlikely to yield much, but they dug anyway out of respect for Kimeu’s instincts and to their astonishment found a nearly completeHomo erectus skeleton. It was from a boy aged between about nine and twelve who had died 1.54 million years ago. The skeleton had “an entirely modern body structure,” says Tattersall, in a way that was without precedent. The Turkana boy was “very emphatically one of us.”

Also found at Lake Turkana by Kimeu was KNM-ER 1808, a female 1.7 million years old, which gave scientists their first clue thatHomo erectus was more interesting and complex than previously thought. The woman’s bones were deformed and covered in coarse growths, the result of an agonizing condition called hypervitaminosis A, which can come only from eating the liver of a carnivore. This told us first of all thatHomo erectus was eating meat. Even more surprising was that the amount of growth showed that she had lived weeks or even months with the disease. Someone had looked after her. It was the first sign of tenderness in hominid evolution.

It was also discovered thatHomo erectus skulls contained (or, in the view of some, possibly contained) a Broca’s area, a region of the frontal lobe of the brain associated with speech. Chimps don’t have such a feature. Alan Walker thinks the spinal canal didn’t have the size and complexity to enable speech, that they probably would have communicated about as well as modern chimps. Others, notably Richard Leakey, are convinced they could speak.

For a time, it appears,Homo erectus was the only hominid species on Earth. It was hugely adventurous and spread across the globe with what seems to have been breathtaking rapidity. The fossil evidence, if taken literally, suggests that some members of the species reached Java at about the same time as, or even slightly before, they left Africa. This has led some hopeful scientists to suggest that perhaps modern people arose not in Africa at all, but in Asia—which would be remarkable, not to say miraculous, as no possible precursor species have ever been found anywhere outside Africa. The Asian hominids would have had to appear, as it were, spontaneously. And anyway an Asian beginning would merely reverse the problem of their spread; you would still have to explain how the Java people then got to Africa so quickly.

There are several more plausible alternative explanations for howHomo erectus managed to turn up in Asia so soon after its first appearance in Africa. First, a lot of plus-or-minusing goes into the dating of early human remains. If the actual age of the African bones is at the higher end of the range of estimates or the Javan ones at the lower end, or both, then there is plenty of time for African erects to find their way to Asia. It is also entirely possible that oldererectus bones await discovery in Africa. In addition, the Javan dates could be wrong altogether.

Now for the doubts. Some authorities don’t believe that the Turkana finds areHomo erectus at all. The snag, ironically, was that although the Turkana skeletons were admirably extensive, all othererectusfossils are inconclusively fragmentary. As Tattersall and Jeffrey Schwartz note inExtinct Humans , most of the Turkana skeleton “couldn’t be compared with anything else closely related to it because the comparable parts weren’t known!” The Turkana skeletons, they say, look nothing like any AsianHomo erectus and would never have been considered the same species except that they were contemporaries. Some authorities insist on calling the Turkana specimens (and any others from the same period)Homo ergaster . Tattersall and Schwartz don’t believe that goes nearly far enough. They believe it wasergaster“or a reasonably close relative” that spread to Asia from Africa, evolved intoHomo erectus,and then died out.

What is certain is that sometime well over a million years ago, some new, comparatively modern, upright beings left Africa and boldly spread out across much of the globe. They possibly did so quite rapidly, increasing their range by as much as twenty-five miles a year on average, all while dealing with mountain ranges, rivers, deserts, and other impediments and adapting to differences in climate and food sources. A particular mystery is how they passed along the west side of the Red Sea, an area of famously punishing aridity now, but even drier in the past. It is a curious irony that the conditions that prompted them to leave Africa would have made it much more difficult to do so. Yet somehow they managed to find their way around every barrier and to thrive in the lands beyond.

And that, I’m afraid, is where all agreement ends. What happened next in the history of human development is a matter of long and rancorous debate, as we shall see in the next chapter.

But it is worth remembering, before we move on, that all of these evolutionary jostlings over five million years, from distant, puzzled australopithecine to fully modern human, produced a creature that is still 98.4 percent genetically indistinguishable from the modern chimpanzee. There is more difference between a zebra and a horse, or between a dolphin and a porpoise, than there is between you and the furry creatures your distant ancestors left behind when they set out to take over the world.