Darwin’s African Guess

As Darwin was putting together his theory of natural selection, he couldn’t help but wonder how humans had come to be. There were no million‑year‑old hand axes yet known for him to examine; in fact, before the late 1850s there were no recognized fossils of ancient humans whatsoever. He sometimes jotted his thoughts in a notebook, but he did not dare make them public. In 1857, two years before Darwin published Origin of Species, Wallace asked him in a letter if he would discuss the origin of mankind in the book. Darwin replied, “I think I shall avoid the whole subject, as so surrounded with prejudices, though I fully admit that it is the highest and most interesting problem for the naturalist.”

His silence was purely strategic. Humans must have evolved, like any other animal. But Darwin didn’t delve into that ramification of his theory, hoping that he could get a fair hearing. Yet as cautious as Darwin was in writing Origin of Species, many of his readers immediately wondered where humans fit into his theory. Making the question all the more pressing, explorers were returning from the jungles of Africa at the time with chimpanzees and gorillas. Huxley and other biologists examined them and showed that they were even more like humans than orangutans were. In 1860, Darwin wrote to Wallace to say that he had changed his mind: he would write an essay on man.

It would take Darwin 11 years to finish it. In the interim he was bogged down by new editions of Origin of Species and his book on orchids; a book on the domestication of animals and plants exploded into a two‑volume monster; he fell sick for months at a stretch. But through all those distractions, the pressure to speak about human evolution only grew. How could natural selection spontaneously produce human beings in all their wonder, with their ability to speak and reason, to love and explore? Even Wallace gave up. He decided that our oversized brains were far more powerful than necessary–we could easily survive with minds slightly more advanced than an ape’s. The creation of humans must, he concluded, be the work of divine intervention.

Darwin did not agree, and in 1871 he finally set forth his argument for human evolution in The Descent of Man and Selection in Relation to Sex. It was a hodgepodge of a book. Darwin used a few chapters to introduce readers to the theory of sexual selection, which he thought was responsible for the differences between human races. (Even Darwin came up with a few clunkers.) In a book that was supposed to be the story of human evolution, Darwin spent hundreds of pages detailing how sexual selection might work on other animals. But he also managed to include evidence suggesting that humans had evolved into their current form from apelike ancestors.

By the time Darwin got around to writing The Descent of Man, only a few hints of our antiquity had emerged, and they were ambiguous ones at that. In 1856 a miner in the Neander Valley in Germany unearthed pieces of a skeleton, which was dubbed Neanderthal Man. Its brow was massive and low, which raised the question of whether it was a separate species or–as Huxley claimed–was at one extreme of human variation. Other scientists had found not fossils but tools–flints and stone scrapers–alongside the fossils of extinct hyenas in England and France. They spoke of humanity’s great antiquity but could say little more.

Because the fossils and tools shed so little light on human evolution, Darwin instead compared humans to great apes. Bone for bone, they are almost identical. As human embryos develop, they pass through virtually identical stages as gorillas or chimps. Only relatively late in their development do they start to diverge, taking on different proportions. These similarities, Darwin argued, were signs that apes and humans descended from some ancient common ancestor. After our ancestors diverged to a branch of their own, they gradually evolved all the traits that make us uniquely human. Since humans are so similar to gorillas and chimpanzees, and gorillas and chimpanzees both live in Africa, Darwin made a guess as to the land of our origins: “It is somewhat more probable that our early progenitors lived on the African continent than elsewhere.”

In 1871 Darwin’s readers may have thought he was firing a scientific shot in the dark. But 130 years later he has been vindicated by a wealth of evidence. Researchers now know that the similarity between the genes of humans and African apes is just as striking as their anatomy. In 1999 an international group of scientists offered an evolutionary tree of humans, based on the most extensive study of our genes to date. Humanity forms a little tuft nestled alongside the chimpanzee lineages. Their tree demonstrates that, genetically speaking, we are practically a subspecies of chimp.

By gauging the rate at which our genes mutate, scientists estimate that the last common ancestor of chimpanzees and humans lived 5 million years ago. Since Darwin’s day, paleoanthropologists have discovered many fossils of ancient humans, as well as a dozen other human‑like species (known as hominids). These fossils show that human evolution was marked by five great transitions. The first, which began about 5 million years ago, gradually pushed our ancestors out onto the African savannas. The second saw the invention of the first stone tools about 2.5 million years ago, and the third came a million years later, as crude blades were transformed into massive hand axes. Half a million years ago, our ancestors went through a fourth transition, mastering fire and becoming more adept at making spears and other tools. And finally, 50,000 years ago, humans began leaving behind signs of truly modern minds–paintings on cave walls, carved jewelry, intricate weapons, and elaborate burials.

The oldest and most chimplike fossil of a hominid was discovered in the early 1990s by a team of scientists working in Ethiopia. There they unearthed a collection of teeth, bits of a skull, and some arm bones dating back 4.4 million years. The fossils were apelike but had some features that were more like humans than chimps. When its mouth closed, some of the skull’s upper and lower teeth fit together in a human‑like way. Its spine contacted the bottom of its skull, as our own spine does. (In chimpanzees and other apes, the point of contact is closer to the back of the head.) But at the same time, the Ethiopian creature had some distinctly chimpish traits. It had massive canine teeth, as chimps do, covered by only a thin layer of tooth enamel. It wouldn’t have been able to eat much meat or tough plants; it presumably ate only soft fruits and tender leaves, as chimps do today.

We have met this kind of strange mixing of traits before–in the walking whales, the fish with legs and toes, the invertebrates with glimmerings of the vertebrate brain. This Ethiopian creature, known as Ardipithecus ramidus, is not a missing link between man and chimp, but it lies on a branch close to the split between our ancestors and theirs.

While A. ramidus remains the oldest known hominid, other scientists have found fossils of several other hominid species dating back well over 3 million years, all of them in East Africa. On the shores of Lake Turkana in Kenya, paleoanthropologist Maeve Leakey discovered a 4.2‑million‑year‑old hominid, which she dubbed Australopithecus anamensis. In Ethiopia, Kenya, and Tanzania, several teams of scientists have dug up a species called A. afarensis, which endured from about 3.9 to 3 million years ago. (This is the most famous early hominid, including in its ranks a specimen named Lucy, the nearly complete skeleton of a female A. afarensis discovered by Donald Johanson.) Other fragments of ancient hominids have also been found in the same region of East Africa, and may turn out to be species in their own right.

These early hominids lived during tumultuous times. A cooling global climate was turning the wide carpet of jungles across sub‑Saharan Africa into a ragged quilt of patchy forests and open woodlands. Chimpanzees and hominids appear to have adapted to the change in very different ways. The chimps clung to the dense forests that survived the climate change in central and western Africa. The hominids, meanwhile, adapted to the more open habitats of East Africa.

As the climate cooled, the bodies of our ancestors changed. Their toes became less like fingers. Their legs grew longer. They held their heads and backs more upright. Kevin Hunt of Indiana University has proposed that these changes occurred as hominids shifted to a new sort of diet. Earlier hominids may have climbed into trees in their jungle home to find food, much as chimps do today. But as their forest habitat became less dense, Hunt proposes that our ancestors began to gather the fruit that hung from low trees. By standing on two legs, an early hominid could brace itself by holding on to a branch with one arm as it grabbed fruit with the other. These changes in diet also altered the way hominids walked. The first hominids probably walked slowly on all fours, using the knuckles of their hands to support their weight as chimps still do. But with longer legs, hominids began to move around bipedally without the help of knuckles.

Walking upright was one of the biggest changes that our ancestors underwent, but the first bipedal hominids could hardly stroll as we do today. An average human walking at a comfortable pace travels about 3 miles per hour. With their short legs, early hominids would have had to run to match that speed. They were forced to walk more slowly, and as a result they covered only a short distance each day. Early hominids may have walked only from one tree to the next, sometimes picking the lowest fruit while standing on the ground, and sometimes climbing into the trees using their long arms and curved fingers to grip the branches. (They probably also scrambled off the ground from time to time to get away from saber‑toothed cats and other predators.)

As one millennium followed another, hominids spread out across a broader range. New hominid species emerged, leaving their fossils as far north as Chad and as far south as South Africa. And by 2.5 million years ago, they were leaving behind something altogether new in the fossil record: stone tools.

Hominids made tools by banging rocks together to chip off their edges. In the process, they created simple blades, which they could use to chop or scrape. Hominids are not the only apes who make and use tools. Orangutans will strip off branches to probe for honey or termites inside trees. Chimps are even more versatile: they can use sticks as probes; they can also place nuts on a rock and smash them with a second rock, like a blacksmith pounding on an anvil. They can use leaves like sponges to soak up water, or as umbrellas in the rain, or as a dry seat on mud. But the tools that hominids invented 2.5 million years ago were beyond the abilities of their ape relatives.

The limits of chimps were put on display by Nicholas Toth of Indiana University in the early 1990s. He tried training a clever captive bonobo named Kanzi to make stone tools. For months Kanzi banged rocks together, but in the end he made no progress. Part of his trouble was that his thumbs didn’t have the range of motion of humans and some other hominids. As a result he couldn’t deliver the precise blows required to shape a rock. But just as importantly, his brain couldn’t master all the variables involved in chipping a stone with another stone–how much force to use, where to hit one stone against the other, and so on. By 2.5 million years ago, however, our ancestors had figured all this out.

As with bipedality, toolmaking may have had its origins in climate changes. Between 2 and 3 million years ago, East and South Africa turned much drier than in the past, and grasslands replaced many of the old woodlands. Hominids evolved an even more upright stance, which may have served as an adaptation for surviving in hot open habitats. The tropical rays of the sun couldn’t hit as much of their bodies if they stood, and an upright body could be cooled more by passing breezes. Antelopes and gazelles, well‑adapted for the new habitats, spread across the savanna, and some of their fossils show signs of being hacked open or scraped clean of meat. Hominids may have been following these mammals across the savannas, eating their flesh, either by scavenging carcasses left behind by lions and other predators or by scaring the animals off their kills.

When the oldest stone tools were first chipped, there were at least four species of hominids alive in Africa. The most likely candidates for the original toolmakers were the first members of our own genus, Homo. The earliest known Homo first appear in the fossil record about 2.5 million years ago, around the age of the oldest known stone tools. They’re different from other hominids in some striking ways. They have opposable thumbs, and big brains. Judging from the cavities in their skulls, the brains of early Homo were 50 percent bigger than the earliest hominids, relative to their body size.

With stone tools, hominids could add much more meat to their diet, even if they didn’t have the jaws of a hyena or the claws of a lion. The evolution of big brains raced on, and in a few hundred thousand years, hominid brains were double the size of a chimp’s, housed in long‑legged bodies that could reach 6 feet in height. All traces of tree climbing were now gone. These hominids, known as Homo ergaster, were the first to warrant the title of human beings. Like modern humans, they had a wanderlust, and before long they had left Africa altogether. By 1.7 million years ago H. ergaster had reached what is now the Republic of Georgia near the Caspian Sea, where they left behind skulls and tools.

But the tools that these Georgians used–the standard chipped rocks that hominids had used for at least 800,000 years–were about to become obsolete. The hominids that remained in Africa took another technological leap around 1.5 million years ago, inventing hand axes. These new tools took far more skill to make than the earlier models. In order to make a hand axe, a hominid had to flake a rock on both sides, giving it a much sharper edge. Whoever made them didn’t just bang rocks until they could be used to cut. They had a particular tool in mind.

The invention of hand axes and other new stone tools allowed the African hominids to fuel their hungry minds. Brain tissue demands 22 times more energy than an equivalent piece of muscle at rest, and hominids now had gigantic brains to feed. Hominids may have used their new tools to get more meat, butchering bigger animals with tougher hides. But if living hunter‑gatherers are any indication, these hominids did not survive on meat alone.

Even today, hunter‑gatherers with much more sophisticated weapons like poison‑tipped arrows do not catch enough game to feed their families. Kristen Hawkes, an anthropologist at the University of Utah, has studied the diet of the Hadza, a group that lives on the East African savannas. While they occasionally eat a gazelle or some other big animal, they depend on roots and tubers for a steady supply of calories. Hawkes has proposed that early Homo women began to use the improved stone tools 1.5 million years ago to craft digging sticks with which they could excavate roots.

It was not long after the invention of these new tools that even bigger waves of hominid migration surged out of Africa. Around million years ago, hominids began moving into Asia and Europe, bringing their tools with them. By 800,000 years ago hominids spanned the Old World, from Spain in the west to Indonesia in the east. But these hominids did not move farther than about 50 degrees north the southern edge of England. It would be hundreds of thousands of years before they would push farther north. As Hawkes has pointed out, 50 degrees marks the line beyond which the weather is too cold for many tubers to grow. If hominids were an army that marched on its stomach, they had to grind to a halt at that barrier.