Why Evolution Is True (34 page)

How can we tell? From the way that the femur (thighbone) connects to the pelvis at one end and to the knee at its other (figure 26). In a bipedally walking primate like ourselves, the femurs angle in toward each other from the hips so that the center of gravity stays in one place while walking, allowing an efficient fore-and-aft bipedal stride. In knuckle-walking apes, the femurs are slightly splayed out, making them bowlegged. When they try to walk upright, they waddle awkwardly, like Charlie Chaplin’s little tramp.
⁴⁵
If you take a primate fossil, then, and look at how the femur fits together with the pelvis, you can tell whether the creature walked on two legs or four. If the femurs angle toward the middle, it’s bipedal. And Lucy’s angle in—at almost the same angle as that of modern humans. She walked upright. Her pelvis too resembles that of modern humans far more than that of modern chimps.

FIGURE 26
. The attachment of the femur (long leg bone) to the pelvis in modern humans, chimps, and
Australopithecus afarensis.
The pelvis of A.
afarensis
is intermediate to the other two, but its inward-pointing femur—a sign of upright walking—resembles that of humans and contrasts with the splayed femur of the knuckle-walking chimp.

 

A team of paleoanthropologists led by Mary Leakey confirmed the bipedality of A. afarensis with another remarkable find in Tanzania: the famous “Laetoli footprints.” In 1976, Andrew Hill and another member of the team were taking a break by indulging in a favorite field pastime: pelting each other with chunks of dried elephant dung. Looking for ammunition in a dry stream bed, Hill stumbled upon a line of fossilized footprints. After careful excavation, the footprints turned out to be an eighty-foot trail made by two hominins who had clearly been walking on two legs (there were no impressions of knuckles) during an ash storm from an erupting volcano. That storm was followed by a rain, which turned the ash into a cementlike layer that was later sealed in by another layer of dry ash, preserving the footprints.

The Laetoli footprints are virtually identical to those made by modern humans walking on soft ground. And the feet were almost certainly from Lucy’s kin: the tracks are the right size, and the trail dates from around 3.6 million years ago, a time when A.
afarensis
was the only hominin of record. What we have here is that rarest of finds—fossilized human behavior.
⁴⁶
One of the tracks is larger than the other, so they were probably made by a male and female (other
afarensis
fossils have shown sexual dimorphism in size). The female’s footprints seem a bit deeper on one side than on the other, so she may have been carrying an infant on her hip. The trail evokes visions of a small, hairy couple making their way across the plain during a volcanic eruption. Were they frightened, and holding hands?

Like other australopithecines, Lucy had a very apelike head with a chimp-sized braincase. But her skull shows more humanlike traces too, such as a semiparabolic tooth row and reduced canine teeth (figures 25 and 27). Between the head and pelvis she had a mixture of apelike and humanlike traits: the arms were relatively longer than those of modern humans, but shorter than those of chimps, and the finger bones were somewhat curved, like those of apes. This has led to the suggestion that
afarensis
might have spent at least some time in the trees.

One could not ask for a better transitional form between humans and ancient apes than Lucy. From the neck up, she’s apelike; in the middle, she’s a mixture; and from the waist down, she’s almost a modern human. And she tells us a critical fact about our evolution: our upright posture evolved long before our big brain. When this was discovered, it went against the conventional wisdom that larger brains evolved first, and made us rethink the way that natural selection may have shaped modern humans.

After A.
afarensis,
the fossil record shows a confusing mélange of gracile australopithecine species lasting up to about two million years ago. Viewed chronologically, they show a progression to a more modern human form: the tooth row gets more parabolic, the brain gets larger, and the skeleton loses its apelike features.

FIGURE 27
. The skeletons and dental arcades of modern
Homo
sapiens,
Australopithecus afarensis
(“Lucy”), and a chimpanzee. While chimps are not the ancestors of the human lineage, they probably resemble the common ancestor more than do humans. In many respects A.
afarensis
is intermediate between the apelike and human morphology.

 

Then things get even messier, for two million years ago marks the borderline between fossils placed in the genus
Australopithecus
and those placed in the more modern genus Homo. We shouldn’t think, though, that this change of names means that something momentous happened—that “real humans” suddenly evolved. Whether a fossil is called one name or another depends on whether it has a larger
(Homo)
or smaller
(Australopithecus)
brain, usually with a somewhat arbitrary cutoff of around 600 cubic centimeters. Some australopithecine fossils, like A.
rudolfensis,
appear so intermediate in brain size that scientists argue hotly about whether they should be called
Homo
or
Australopithecus.
This naming problem is compounded by the fact that even within a single species we see considerable variation in brain size. (Modern humans, for example, span a very wide range: between 1,000 and 2,000 cubic centimeters, which doesn’t, by the way, correlate with intelligence.) But the semantic difficulties shouldn’t distract us from realizing that the late australopithecines, already bipedal, were beginning to show changes in teeth, skull, and brain that presage modern humans. It is very likely that the lineage that gave rise to modern humans included at least one of these species.

Another great leap forward in human evolution was the ability to make and use tools. Although chimpanzees use simple tools, including sticks to extract termites from their mounds, using more elaborate tools probably required more flexible thumbs and an erect posture that freed the hands. The first unequivocally tool-using human was
Homo habilis
(figure 25), whose remains first appear about 2.5 million years ago. H.
habilis
means “handy man,” and his fossils are associated with a variety of flaked stone tools used for chopping, scraping, and butchering. We’re not sure if this species was a direct ancestor of H.
sapiens,
but
habilis
does show changes toward a more humanlike condition, including reduced back teeth and a brain larger than that of the australopithecines. A cast of one brain shows distinct swellings corresponding to Broca’s area and Wernicke’s area, parts of the brain’s left lobe associated with speech production and comprehension. These bumps raise the possibility—still far from certain—that
habilis
was the first species with spoken language.

We do know that H.
habilis
coexisted—in time if not in space—with a whole host of other hominins. The most famous are the East African “robust” (as opposed to gracile) hominins. There were at least three of these—
Paranthropus
(or
Australopithecus) boisei
(figure 25),
P. robustus,
and
P. aethiopicus,
all with massive skulls, heavy chewing teeth (some of the molars were nearly an inch across), sturdy bones, and relatively small brains. They also sported sagittal crests: a ridge of bone atop the skull that anchored large chewing muscles. Such robust species probably subsisted on coarse food like roots, nuts, and tubers (P. boisei, discovered by Louis Leakey, was nicknamed “Nutcracker man”). All three species went extinct by 1.1 million years ago, leaving no descendants.

But
H. habilis
may have lived alongside three species
of Homo
as well:
H. ergaster, H. rudolfensis, and H. erectus,
although each of these species shows considerable variation and their relationships are disputed.
H. erectus
(“upright man”) holds the distinction of being the first hominin to leave Africa: its remains have been found in China (“Peking man”), Indonesia (“Java man”), Europe, and the Middle East. It is likely that as its populations in Africa expanded,
erectus
simply sought new places to live.

By the time of this diaspora, the brain size of
erectus
was nearly equal to that of modern humans. Their skeletons were also nearly identical to ours, though they still had a flattened, chinless face (the chin is a hallmark of modern H.
sapiens).
Their tools were complex, particularly those of late
erectus,
who fashioned complex stone axes and scrapers with intricate flaking. The species also seems responsible for one of the most momentous events in human cultural history: the control of fire. In a cave at Swartkrans, in South Africa, scientists found erectus remains alongside burned bones—bones heated at a temperature too high to have come from a brushfire. These could be the remains of animals cooked over a campfire or hearth.

H. erectus
was a highly successful species, not only in population size but in longevity. It was around for one and a half million years, disappearing from the fossil record about 300,000 years ago. It may, though, have left two famous descendants:
H. heidelbergensis
and
H. neanderthalensis,
known respectively as “archaic
H. sapiens”
and the famous “Neanderthal man.” Both of these are sometimes classified as subspecies (differentiated but interbreeding populations) of
H. sapiens,
though we have no idea whether either contributed to the gene pool of modern humans.

Living in what is now Germany, Greece, and France, as well as Africa,
H. heidelbergensis
first appears half a million years ago, showing a mixture of modern human and
H. erectus
features. Neanderthals show up a bit later—230,000 years ago—and lived all over Europe and the Middle East. They had large brains—even bigger than those of modern humans—and were excellent toolmakers, as well as adept hunters. Some skeletons bear traces of the pigment ochre, and are accompanied by “grave goods” such as animal bones and tools. This suggests that Neanderthals ceremonially buried their dead: perhaps the first inkling of human religion.

But around 28,000 years ago, the Neanderthal fossils vanish. When I was a student, I was taught that they simply evolved into modern humans. This idea now seems incorrect. What really happened to them is arguably the biggest unknown about human evolution. Their disappearance may have been associated with the spread of another form originating in Africa:
Homo sapiens.
As we learned, by about 1.5 million years ago
H. erectus
had spread all the way from Africa to Indonesia. And within this species there were different “races,” that is, populations that differed in some of their traits.
(H. erectus
from China, for example, had shovel-shaped incisor teeth not seen in other populations.) Then, about 60,000 years ago, every
H. erectus
population suddenly vanished and was replaced by fossils of “anatomically modern”
H. sapiens,
who had skeletons nearly identical to those of living humans. Neanderthals hung on awhile longer, but then, after finding a last redoubt in caves overlooking the Strait of Gibraltar, they too gave way to modern
H. sapiens.
In other words,
Homo sapiens
apparently elbowed out every other hominin on earth.

What happened? There are two theories. The first, called the “multiregional” theory, proposes an
evolutionary
replacement: H. erectus (and perhaps H.
neanderthatensis)
simply evolved into
H. sapiens independently in several areas,
perhaps because natural selection was acting in the same way all over Asia, Europe, and Africa.

The second idea, dubbed the “out of Africa” theory,
⁴⁷
proposes that modern
H. sapiens
originated in Africa and spread,
physically
replacing
H. erectus
and the Neanderthals, perhaps by outcompeting them for food or killing them.