Last Ape Standing: The Seven-Million-Year Story of How and Why We Survived (7 page)

Despite decades of sweltering work, paleoanthropologists have yet to categorically determine which of these humans who trod the shores of Turkana led directly to us, but it is possible to make an informed guess, at least based on the limited evidence scientists have to work with. We already know
Homo habilis
is out of the question, an evolutionary dead end unrelated to
Homo erectus. Homo rudolfensis
is also unlikely because he bears such a strong resemblance to
Paranthropus boisei
and his robust ancestors. He may have been a bridge species of some sort.
Boisei
himself would seem not to qualify given that he wasn’t gracile (we are) and possessed the smallest brain of the group, the largest jaws, and the most apelike features.

That leaves
Homo ergaster
, “the worker” (
ergaster
derives from the Greek word
, meaning “workman”), formerly considered
an example of
Homo erectus
. Truthfully,
ergaster
wouldn’t seem to be a promising candidate for a direct ancestor either, except for one remarkable fossil find that has been, after some heated debate, assigned to the
ergaster
line. In the scientific literature he is known as Turkana (or sometimes Nariokotome) Boy because Kamoya Kimeu, a paleoanthropologist who was working at the time with Richard Leakey, came across him on the western shore of Lake Turkana.

His discovery first stunned his fellow anthropologists and then the world with the completeness of what he had found. In a scientific field where scraping up a tooth or a jaw fragment, or a wrecked piece of tibia, can be cause for wild jubilation, Kimeu and his colleagues uncovered not only a skull, but a rib cage, a complete backbone, pelvis, and legs, right down to the ankles. There, in the brittle detritus of the Dark Continent, lay the nearly complete remains of a boy who had lived 1.5 million years ago and died in the swamps of the lake somewhere between the ages of seven and fifteen. It was nothing short of remarkable.

You may have noticed the wide range of the boy’s possible age. There’s a reason for that. Despite being among the most studied fossils in the annals of paleoanthropology, scientists cannot seem to universally agree on the age of their owner, a mystery that brings us back to the issue of long and lengthening childhoods. The boy’s age is elusive because we have only two living examples of primates that we can use as benchmarks to determine his age when he died—forest apes and us. But Turkana Boy is neither. With an adult brain that would likely have been about 880 cc, he falls almost midway between the two extremes. Take away half the mass of his brain, and it would be about the size of a chimpanzee’s. Add the same amount and he would be within the range of most modern humans.

When scientists first inspected the boy’s fossilized teeth, they immediately realized he was, in fact, a boy because several of them had not yet entirely arrived. In his lower jaw a few permanent incisors, canines, and molars had formed, but not all of them were fully grown. In his upper jaw he still had his baby, or milk, canines and no third molar. If a dentist were looking into a mouth like that today, she would conclude she was dealing with an eleven–year–old. But if the mouth belonged to a chimpanzee, seven would be a better guess.

Teeth represent one type of clue scientists use to help estimate the
age of a skeleton (or more precisely, the skeleton’s former owner) when he died. Another is growth plates. Long bones like those in our arms and legs don’t fuse permanently with the joints attached to them until they are fully grown. The state of growth plates reliably predicts age. Turkana Boy’s long leg bones were still growing and had not yet fused, particularly at the hips, although one of his shoulder and elbow joints was fusing. Given the state of his growth plates, researchers concluded the boy could have been as young as eleven or as old as fifteen the day he met his untimely end,
if
he was human. Or a mere seven if he was a chimpanzee.

A final feature that helps determine age is height. Nariokotome Boy’s thighbone is seventeen inches long, which would have made him roughly five feet three inches tall, about the size of an average fifteen–year–old
Homo sapiens
, or a full–grown chimpanzee. Compared with other fossil primates, australopithecines or even his Turkana contemporaries like
Homo habilis
and
rudolfensis
, for example, Nariokotome was tall, and depending on his exact age, he might have grown considerably taller, had he survived. So how old
was
the “working” boy?

Viewed from either end of the spectrum, none of the clues about his age have made much sense to the teams of scientists who have labored over them. Each was out of sync with the other. Some life events were happening too soon, some too late, none strictly adhering to the growth schedules of either modern humans or forest apes. Still, the skeleton’s desynchronized features strongly suggested that the relatives of this denizen of Lake Turkana were almost certainly being born “younger,” elongating their childhoods and postponing their adolescence. Apes may be adolescents at age seven and humans at age eleven, but this creature fell somewhere in between.

If the Rubicon theory is correct, and an adult brain of 850 cc marked the time when newborns begin to struggle to successfully make their journey through the birth canal,
ergaster
children were likely already coming into the world earlier than the rain–forest primates that preceded them five million years earlier. On the other hand, Turkana Boy was not being born as “young” as we are. His large brain, as large as any other in the human world at that time, and his slim hips, optimized for upright walking and running, reinforce the evidence. He must have been born “premature” or he wouldn’t have been born at all. But if he was being born earlier, how much earlier?

Suppose the brain of a fully grown Turkana Boy was 60 percent the size of our brain today. (We have to suppose because we have no adult
ergaster
cranium to consult.) And let’s assume
ergaster
children would have come into the world after fourteen months of gestation, approximately 30 percent sooner than a chimp. This isn’t as drastically different as the eleven–month disparity between other primates and us modern humans, but it would have represented the beginning of a significant human childhood, and it would have begun to upend the daily lives of our ancestors in almost every way.

Why? First, there would have been more death in a world where, unfortunately, death was no stranger. Many “early borns” would have died after birth, unable, unlike today’s chimps and gorillas, to quickly fend for themselves. Because gorilla and chimp newborns are more physically mature than human newborns, they often help pull themselves out of the birth canal and quickly crawl into their mother’s arms or up onto her back. It’s unlikely that
ergaster
infants were capable of this. Of all primates, human newborns are by far the most helpless. When we arrive, we are utterly incapable of walking or crawling. We can’t see well or even hold our heads up. Without immediate and almost constant care, we would certainly die within a day or two. Though these “preemies” were not likely as defenseless at birth as we are, they would have been far less physically developed than their jungle or even early savanna predecessors.

But even if the newborns didn’t die in childbirth, their mothers might have, their narrow pelvises unable to handle what scientists call the expanding “encephalization quotients” of their babies. To compensate,
ergaster
newborns may have begun to turn in the birth canal so that they were born faceup, a revolutionary event in human birth. Unlike other primates, our upright posture makes it necessary for babies to rotate like a screw so they emerge faceup. If they came out with their faces looking at their mother’s rump as chimps and gorilla infants do, their backs would snap during birth.

The job of bringing a child into the world would not only have become more complicated, but imagine life for the mothers of these offspring, assuming both survived the ordeal of birth. They were already living a precarious existence in a menacing world—open grasslands or at best thick brush with occasional clusters of forest. Predators such as striped hyenas and the sythe–toothed
Homotherium
had appetites and needs, too. There was no such thing as a campfire to keep
predators at bay. Fire had yet to be mastered. When night fell, it was black and total with nothing more than the puny illumination provided by the long spine of the Milky Way, a fickle moon, or an occasional wildfire in the distance sparked suddenly and inexplicably by lightning or an ill–tempered volcano. And the big cats of the savanna like to hunt when the sun has set.

Not only were these new human infants more helpless than ever, but their neurons were proliferating outside the womb at the same white–hot rate they once did inside. Rapidly growing brains demand serious nutrition. Studies show that children five and younger use 40 to 85 percent of their standing metabolic rate to maintain their brains. Adults, by comparison, use 16 to 25 percent.
⁹
Even for
ergaster
children, a lack of food in the first few years of life would often have led to premature death. Nariokotome Boy might have been undernourished himself. His ancient teeth reveal he was suffering from an abscess. His immune system may not have been strong enough to defeat the infection, and lacking antibiotics, scientists theorize blood poisoning abbreviated his life. He was probably not the first among his kind to die this way.

In every way, early borns would have made life on the savanna more difficult, more dangerous, and more unpredictable for their parents and other members of the troop. So why should evolution opt for larger brains and earlier births? And how did it manage to make a success of it?

Difficult question to answer. Looking back on the scarce orts of information science has so far gathered together, premature birth doesn’t make an ounce of evolutionary sense. Not on the surface. Darwinian adaptations succeed for one reason—they help ensure the continuation of the species. That means if your kind misplaces the habit of living long enough to have sex successfully, extinction will swiftly follow. Since this is the ultimate fate of 99.9 percent of all life on earth, it is difficult to fathom how the mountains of challenge that early–arriving newborns heaped on the backs of their gracile ape parents could possibly help them successfully struggle to stay even a single step ahead of the grim reaper.

It certainly wouldn’t seem to make much sense to lengthen the time between birth and sex. Keeping that time as brief as possible has immense advantages after all. It’s a powerful way to maximize the number of newborns either by having large numbers of them at once or by
having them often, or both. Dogs, for example often enter the world in bundles of five or six at a time, are weaned by six weeks, and ready to mate as early as six months. They aren’t puppies long, and once they are done breast–feeding, they are soon prepared to fend for themselves. For mice the process is even more compressed. The result is that mothers bear more children with every birth, do it more often, and those offspring are quickly ready to mate and repeat the cycle. All of this accelerates the proliferation of the species
and
improves its chances of survival.

We humans, however, wait an average of nineteen years before bearing our first child. Why? If shortening the time between being born and bearing as many offspring as often as possible works so well for other mammals, for what reasons would evolution twist itself backward with Africa’s struggling troops of savanna apes? Why bring increasingly defenseless infants into the world? Why expose their parents to greater danger to feed and protect them? Why insert this extra, unprecedented cycle of growth, this thing we call childhood, into a life—a time when we rely utterly on other adults to take care of us? And what advantage is there in taking nearly two decades to bring the first of the next generation into the fold?
¹⁰

In his landmark book
Ontogeny and Phylogeny
, Stephen Jay Gould spends considerable time discussing two types of environments that drive different varieties of evolutionary selection. One he calls
r
selection, which takes place in environments that provide plenty of space and food and little competition. A kind of animal Valhalla. The other is called
K
selection, environments where space and resources are scarce and competition is nasty and formidable.

R
selection (Gould points out many studies that back this up) encourages species to have plenty of offspring as quickly as possible (think rabbits, ants, or bacteria) to take advantage of the lavish resources at hand. But
K
–style environments require species to slow down, create fewer offspring, and take more time doing it because it reduces stress on the environment and the competition among those trying to survive in it. By random chance, evolution begins to favor the creation of fewer competitors within a species who will only die off from lack of resources.
¹¹
By reducing death and lengthening life, in particular early life,
K
selection also provides species extra time to develop in ways that make them more adaptable. In our case, as Gould put it,
K
selection made us “an order of mammals distinguished by
their propensity for repeated single births, intense parental care, long life spans, late maturation, and a high degree of socialization.” Today you and I stand as the poster children for
K
–strategy evolution. Yet, while the simple fact that we are walking around today provides conspicuous proof that
K
strategies can succeed, it still fails to explain
why
they succeed.
¹²