Last Ape Standing: The Seven-Million-Year Story of How and Why We Survived (6 page)

The different ways some parts of us seem to accelerate and mature while others bide their time or halt altogether has generated a flock of terms related to
neoteny—paedomorphosis, heterochrony, progenesis, hypermorphosis
, and
recapitulation
. The debate is ongoing about what exactly
neoteny
and the rest of all of these labels truly mean. In the end, however, it comes down to this—each represents an evolution of evolution itself, an exceptional and rare combination of adaptations that changed our ancestors so fundamentally that it led to an ape (us) capable of changing the very planet that brought it into existence.
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Put another way, it changed everything.

Mostly we think of Darwin’s “descent by natural selection” as a chance transformation of newly arrived mutations—usually physical—into an asset rather than a liability, which is then passed along to the next generation. So paws become fins in mammals that have taken to the sea. The spindly arms of certain dinosaurs evolve into the wings of today’s birds. The ballasting bladders of ancient fish become the predecessors of land animals’ lungs. All of that is true. But what neoteny (and paedomorphosis and all the rest) illustrate is that the forces of evolution don’t simply play with physical attributes, they play with time, too, or more accurately they can shift the times when genes are expressed and hormones flow, which not only alters looks but behavior, with fascinating results.

Evolution manages this by not affecting solely
what
traits it reveals, but
when
it reveals them. It moves abilities, physical features, and behaviors forward or backward, or stops them altogether by altering the expression of genes that affect developmental hormones. It plays with time like a boulevard–game master plays a shill game with walnut shells and peas. So in us, our big toe remains straight throughout our lives rather than crooking thumblike before birth as it does for chimps and gorillas. We remain relatively hairless, like fetal apes.
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Our jaws stay square and our foreheads flat throughout our lives rather than sloping backward as we leave our early years behind. And instead of decelerating brain growth after birth like orangutans, chimps, and gorillas, the genes that control the amount and interconnections of neurons act as though we are still in the womb and continue to fervently multiply.

Put another way, after birth, processes that were once
pre
natal in our ancestors become
post
natal in us. By being born “early,” our youth is amplified and elongated, and it continues to stretch out across our lives into the extended childhood that makes us so different from the other primates that preceded us. We see it in the fossil record. Almost without exception, the dusty bones scientists have unearthed and fitted together reveal that the faces of gracile primates such as
habilis, rudolfensis
, and
ergaster
, while still plenty simian, grew step by step to increasingly resemble us. Their snouts were flattening, their foreheads were growing higher and less sloped, their chins stronger. Features that once existed only in fetal forest apes like big toes and heads that rested upright on shoulders now not only existed in youth but also persisted into adulthood.

Exactly how all of this unfolded on the wild and sprawling plains of Africa isn’t clear precisely, but there can be no doubt that it did. We stand as the indisputable proof. All of the evidence emphatically points to our direct, gracile ape ancestors steadily extending their youth. They were inventing childhood. But most important, to us at least, in the inventing they were becoming more adept at avoiding extinction’s sharp and remorseless scythe. And the main reason that was happening was because the childhood that was evolving enabled the development of a remarkably flexible brain. That is where the grand story of our evolution made an extraordinary turn.

The clustered neurons that together compose the brains of all primates grow at a rate before birth that even the most objective laboratory researcher could only call exuberant, maybe even scary. Within a month of gestation primate brain cells are blooming by the thousands per
second
. But for most species that growth slows markedly after birth. The brain of a monkey fetus, for example, arrives on its birthday with 70 percent of its cerebral development already behind it, and the remaining 30 percent is finished off in the next six months. A chimpanzee completes all of its brain growth within twelve months of birth. You and I, however, came into the world with a brain that weighed a mere 23 percent of what it would become in adulthood. Over the first three years of your life it tripled in size, continued to grow for three more years until age six, underwent massive rewiring again in adolescence, and finally completed most, but not all, of its development by the time you reached your second decade (assuming that as you read this you
have
reached your second decade).

Being born so “young,” you might conclude our brains arrive comparatively underdeveloped at birth, but that is not the case. Despite our early arrival we still come into the world bigheaded, even compared with our more mature cousin primates. At birth the brains of apes constitute 9 percent of their total body weight, hefty by the standards of most mammals. We, however, weigh in at a strapping 12 percent, which makes our brain 1.33 times larger than an infant ape’s, relatively speaking, despite our abbreviated sojourn in the womb. In other words even arriving in our early, fetal state, with less than a quarter of our brain development under our belts, we are still born with remarkably large brains.

Keep in mind that this approach to brain development is so extraordinarily
strange and rare that it is unique in nature. And dangerous. If an engineer were planning the optimum size of a brain at birth, it would clearly be illogical to bring newborns into the world this cerebrally incomplete. Too fragile, and too likely to fail. Far more practical to do all the work in the safety of a mother’s body. But evolution doesn’t plan. It simply modifies randomly and moves forward. And in this case, remember, remaining in the womb full term was out of the question. For us it was be born early, or don’t be born.

As much as we might like to know the answer, exactly when it became necessary for our ancestors to exit the birth canal “younger” is frankly impossible to say. Since we
Homo sapiens
are the only human species to still be walking the planet since Africa’s retreating jungles orphaned the rain–forest apes that preceded us, and since the skeletal remains of those who came before us are rare and difficult to decipher, we simply haven’t yet gathered enough clues to know precisely when an early birth became unavoidable. There are, however, a few theories.

Some scientists believe earlier births would have begun when the adult brain of some predecessor or another reached 850 cc.
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Anthropologist Robert D. Martin calls this the “cerebral Rubicon,” a line that once crossed would have required that some sort of longer, humanstyle childhood become part of that creature’s life. If that’s true, that narrows the candidates to those human species living between 1.8 and 2 million years ago—species like
Homo rudolfensis
or
Homo ergaster
. Until recently scientists felt
Homo habilis
(Handyman) was the best candidate, but new evidence has caused some realignment of the human family tree. For decades the common wisdom had it that we descended from
Homo habilis
by way of
Homo erectus
, which in turn evolved into what paleoanthropologists call “anatomically modern humans” (AMH), our kind. But new fossil finds now indicate that
erectus
and
habilis
were East African contemporaries for nearly a half million years, making it rather difficult to have descended from one another. Furthermore,
ergaster
and
rudolfensis
, which were often tossed in with
Homo erectus
, are now more often considered to be their own separate species.

This means that in the ever–shifting drama (and nomenclature) of human evolution, Handyman now represents an evolutionary dead end and
Homo erectus
may turn out to be not one species, but many, with only one particular representative leading directly to us, if that.
Whatever the case, around this time, when humans began to grow adult brains about three quarters of the size that ours are today, the offspring of upright walking humans may have been forced to arrive prematurely as the fit between head and pelvis grew increasingly tight. Who, the question then becomes, were the people from whom we directly descended, and where can we suppose they lived?

Some history might be in order.

Thirty–five million years ago the northeast corner of Africa was being carried on the back of a tectonic plate determined to make its way eastward toward Asia, while the rest of the continent was steadfastly refusing to go along. One consequence of this dogged parting of the ways was the emergence of the Arabian Sea and peninsula (with, coincidentally, all its oil beneath it). Another was the formation of a long and immense lake in East Africa made possible by the three substantial rivers that drained into it from the surrounding mountains. Two million years ago, the evidence of this great African rift, and the lake it created, was still all around. The ruptured land had left behind dozens of volcanoes, smoldering ominously and erupting unpredictably. One even rose defiantly out of the great lake itself, a disdainful sentinel that stood unfazed by the storms that howled when the seasons changed or the dust devils that spun along its flanks in the hot summer months.

If you check a map of Africa today, you will notice the slender imprint of this lake we now call Turkana (formerly known as Lake Rudolf). It is still vast, a long, liquid gem that lies on the breast of East Africa, most of it in northern Kenya with just its upper nose nudging the highlands of southern Ethiopia. Today Lake Turkana fails to be as hospitable as it was earlier in its life. The rivers that once drained it are gone, so evaporation is the only exit for Turkana’s waters. That has turned it a splendid jade color and made it the world’s largest alkaline lake. These days the land that surrounds it is mostly dry, harsh, and remote. However, 1.8 million years ago it was an exceedingly fine place to set up housekeeping.

Life of every kind thrived along Turkana’s shores in the early days of the Pleistocene epoch, despite the occasional ferocity of the weather and the ominous belching of its volcanoes.
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Crocodiles bathed in its warm waters;
Deinotherium
, an ancient version of the elephant, and both black and white rhinoceroses grazed among the grasslands.
Hyenas yelped and hooted, scavenging what they could and hunting flamingos that fed in the shallows, while the grandcousins of lions, tigers, and panthers harvested dinner from herds of an early, three–toed horse called
Hipparion
. The lake, the streams and the rivers that fed it, and the variability of the weather made the area a kind of smorgasbord of biomes—grasslands, desert, verdant shorelines, clusters of forest and thick scrub. The bones of the extinct beasts that lie by the millions in the layers of volcanic ash beyond the shores of Lake Turkana today attest to its ancient popularity.

The existence of a habitat this lush and hospitable wasn’t lost on our ancestors any more than it was on the elephants, tigers, and antelope that roamed its valleys. In fact it was so well liked that
Homo ergaster
(left),
Homo habilis
, and
Homo rudolfensis
were all ranging among its eastern and northern shores 1.8 million years ago, sharing the benefits of the basin with their robust cousin
Paranthropus boisei
. As many as a million years earlier,
Paranthropus aethiopicus
came and went along the northwestern fringe of the lake, and half a million years before that the flat–faced one,
Kenyanthropus platyops
, braved Turkana’s winds and watched its volcanoes rumble and spew.

Homo ergaster