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Authors: Chip Walter

Tags: #Science, #Non-Fiction, #History

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BOOK: Last Ape Standing: The Seven-Million-Year Story of How and Why We Survived
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When an animal is having a chronically difficult time filling its belly, something intriguing happens in its body at a molecular level. Aging slows down, and cells don’t die as quickly as they do when food is available. Contrary to what you might think, a cell’s health in this situation doesn’t deteriorate. It improves. The body, sensing deprivation, seems to call all hands on deck, husbands its energy, and prepares for the worst. In a sense each cell grows tougher and more wary. This is thanks largely to a class of proteins called sirtuins, which some scientists suspect reduce the rate of cell growth.
3

Numerous studies show that reducing the normal diets of creatures as different as fruit flies, mice, rats, and dogs by 35 to 40 percent will increase life span as much as 30 percent. (Scientists can’t ethically perform
these sorts of experiments on humans, but all indications are that the same holds true for us.) When food is scarce, fertility also drops and animals mate less frequently, an additional way of slowing down the cycle of life. While the deprivation makes life horrible for the creatures enduring it, from an evolutionary point of view it carries with it the quality of pure genius. Nutritional penury not only extends the life of an animal, but fewer offspring improves the chances of the whole species remaining in the evolutionary sweepstakes. Fewer offspring also places less stress on already overburdened food resources. The whole process of living decides, it seems, to bide its time until the storm passes. Cell growth on every level slows except for one key and remarkable exception: brain–cell growth increases.

There the cells last longer,
and
they begin to make new versions of themselves faster, or at least the neurotrophins generated by the hypothalamus, which are the precursors of new brain cells, do. Not only that, other experiments show that food deprivation increases an appetite–stimulating peptide called ghrelin, which enables synapses to transform themselves by some molecular magic into cortical neurons. You could say the body and the brain strike a bargain. To compensate for the aggressive growth of new neurons, the rest of the anatomy fasts, stretching scarce nutritional resources that it then redirects to the brain. Or put another way, the body slows down aging and accelerates intelligence. This means that 3.5 million years ago, by the time Lucy and her contemporaries were desperately scavenging at the margins of an unpredictable land, the chronic deprivation they were facing was accelerating the growth of their brains.
4

So our ancestors had two assets going for them. Upright walking made them more mobile and efficient, able to cover more ground and better equipped to evade the predators that were evolving along with them. Their larger brains meanwhile made them more capable of adapting to dangerous situations on the fly, more adept scavengers, and better at collaborating successfully with one another. All good in these strange and dangerous environments. That they survived despite their desperate circumstances proves that the combination of the two adaptations was succeeding. But there was now a new challenge: the two trends were on a collision course and bound, in time, to make it impossible to survive. Something had to give.

Chapter Two
The Invention of
Childhood (Or Why It Hurts to Have a Baby)

My mother groaned, my father wept,
into the dangerous world I leapt
.

—William Blake

The human birth canal inlet is larger transversely than it is anteroposteriorly
(front to back) because bipedal efficiency favors a shorter anteroposterior
distance between a line that passes through both hip joints and the
sacrum … This size relationship, along with a twisted birth canal shape,
makes human parturition mechanically difficult
.

—Robert G. Franciscus

“When Did the Modern Pattern of Childbirth Arise?,”
Proceedings of the National Academy of Sciences

Two and a half million years ago, around the end of August in the Human Evolutionary Calendar, primates like
Kenyanthropus platyops, Australopithecus afarensis
, and
Australopithecus africanus
begin to disappear from the fossil record. They may not actually have disappeared, but the evidence of them does. Either way, their evolutionary run was apparently nearing an end. However, with their disappearance a new, rich wave of human species began to crest and break on Africa’s broad and windy plains. In the space of one million years, nine new varieties of humans emerged. Stepping back and looking at the aggregated remains scientists have labored to pick out of the hills,
valleys, and ancient lake beds of Africa like so many needles from an incomprehensibly huge haystack, these species give you the impression that the savanna apes that had been struggling so long to survive were finally getting the hang of living in their new environment, fanning out in more directions, deepening the peculiar evolutionary experiment we call humanity.

And it
was
an experiment; make no mistake, because not all branches of the human family were evolving along the same lines. More precisely, species were striding down two distinctly different roads—one that included smaller, slimmer, so–called gracile apes, and another that embraced bigger, thicker humans with large jaws and teeth, known in the world of paleoanthropology as robust apes. Each approach had its advantages. But in the long run, only one would succeed.

The members of the robust branch of the human family first showed their simian faces in late August among the flooded grassland along the Omo River in southern Ethiopia and the western shores of Lake Turkana in northern Kenya. Scientists call this specimen
Paranthropus aethiopicus
, and he is perplexing because he combines so many contradictory characteristics. His bone structure seems to say that he more often than not walked on all fours among the elephants, saber–toothed cats, and hyenas with which he passed his days. Yet he lived in wet, open grasslands munching on tubers and roots with his big, flat teeth and ample jaws, rather than in wooded areas where you might think knuckle–walking would make more sense. Despite his chimplike anatomy and relatively small brain (no more than 450 cc in adulthood), he may have been the first to pull off the astounding trick of fashioning the first stone tools, preceding even the famous feats of “Handyman” (
Homo habilis
), who followed him and is generally considered the inventor of the first Neolithic technology. (Scientists are debating anew who should get credit for this remarkable advance.)

Whatever
aethiopicus
accomplished, more like him were to follow. Later in the calendar year—the middle of October—two other
Paranthropus
species,
boisei
and
robustus
(also known as
crassidens
in the ever–changing argot of paleoanthropology), arrived, also generously jawed, large headed, and big of tooth, like
aethiopicus
.

Paranthropus
humans represent an evolutionary “strategy” that modified the behaviors of jungle apes, but didn’t leap dangerously far from them. Of the two routes down which evolution was walking earth’s humans, this was the safer, more conservative one. Like their predecessors in the rain forests, troops of robust apes roamed from location to location, gathering what food they could find in the thinning forests, bush, and grasslands where they lived. Because of the sorts of foods they ate,
Paranthropus
possessed heads that sported thick, sagittal crests like the ones you see on the silver–backed gorillas at your local zoo, though they were more chimp–size than gorilla–size. The crests are a stout, jagged line of bone that runs from the top of the forehead to the back of the neck like the metal rim of a medieval helmet. Anchored to these were thick ropes of muscle that ran to their massive jaws and dense necks so that the broad, square rows of teeth in their mouths could crush the cement–hard shells of the nuts they consumed, pulverize bark and seedy berries, crunch the exoskeletons of large insects, or masticate the bones of an unfortunate small animal they might have been lucky enough to snatch up.

Beneath these crests the brains of
boisei
and
crassidens
had expanded roughly a third during the four million years that had passed since the first human emerged from Africa’s rain forests. They were undoubtedly resourceful and even more socially bonded than the apes from which they had descended, mostly thanks to the menace that surrounded them. Danger breeds reliance and cooperation. Day–to–day living would have been unimaginably harsh: a life of slow migration, eating to gather the strength to move forward and moving forward to gather more food to eat. Despite its hardships, however, this was by no means an unsuccessful evolutionary path. By current accounts,
boisei
roamed the plains of Africa for a million years, foraging the foods at hand and getting along, if not famously, then well enough. If
we measure success by how long species survive, we
Homo sapiens
, amount to little more than rookies still wet behind the ears. We have been in the game of life a scant two hundred thousand years.
Boisei
held sway on the Horn of Africa five times as long before exiting the gene pool. If we become this lucky, we will someday be dating our letters July 12, 802013.

The other path plotted by the combinations of genes, environment and random chance was the one taken by members of a branch of the human family paleoanthropologists like to call gracile. This includes
Australopithecus garhi
, a creature who, along with
aethiopicus
, made his debut on the Horn of Africa about 2.5 to 3 million years ago. There is some slim evidence that
garhi
may also have fashioned simple stone tools, but as in the case of
aethiopicus
that’s a controversial and unresolved theory. At best,
garhi
may have used crude stone hammers to break open bones to get at the marrow inside, or sharp flint to scrape and hack meat away from a bone left behind by larger predators. But even these uses of rock mark a colossal technological advance.

About 1.9 million years ago another gracile human, dubbed
Homo rudolfensis
, appeared along the shores of Lake Rudolph, now known as Lake Turkana, a long body of water that runs in the shape of an index finger from southern Ethiopia into the western heart of Kenya.
Homo habilis
and
Homo ergaster
soon followed, both slim and light–boned, both also passing their time in East Africa.

In 1991 scientists scrounging among rocks near Dmanisi, Georgia, west of the Caspian Sea, unearthed the remains of still another species of gracile human from this epoch—
Homo georgicus
. While he remained simian in his looks, his face was flatter, a step closer to ours. Like
Homo habilis, georgicus
was a lean toolmaker, but with a considerably more advanced case of wanderlust. He lived in a river valley more than twenty-five hundred miles north of the grasslands where
Homo habilis
passed his days. He may be an indicator that other species, so far unknown, also strode beyond the borderlands of the Dark Continent, settling who knows where, still awaiting discovery.

Although all of these species came upon the world clustered, like a posse, information about the majority of them is sketchy. Drawing any conclusions about them is a little like drawing conclusions about a long–lost family relative who headed off to the merchant marine or the French Foreign Legion. The best we have in most cases is a few battered bones that offer scant insights into the creatures’ lifestyles or appearance.
Georgicus
, for example, has seen fit to provide us with three skulls—one with jaws attached, one with a solitary jawbone, and one missing its jaws altogether. Nor did they leave anything much in the way of teeth, let alone whole limbs or vertebrae.
Homo rudolfensis
has bequeathed a similarly ungenerous array of jaws and skulls, and a scattering of other fragments that may not belong to the species. What we know of
ergaster
(the Workman) is based on a bundle of six or so skulls, jawbones, and a few teeth, several of which don’t much resemble one another, creating some lively academic brawls about exactly which species is which.

BOOK: Last Ape Standing: The Seven-Million-Year Story of How and Why We Survived
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