Last Ape Standing: The Seven-Million-Year Story of How and Why We Survived (5 page)
Read Last Ape Standing: The Seven-Million-Year Story of How and Why We Survived Online
Authors: Chip Walter
Tags: #Science, #Non-Fiction, #History
The stinginess of these creatures makes them mysterious, even among our ancestors, humans who have steadfastly held the cards of their pasts close to their primeval vests. Of all these slender primates, however, one has been a little less secretive—
Homo habilis
, otherwise famously known as Handyman, long thought to be our direct ancestor and the first toolmaking primate. We have been able to infer a little more about the life of
habilis
only because we have been lucky enough to have stumbled across more parts of his body than his other contemporaries—several skulls, a hand bone complete with fingers, and multiple leg and foot bones that can’t conclusively be connected with the skulls, but at least provide some clues about the creature’s size and gait. Together the evidence tells us that
habilis
, though slight in stature, walked upright all the time and possessed considerably larger brains than the first ancient humans, as spacious as 950 cc, depending on which skull you inspect. The shapes of their heads and jaws indicate that unlike their robust cousins, they didn’t care much for nuts, bark, and berries, but had developed an appetite for meat, and the protein it provided, which may account for their larger brains. (See sidebar “Big Guts vs. Big Brains” p. 21.) Nor did they sport great sagittal crests, or huge, square teeth made for grinding. Their teeth were better at tearing. Chances are they hunted small game in packs, not unlike the way chimpanzees sometimes do. And they helped themselves to savanna carrion and whatever other more adept and deadly predators left behind in the way of their prey’s remains.
Big Guts vs. Big Brains
Cows, as we all learned in grade school, have four stomachs. They do because it requires a lot of work to extract enough nutrients from grass to transform it into beef and milk. The same was true of our early savanna–roaming ancestors, at least some of them. Subsisting on a diet of nuts, roots, thistles, berries, and other plants required long intestines and strong stomachs if they hoped to squeeze enough nutrients from them to stay alive.
As the climate changed in Africa and the savannas became broader and drier, the old jungle ways of gathering low–hanging fruit from nearby trees and not moving very far from day to day simply didn’t work. Fruit and foliage became increasingly rare, and three humans had to cover more distance to gather it, which required still more energy. Ultimately that was not a sustainable survival strategy.
But if you could get your hands on some meat! Then you were instantly rewarded with much more nutritional bang for your hunting–and–gathering buck. That is precisely what the robust lines of savanna humans did. But this choice paid an additional, unexpected dividend. A diet of meat of any kind (even dining on termites and small rodents) made larger brains possible, and less cowlike intestinal tracts necessary. This is something paleoanthropologist Leslie Aiello dubbed the Expensive Tissue Hypothesis when she first came up with the idea in the early 1990s. What this meant, and what fossil finds reveal, is that as our ancestors began to consume more meat, their bodies could redirect the energy those complex intestinal tracts demanded to the business of constructing larger brains. It was a close question two million years ago which approach might work best. Both experiments were tried, and for hundreds of thousands of years both worked. Ultimately, though, larger brains turned out to be a more effective survival tool than longer intestines, something the fossil record bears out. While australopithecines and the robust members of the human family were relatively small brained, often not much more cerebrally endowed than a chimpanzee,
Homo ergaster
’s brain size ballooned to 900 cc or so. After a run of more than one million
years, the last of the robust humans finally made their exit 1.2 million years ago.
This evolutionary path had other ramifications as well. We aren’t as strong as our primate cousins—chimps, gorillas, orangutans—for example. We seem to have exchanged brawn for brains. Richard Wrangham has argued that mastering fire and cooking made meat and other foods of all kinds easier to digest, increasing the protein we could consume and reducing the need for longer intestinal tracts even further. In time bigger brains delivered better weapons, and more strategic ways of hunting. And that likely led to bigger game, more meat, more protein, more cerebral horsepower. The result? Over the past two million years, the brains of the gracile line of humans nearly doubled their size.
The scattered fossils of both of these sides of the human family tell us that evolution was putting a series of unstated questions on the table 1.5 million years ago: Which approach is best? Gracile or robust? A steady diet of tubers, nuts, and berries? Or a sparse, starvation diet of scavenged carrion along with whatever else could be scraped from nature’s table. A serviceable brain with a cast–iron stomach, or a great brain with a simpler, less sturdy digestive system?
If you were a betting primate, you couldn’t be blamed for putting your money on the robust branch of the family. At the time, they looked to be winning the battle. They were strong and durable and had adapted the jungle ways of their antecedents to the savanna exceedingly well, meandering through flooded grasslands and clusters of forests, sometimes upright, sometimes on all fours, consuming, if not jungle fruit like their more gorilla–like ancestors, then the next–closest foods that give the term
high fiber
a whole new meaning.
Their stomachs had to be large and their intestines long to digest these foods, and their eating would itself have required liberal funds of energy. In some ways they were consuming so they would have the energy to consume. According to one theory, this explains why their
brains did not grow as large and as fast as their gracile cousins’. Hardworking stomachs can’t afford to redirect energy to cerebral growth. But their arrested development may have saved them and been the secret to the success of their million–year run.
Gracile apes on the other hand looked less likely to succeed. They were smarter—given their increased brain size they had to be—but their diet was unpredictable. They used less energy because they walked upright all the time, but they had to make do with whatever else their smaller, less sturdy stomachs could handle. The robust approach was stable. The gracile approach was risky.
Sometimes, however, risk pays off. A high–stakes wager placed on
Paranthropus
would have paid nothing, yet against all odds, several underdog gracile species remained in the hunt. Good news for us because it was from one of these lines that you and I descended. Still, trouble loomed. Just as it appeared gracile apes were succeeding, finally brainy and efficient enough to outfox the rough treatment their savanna environment was dishing out, the self–same adaptations that were saving them—an upright gait and bigger brains—were also aligning to become the agents of their doom.
1
Striding on two legs efficiently—not waddling the way a chimp or gorilla does when it walks upright—requires, among other adaptations, a fundamental rearrangement of pelvic architecture. An upright stride narrows the hips, and for females, narrowing the hips narrows the birth canal, and a slimmer birth canal makes for increasingly snug trips for newborns out of the womb. Despite the many advantages that upright walking delivered, it creates problems when one is simultaneously evolving bigger brains and larger heads, which was precisely what our gracile ancestors were up to. Yet, since both adaptations were working, what could be done? Each was an evolutionary blessing, yet both were on a collision course. Something would have to give.
Lucky for us, the forces of evolution worked out an exceedingly clever solution: gracile humans began to bring their children into the world early. We know this because you and I, being extreme versions of gracile apes, are the living, breathing proof. If you, for example, were to be born as physically mature and as ready to take on the world as a gorilla newborn, you would have to spend not nine months in the womb, but twenty, and that would clearly be unacceptable to your mother. Or, looked at from a gorilla’s point of view, we humans
are born eleven months “premature.” We do not reach full term, which makes us fetal apes. Of course if we didn’t make our departure from the womb ahead of schedule, we wouldn’t be born at all because our heads, after nearly two years in the womb, would be far too large too make an exit. We would be, literally, unbearable.
It’s impossible to overstate the colossal impact this turn of events had on our evolution, but it requires some context to fully appreciate what it means. Our habit of being born early is part of a larger, stranger phenomenon that scientists call
neoteny
, a term that covers a lot of evolutionary sins at the same time it explains so much of what makes us the unique, even bizarre creatures we are.
The dictionary defines
neoteny
as “the retention of juvenile features in the adult animal.” The term comes from two Greek words,
neos
, meaning “new” (in the sense of “juvenile”), and
teinein
, meaning to “extend.” In our case it meant that our ancestors, rather remarkably, passed along to us a way to stretch youth farther into life. The question is, why, and how, did it happen?
When faced with resolute obstacles, evolution—always in the service of survival—has a marvelous way of selecting astonishingly diverse solutions cooked up entirely by random chance. This is how the planet has found itself with the unearthly–looking aye–aye of Madagascar, Borneo’s clownish proboscis monkey, the squashed and unappetizing blobfish of Tasmania, and the rapier–nosed narwhals of the arctic seas. It also helps explain the bizarre mating rituals of porcupines, and male anglerfish, not to mention the torturous eating habits of ichneumon wasps. Each of these creatures is a living testament to the marvelous, if accidental, creativity natural selection conjures, again and again. But as remarkable as these evolutionary banks and turns have been, neoteny can count itself as one of the strangest, and we
Homo sapiens
are by far the most dramatic and extreme example.
2
The term
neoteny
was coined by Julius Kollmann—a groundbreaking German embryologist and a contemporary of Charles Darwin’s. Kollman had nothing like human beings in mind when he created the term. He conceived it to describe the retention of larval features in the Mexican axolotl (
Ambystoma mexicanum
), and other species of salamanders like the mud puppy (
Necturus maculosus
) and the olm (
Proteus
), all of which refuse in their lives to fully grow up and out of their larval stage, even in their adulthood. They mature normally and sexually, but all within the body of their youth. This would be a little bit like a
two–year–old boy behaving in every way like a fully grown, sexually mature twenty-five–year–old. In humans, neoteny isn’t quite that pronounced (probably a good thing), but it is nevertheless remarkable, and remarkably odd, if you are willing to circle around and look at it fresh.
The idea of neoteny predates even Darwin and was explored as far back as 1836, when Étienne Geoffroy Saint–Hilaire, a French scientific prodigy and compatriot of Napoléon’s, first pointed out how astonishing it was that the young orangutans that had recently arrived from Asia at the Paris zoo resembled “the childlike and gracious features of man.”
In the twentieth century a handful of other scientists and evolutionary thinkers adopted Kollmann’s term and Geoffroy’s sentiments when they began applying the idea of neoteny to humans, observing that infant apes bore a striking resemblance to adult humans especially in the shapes of their faces and heads. Naturally this raised a few questions: Was this simply a coincidence? Why would we resemble baby apes? And did this have anything to do with our own evolution?
A professor of anatomy in Amsterdam named Louis Bolk became nearly obsessed with those questions. Between 1915 and 1929 he penned six detailed scientific papers and one entire pamphlet on the subject with the ambitious title
Das Problem der Menschwerdung
(“On the Problem of Anthropogenesis”). He argued that a surprisingly high number of human physical traits “have all one feature in common, they are fetal conditions [seen in apes] that have become permanent [in adult humans].”
3
In one paper Bolk even enumerated twenty-five specific fetal or juvenile features that disappear in apes as they grow to adulthood, but persist in humans right up to death. The flatter faces and high foreheads that we and infant chimps share, for example. Our lack of body hair compared with chimpanzees and gorillas (fetal apes have little body hair). The form of our ears, the absence of large brow ridges over our eyes, a skull that sits facing forward on our necks, a straight rather than thumblike big toe, and the large size of our heads compared with the rest of our bodies. The list is long and Bolk’s observations were absolutely accurate.
c
You can find every one of these traits in fetal, infant, or toddling apes, and all modern human adults. No less than evolutionary biologist Stephen Jay Gould agreed with Bolk
in his own landmark book,
Ontogeny and Phylogeny
(though he didn’t agree with the elder scientist’s reasons for coming to those conclusions, which were tainted with racism and convoluted views of evolution). Gould called our peculiar brand of neoteny one of the most important twists in all the turns that human evolution has taken.
4
Given its dictionary definition, you might think that neoteny is simply a matter of a species holding on to as many youthful traits of an ancestor as long into adulthood as possible (a little like Joan Rivers or Cher). But it’s not that simple. Undeniably, in some ways we are childlike versions of our pongid ancestors, but in others our maturity is accelerated, rather than stunted. For example, while our faces and heads may not change as radically as an ape’s as we enter adulthood, our bodies still continue to grow and change. We don’t retain the three–foot stature of a two–year–old toddler. In fact at an average (worldwide) male height of five feet nine inches, give or take a few centimeters, we are among the largest gracile apes to have ever evolved. Nor is our sexual maturity slowed, though it is delayed compared with other human species (including Neanderthals, as we will see soon). And our brain development is anything but arrested. In fact, just the opposite. As I said, complicated.