There's been no biological change in humans in 40,000 or 50,000 years. Everything we call culture and civilization we've built with the same body and brain.
—Stephen Jay Gould
Something must have happened to weaken the selective pressure drastically. We cannot escape the conclusion that man's evolution towards manness suddenly came to a halt.
We intend to make the case that human evolution has accelerated in the past 10,000 years, rather than slowing or stopping, and is now happening about 100 times faster than its long-term average over the 6 million years of our existence. The pace has been so rapid that humans have changed significantly in body and mind over recorded history. Sargon and Imhotep1 were different from you genetically as well as culturally. This is a radical idea and hard to believe—it's rather like trees growing noticeably as you watch. But as we will show in the following pages, the evidence is there.
Scientists have long believed that the "great leap forward," some 40,000 to 50,000 years ago in Europe, marked the advent of cultural evolution and the end of significant biological evolution in humans. At this time, the theory goes, humans developed culture, as shown by the sophisticated new tools, art, and forms of personal decoration that emerged in the Upper Paleolithic. Culture then freed the human race from the pressures of natural selection: We made clothes rather than growing fur and built better weapons rather than becoming stronger.
The argument that the advent of behavioral modernity somehow froze human evolution is dependent on the notion of a static environment.2 In other words, if a population—of humans, wolves, crabgrass, you name it—experiences a stable environment for a long time, it will eventually become genetically well matched to that environment. Simple genetic changes then do little to improve individual fitness, because the species is close to an optimum. An economist would say that all the $100 bills have already been picked up off the sidewalk. In that situation, evolution slows to a crawl. That's not to say that a stable species has reached perfection, but that its life strategy is well implemented. For example, hopping may not be as efficient as walking on all fours (four legs good, two legs bad!), but kangaroos are good at hopping; their bodies are well suited to their style of locomotion. The match of the population to its environment can never be exact, since environments fluctuate, but it can be quite close. For example, there are orchids that imitate a bee so closely in appearance and in odor that bees try to mate with them, and so pollinate the orchids. Some creatures that are well suited to their environment, such as horseshoe crabs, have managed to stay much the same for hundreds of millions of years. They're literally older than the hills.
However, modern humans have experienced a storm of change over the past 50,000 years. We left Africa and settled every continent other than Antarctica. We encountered and displaced archaic humans like Neanderthals—and probably picked up genes from them in the process. An ever-accelerating cultural explosion from the Upper Paleolithic to the Neolithic and beyond led to new technologies and new social forms. Indeed, culture itself has been an increasingly important part of the human environment.
Geographic expansion (which placed us in new environments) and cultural innovation both changed the selective pressures humans experienced. The payoff of many traits changed, and so did optimal life strategy. For example, when humans hunted big game 100,000 years ago, they relied on close-in attacks with thrusting spears. Such attacks were highly dangerous and physically taxing, so in those days, hunters had to be heavily muscled and have thick bones. That kind of body had its disadvantages—if nothing else, it required more food—but on the whole, it was the best solution in that situation. But new weapons like the atlatl (a spearthrower) and the bow effectively stored muscle-generated energy, which meant that hunters could kill big game without big biceps and robust skeletons. Once that happened, lightly built people, who were better runners and did not need as much food, became competitively superior. A heavy build was yesterday's solution: expensive, but no longer necessary. The Bushmen of southern Africa lived as hunter-gatherers until very recently, hunting game with bows and poisoned arrows for thousands of years in that region. They are a small, tough, lean people, less than five feet tall. It seems likely that the tools made the man—the bow begat the Bushmen.
With the invention of nets and harpoons, fish became a more important part of the diet in many areas of the world, and metabolic changes that better suited humans to that diet were favored. Close-fitting clothing provided better protection against cold, allowing people to venture farther north. In cool areas, people needed fewer physiological defenses against low temperatures, while in the newly settled colder regions they needed more such defenses, such as shorter arms and legs, higher basal metabolism, and smaller noses. With the advent of new methods of food preparation, such as the use of fire for cooking, teeth began to shrink, and they continued to do so over many generations. Pottery, which allowed storage of liquid foods, accelerated that shrinkage. Complex biological functions tend to slowly deteriorate when they no longer matter, since mutations that interfere with the function no longer reduce reproductive fitness, and you might think that this would explain these dental changes. However, this trend, which we call "relaxed selection," happens too slowly to be the explanation. Instead, the changes in tooth size must have been driven by positive advantages—possibly because small teeth are metabol-ically cheaper than large ones.
As the complexity of human speech approached modern levels, there must have been selection for changes in hearing (both changes in the ear and in how the brain processes sounds) that allowed better discrimination of speech sounds. Think of the potential advantages in being just a bit better at deciphering a hard-to-understand verbal message than other people: Eavesdropping can be a life-or-death affair. We have evidence of this, since a number of genes affecting the inner ear show signs of recent selection.3 Such genes are easy to recognize, since radical changes in them cause deafness. Combined with an increased capacity for innovation, complex speech must also have entailed an increase in the capacity for deception—and must have resulted in selective pressures for changes in personality and cognition that helped people resist Paleolithic con men.
There's a common impression that evolutionary change is inherently very slow, so that significant change always takes millions of years. A more detailed look at the fossil record, combined with evidence from contemporary examples of natural selection, makes it clear that natural selection can proceed quite rapidly, and that the past consists of long periods of near-stasis (in populations that were well matched to their environments) interspersed with occasional periods of very rapid change. Those brief periods of rapid change are poorly represented in the fossil record, since fossilization is rare.
Stephen Jay Gould's position that 50,000 or 100,000 years is an "eye blink," far too short a time to see "anything in the way of evolutionary difference," is simply incorrect.4 We are surrounded by cases in which selection has caused big changes over shorter time spans, often far shorter; everything from the dog at your feet to corn on the cob is the product of recent evolution.
The most accessible examples are the products of domestication. Domesticated animals and plants often look and act very different from their wild ancestors, and in every such case, the changes took place in far less than 100,000 years. For example, dogs were domesticated from wolves around 15,000 years ago; they now come in more varied shapes and sizes than any other mammal.
Their behavior has changed as well: Dogs are good at reading human voice and gestures, while wolves can't understand us at all. Male wolves pair-bond with females and put a lot of effort into helping raise their pups, but male dogs—well, call them irresponsible. There have been substantial changes in dogs in just the past couple of centuries: Most of the breeds we know today are no older than that.
In an extreme example, the Russian scientist Dmitri Belyaev succeeded in developing a domesticated fox in only forty years.5 In each generation he selected for tameness (and only tame-ness); this eventually resulted in foxes that were friendly and enjoyed human contact, in strong contrast to wild foxes. This strain of tame foxes also changed in other ways: Their coat color lightened, their skulls became rounder, and some of them were born with floppy ears. It seems that some of the genes influencing behavior (tameness in this case) also affect other traits—so when Belyaev selected for tameness, he automatically got changes in those other traits as well. Many of these changes have occurred as side effects of domestication in a number of species—possibly including humans, as we shall see.
Changes in domesticated plants can be just as impressive. Corn, or maize, which is derived from a wild grass named teosinte, has changed wildly in only 7,000 years. It's hard to believe that maize and teosinte are closely related.
Such dramatic responses to selection aren't isolated cases— they've occurred in many domesticated species and continue to occur today. Evolutionary genetics predicts that substantial change in almost any trait is possible in a few tens of generations, and those predictions are confirmed every day. Selection is used routinely in many kinds of agriculture, and it works: It grows more corn, lots more. You can't argue with corn.
That doesn't keep some people from trying, though. One argument is that domesticated animals and plants are examples of artificial selection and so not relevant. But the process in which some gene variants are favored and gradually increase in frequency is the essence of evolutionary change for both natural and artificial selection. There is no fundamental distinction in the process, just a difference in scale. Furthermore, we have on record examples of entirely natural adaptive change over a few thousand years—the time since the end of the Ice Age.
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