The Ice Age ended (or paused, at any rate) some 11,500 years ago. That caused dramatic environmental changes in many parts of the world, especially in the Northern Hemisphere. The American Southwest turned warmer and drier, becoming the desert it is today, and as it did, the creosote bush appeared there.
Originally from Argentina, its seeds were probably transported north by migratory birds. It thrived in the desert, thanks to its resin-coated leaves; dense lateral roots, which starve out competing plants; and taproot, which can grow up to 15 feet deep into the earth. A number of insects now live in and on creosote bushes: Some have become so specialized that they can eat nothing else. The creosote bush walking-stick looks just like the creosote stems, while a grasshopper, which even has silvery patches that match the shine of the plant's resin, mimics the leaves. All of these creosote-specialized insects in the Southwest have North American ancestors, not South American—and so all their specializations have come into existence in the past 10,000 years.6
The end of the Ice Age also brought about a global rise in sea level. Mile-thick continental ice sheets melted, and the sea level rose hundreds of feet. As the waters rose, some mountains became islands, isolating small groups of various species. These islands were too small to sustain populations of large predators, and in their absence the payoff for being huge disappeared. Instead, small elephants had an advantage over large ones, probably because they required less food and reproduced more rapidly. Over a mere 5,000 years, elephants shrank dramatically, from an original height of 12 feet to as little as 3 feet. It is worth noting that elephant generations are roughly twenty years long, similar to those of humans.
But simply getting smaller is hardly a dramatic example of evolution. Indeed, John Tooby and Leda Cosmides (two of the founders of modern evolutionary psychology) have said that "given the long human generation time, and the fact that agriculture represents less than 1 percent of the evolutionary history of the genus Homo, it is unlikely that we have evolved any complex adaptations to an agricultural (or industrial) way of life."7 A complex adaptation is a characteristic contributing to reproductive fitness that involves the coordinated actions of many genes. This means that humans could not have evolved wings, a third eye, or any new and truly complicated adaptive behavior in that time frame. Tooby and Cosmides have argued elsewhere that, therefore, deep mental differences between human populations cannot exist.8
We think that this argument concerning the evolution of new complex adaptations is correct, but it underestimates the importance of simple adaptations, those that involve changes in one or a few genes. The conclusion that all humans are effectively the same is unwarranted. We will see not only that we have been evolving at a rapid rate, but that evolution has taken a different course in different populations. Over time, we have become more and more unlike one another as differences among populations have accumulated.
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