Anthropological progress in the 20th century made this less and less tenable. In several parts of the world, archaeologists accumulated evidence that the domestication of plants and animals had been a long, gradual process of change in which species found wild in local environments came slowly to resemble the domesticated forms of today (e.g., MacNeish 1978). 

Cultural anthropologists, meanwhile, were learning that hunting-gathering peoples possess extensive knowledge of the plant and animal life around them. The fact that plants grow from seeds, for example, was not a profound mystery but common knowledge. Furthermore, the foraging life, even in difficult environments such as the Kalahari Desert in south Africa, proved to be much "cushier" than had been believed (e.g., Lee 1984). None of this seemed to fit at all with the old theory. If foraging for food was usually a relatively easy life style, why did people ever begin growing food? And why, when they finally did (after millions of years of foraging), did it happen so slowly and in so many places? 

The pieces of the puzzle were assembled beautifully by the archaeologist Mark Nathan Cohen. Influenced by earlier writers, especially Ester Boserup (1965), he proposed population pressure as the key. The beginnings of agriculture some ten thousand years ago approximately coincided, Cohen pointed out, with the end of the long process of human expansion throughout the habitable portions of the planet. As population continued to grow with nowhere new to go, global density would have begun to increase rapidly; wild plant and animal food sources gradually were ever less sufficient for human survival. Our ancestors took up farming only when, and to the extent that, they had to. 

An especially nice feature of this theory is that it explains why, after several millions of years of human existence, agriculture cropped up so many places within a mere few thousand years. Study of recent foragers demonstrates that individuals move rather freely between bands, and that the bands themselves move frequently over the landscape. Both kinds of movements often are in response to resource distributions. (Among the Mbuti pygmies, for instance, newlyweds go to live with either the bride's or groom's band, depending usually on where food is most plentiful at the time.) These "flux" mechanisms, then, distribute population relative to resources (Turnbull 1968). During the human expansion out of the tropics into the rest of the world, an expansion that began one or two million years ago, our ancestors had been foragers too; it therefore is a safe bet that flux mechanisms operated day in and day out over the millenia, constantly distributing and redistributing population relative to food resources. When expansion at last had to end, but population kept growing, the pressure on wild resources would have increased sharply all over the world. Cohen's theory thus tied together findings of archaeologists and cultural anthropologists to produce the best general theory we have of this great transformation in cultural evolution. 

One slight problem is the fact that archaeological evidence of hunter-gatherers throughout the New World is not quite as ancient as the theory implies.  However, as continuing research pushes back estimates for the peopling of the New World, Cohen's theory will look better and better.