More advanced tools and techniques could squeeze more food from a given amount of land; but what if traditional methods yielded more food for a given amount of labor? Boserup launched into a detailed study of several forms of agriculture that convinced her that those involving simple tools and long periods of fallow indeed were more labor-efficient. The greatest single reason seemed to be that at low population densities, people could afford to farm only a small portion of their land each year. By the time they returned to a given plot to plant a garden, forest had reclaimed it. This seeming disadvantage was in fact a huge advantage: forested land could be cleared well enough for planting merely by slashing down the young forest vegetation, burning it, and sowing seeds in the ash. Stumps of larger trees were simply planted around. Neither plowing, nor even hoeing, was necessary. Such “low-tech” farming paid off very handsomely for no more labor than it took. Fire was the secret key, low density the necessary precondition.

So why did agriculture change? Probably, Boserup wrote, due to increasing density of human population. When land had to be planted before forest had had time to reclaim it, fire was less effective for clearing because fire does not destroy the roots of thick grass (not a problem once forest has taken over). Hoes, and eventually plows, had to be adopted due to increasing population pressure.

Before Boserup, land had been thought of, by scientists, as either cultivated or uncultivated. An important feature of her analysis was a five-stage sequence based on how frequently a plot was cultivated. The “least intensive” stage involved cropping only about once every twenty years; the most intensive, more than once a year, which usually required much labor for fertilizing and irrigating.

In presenting a set of progressive stages, Boserup’s work harked back to a 19th-century approach, used by such scholars as Lewis Henry Morgan, that had fallen into prolonged disfavor among anthropologists. But Boserup’s stages, by being carefully defined and by allowing land use to be measured more accurately than ever before, proved an asset rather than a liability. Her work deservedly had a huge and continuing impact on anthropology in general, and on cultural evolutionism in particular (see Spooner 1972).