food-sharing would have favored the development of language, social reciprocity and the intellect,” he told a 1982 gathering that marked the centenary of Darwin’s death.
Five patterns of behavior separate humans from our ape relatives, he wrote in his 1978 paper: (1) a bipedal mode of locomotion, (2) a spoken language, (3) regular, systematic sharing of food in a social context, (4) living in home bases, (5) the hunting of large prey. These describe modern human behavior, of course. But, Isaac suggested, by 2 million years ago “various fundamental shifts had begun to take place in hominid social and ecological arrangements.” They were already hunter-gatherers in embryo, living in small, mobile bands and occupying temporary camps from which the males went out to hunt prey and the females to gather plant foods. The camp provided the social focus, at which food was shared. “Although meat was an important component of the diet, it might have been acquired by hunting or by scavenging,” Isaac told me in 1984, a year before his tragically early death. “You would be hard pressed to say which, given the kind of evidence we have from most archeological sites.”
Isaac’s viewpoint strongly influenced the way the archeological record was interpreted. Whenever stone tools were discovered in association with the fossilized bones of animals, it was taken as an indication of an ancient “home base,” the meager litter of perhaps several days’ activity of a band of hunter-gatherers. Isaac’s argument was plausible, and I wrote in my 1981 book The Making of Mankind that “the food-sharing hypothesis is a strong candidate for explaining what set early humans on the road to modern man.” The hypothesis seemed consistent with the way I saw the fossil and archeological records, and it followed sound biological principles. Richard Potts, of the Smithsonian Institution, agreed. In his 1988 book titled Early Hominid Activities at Olduvai , he observed that Isaac’s hypothesis “seemed a very attractive interpretation,” noting:
The home-base, food-sharing hypothesis integrates so many aspects of human behavior and social life that are important to anthropologists—reciprocity systems, exchange, kinship, subsistence, division of labor, and language. Seeing what appeared to be elements of the hunting-and-gathering way of life in the record, in the bones and stones, archeologists inferred that the rest followed. It was a very complete picture.
In the late 1970s and early 1980s, however, this thinking began to change, prompted by Isaac and by the archeologist Lewis Binford, then at the University of New Mexico. Both men realized that much of prevailing interpretation of the prehistoric record was based on unspoken assumptions. Independently, they began to separate what could truly be known from the record from what was simply assumed. It began at the most fundamental level, questioning the significance of finding stones and animal bones in the same place. Did this spatial coincidence imply prehistoric butchery, as had been assumed? And if butchery could be proved, does that imply that the people who did it lived as modern hunter-gatherers do?
Isaac and I talked often about various subsistence hypotheses, and he would create scenarios in which bones and stones might finish up in the same place but have nothing to do with a hunting-and-gathering way of life. For instance, a group of early humans might have spent some time beneath a tree simply for the shade it afforded, knapping stones for some purpose other than butchering carcasses—for example, they might have been making flakes for whittling sticks, which could be used to unearth tubers. Some time later, after the group had moved on, a leopard might have climbed the tree, hauling its kill with it, as leopards often do. Gradually, the carcass would have rotted and the bones would have tumbled to the ground to lie amid the scatter of stones left there by the toolmakers.