Thirty years on the "broad spectrum revolution" and paleolithic demography.

A ll Paleolithic hominids lived by hunting and collecting wild foods, an aspect of existence that began to disappear only with the emergence of the farming and herding societies of the Neolithic #10,000 years ago (10 KYA). What are the roots of this remarkable economic transformation? The answer lies in equally revolutionary changes that took place within certain stone age cultures several millennia before. In 1968, Lewis R. Binford noted what appeared to be substantial diversification of human diets in middleand high-latitude Europe at the end of the Paleolithic, roughly 12–8 KYA (1). Rapid diversification in hunting, food processing, and food storage equipment generally accompanied dietary shifts, symptoms of intensified use of habitats, and fuller exploitation of the potential foodstuffs they contained. Some of this behavior was directed to grinding, drying, and storing nuts, but it also involved small animals (2–6). Kent Flannery pushed these observations further in 1969 with his ‘‘Broad Spectrum Revolution’’ (BSR) hypothesis, proposing that the emergence of the Neolithic in western Asia was prefaced by increases in dietary breadth in foraging societies just before this period (7). He argued that subsistence diversification, mainly by adding new species to the diet, raised the carrying capacity of an environment increasingly constrained by climate instability at the end of the Pleistocene. Binford’s and Flannery’s papers have stimulated much archaeological research over three decades. Inspired by the early works of Odum and Odum (8), Emlen (9), and MacArthur and Pianka (10), both archaeologists argued that economic change resulted from unprecedented demographic crowding in certain regions of the world. Some archaeologists have questioned the role of ‘‘population pressure’’ in human social evolution (6, 11), but most continue to think of demographic factors as one of several ingredients necessary to the forager-to-farmer transition or the Paleolithic to the Neolithic (5, 12–16). If density-dependent effects can play decisive roles in shaping the evolutionary histories of predator–prey systems in general (17–20), why not in cases involving human beings (21–22)? Changes in human population density are bound to influence the rates of interspecific and intraspecific contact and the availability of critical foodstuffs, as well as people’s solutions for getting enough to eat. Rapid technological change and increased densities of archaeological sites during the later Paleolithic lend some credence to this position. Evidence of increasing dietary breadth is expected to take the form of more species in the diet (7) andyor greater proportional evenness among highand low-ranked prey items in response to declining availability of preferred types. A predator can afford to ignore lowerquality prey at little cost if the chance of finding a superior type in the near future is high, which fosters a narrower diet. As the supply of preferred prey dwindles, however, broadening the diet to include common but lower-yield prey types maximizes a predator’s returns per unit expenditure by reducing search time (19). Archaeological evidence for broadening of Paleolithic diets in Eurasia is clear from greater exploitation of energy-rich nuts and large seeds, whose nutritional benefits require considerable work and equipment to extract (5). This trend is most apparent from the proliferation of milling tools after the Last Glacial Maximum (23) and, to a lesser extent, from increasing evidence of storage facilities and preserved plant parts (24, 25). Under chronically lean conditions, people should also have become less selective about what animals to hunt, rather than going hungry. Oddly, the story from the faunal evidence was much less clear than that from plants. Measures of dietary diversity in game use based on Linnean taxonomic categories (counting species or genera) register only one economic transition—that from foragers to farmers in the early Neolithic, marked by a decline in dietary breadth (26–29). What variation could be found in the taxonomic diversity of archaeofaunas over the Middle, Upper, and Epi-Paleolithic was more easily explained by climate-driven environmental changes or geographic variation in animal and plant community composition (30–32). There seemed to be no zooarchaeological support for the BSR hypotheses of expanding diet breadth in the later Paleolithic. The basic idea behind the BSR hypothesis remains a good one. The contradictions between data on plant and animal exploitation actually stem from how zooarchaeologists have tended to categorize prey animals (33). Because the cultures of interest are extinct, prey-ranking systems cannot be inferred from watching people make decisions. The relative values (payoffs) of prey must instead be evaluated from knowledge of modern variants of the animals whose bones occur in archaeological deposits. Species and genera present the most obvious analytical categories, and the most literal expectation of Flannery’s BSR hypothesis is indeed more species in the diet andyor more even emphasis on those species. Thus diet variation normally is examined in terms of indices of taxonomic richness (N-species or N-genera) and taxonomic evenness (proportionality in abundance) (26, 28, 31, 34). Such analyses use either Kintigh’s simulation-based technique (35) or a more longstanding regression approach (36) developed from the work of Fisher, Corbet, and Williams (37) and others for problems of sampling in modern community ecology. The main weakness of diversity approaches that rely on Linnean taxonomic units is their insensitivity to physical and behavioral differences among prey animals. The only qualification normally added to these analyses is prey body size, because all game animals are composed of similar tissues, and large animals yield much more food than small ones, even if they are more difficult to catch. This practice potentially overlooks great differences in prey-handling costs and the longterm price of heavy exploitation among animals that are broadly equivalent in food content and package size. In fact, some distantly related taxa are nearly equivalent from the viewpoint of handling costs because of their locomotor habits or ways of avoiding predators: both tortoises and rock-dwelling marine shellfish are