proportions even at the dawn of animal domestication) fails to make the crash disappear.

Significantly, even though human biomass was rising dramatically at the time, that rise was not enough to balance the biomass lost from the megafauna that were going extinct. Therefore, more was at work than a simple biomass tradeoff among megafauna. The suddenness of the crash, its magnitude, and its distribution across three continents suggest a global threshold event (Scheffer et al., 2001; Scheffer and Carpenter, 2003). Threshold events by definition are sudden changes to alternative ecosystem states induced either by some gradual change reaching a critical value or by abnormally strong perturbations. In either case, the net effect is to push the system from one “basin of attraction” (in this case, a world where megafauna body mass is distributed across many species) into a different one (a world where most megafauna body mass is concentrated around humans). The cause of the QME threshold event may well reflect a synergy of reaching a critical value of human biomass at the same time that ecologically unusual perturbations came into play. The unusual perturbations included increasingly sophisticated hunting of megafauna by people, habitat alteration by growing human populations, climate changes that would have decreased total global ecological energy at least temporarily, and possibly a comet impact.

The potentially dramatic effects of hunting (so-called overkill) have been most convincingly demonstrated by simulations of the effects of Clovis hunters first entering North America (Alroy, 2001), which occurred just as global human biomass began to steeply rise. Those simulations suggest that hunting alone would result in many extinctions, because humans killed megafauna for food (Alroy, 2001).

Similarly comprehensive simulations have not yet been done for other continents, but at least indirect human impacts (including habitat alteration and fragmentation) seem likely, given the coincidence of the megafauna biomass crash after first entry or markedly increasing population sizes of humans into various regions. These coincidences include first entry of humans into South America (near 14.6 kyr B.P.) and the entry and population growth of H. sapiens into Eurasia (from ≈40 kyr B.P.). Entry of humans also precedes the QME in Australia, although there, both were earlier than the worldwide biomass crash. In parts of Australia (Miller et al., 2005), North America (Burney et al., 2005), and South America (Moreno, 2000), the evidence for indirect human impacts includes sedimentary records of increasing fire frequency, potentially indicating widespread habitat alteration through human-set fires. The indirect or direct role of humans in the QME also is suggested by the observation that the main megafauna survivors had habitat preferences that would have kept them farthest from humans (Johnson, 2002).