The hypothesis that megafauna extinction some 10,000 years ago was due to "overkill" by Paleolithic hunters is examined using an economic model of a replenishable resource. The large herding animals that became extinct, such as mammoth, bison, camel, and mastodon, presented low hunting cost and high kill value. The absence of appropriation provided incentives for the wastage killing evident in some kill sites, while the slow growth, long lives, and long maturation of large animals increased their vulnerability to extinction. Free-access hunting is compared with socially optimal hunting and used to interpret the development of conservationist ethics, and controls, in more recent primitive cultures.