Unlike any great apes, humans have expanded into a wide variety of habitats during the course of evolution, beginning with the transition by australopithecines from forest to savanna habitation. Novel environments are likely to have imposed hominids a demographic challenge due to such factors as higher predation risk and scarcer food resources. In fact, recent studies have found a paucity of older relative to younger adults in hominid fossil remains, indicating considerably high adult mortality in australopithecines, early Homo, and Neanderthals. It is not clear to date why only human ancestors among all hominoid species could survive in these harsh environments. In this talk we explore the possibility that hominids had shorter interbirth intervals to enhance fertility than the extant apes. To infer interbirth intervals in fossil hominids, we introduce the notion of the critical interbirth interval, or the threshold length of birth spacing above which a population is expected to go to extinction. We develop a new method to obtain the critical interbirth intervals of hominids based on the observed ratios of older adults to all adults in fossil samples. Our analysis shows that the critical interbirth intervals of australopithecines, early Homo, and Neanderthals are significantly shorter than the observed interbirth intervals of extant great apes. This result suggests that the child care burden of hominid mother was reduced by other group members, including males. In fact, small canine of early hominids may indicate increasing importance of male provisioning, relative to male-male aggression, for males to gain higher reproductive success. To discuss evolutionary causes of these hominid features, we develop a mathematical model to obtain the condition for paternal care to evolve.