Australopithecus afarensis
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Australopithecus afarensis is an extinct hominid which lived between 3.9 and 2.9 million years ago. Similar to the younger Australopithecus africanus, A. afarensis was slenderly built. It has been thought that A. afarensis was an ancestor to both the genus Australopithecus and the genus Homo, which includes the modern human species, Homo sapiens.
[change] Sexual dimorphism in Australopithicus Afarensis
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Afarensis can be considered the evolutionary bridge between the Australopithecines and early Homo. Splitting form this more primitive Australopithecine, two divergent paths emerge: from gracile to more robust species of Australopithecus, and the unknown species that led to early Homo. The earliest known members of Homo were contemporaries to the later A. africanus. A commonly accepted idea is that A. afarensis was sexually dimorphic. Males were thought to be substantially larger than females, complete with heavy cresting. Present in the dentiton of A. afarensis is the diastema common in chimpanzees, suggesting that the males may have possibly had enlarged canines (useful for aggressive display behavior which establishes hierarchical status in social species as well as for mate acquisition). As a whole, A. afarensis seems to have been more ape-like than human in dentition, though it does display a marked reduction of the canines. This reduction is progressive throughout all later hominids. A generally accepted idea is that A. afarensis was more chimp-like than human-like in its reproductive strategy, with high amounts of sexual competition for mates. It has been theorized that, like chimp females, A. afarensis females would’ve been highly promiscuous. Males would compete for breeding rights with aggressive display behavior. Sperm competition would be appropriately high, a common feature in promiscuous primate species. However, there are many within the scientific community who doubt a chimp/ A. afarensis homologue in breeding strategies, in that they diverge from chimpanzees in many key respects. Most notably, A. afarensis utilized a completely different locomotor apparatus (i.e. bipedalism), had substantially larger brains, and lived in a vastly different environment than modern chimps. Recent studies have suggested that previous assumptions of marked sexual dimorphism within A. afarensis may have been wrong. Evolutionary biologists Philip Reno and Owen Lovejoy have posited the possibility of more human-like differences between the sexes. The implications for a more monomorphic model of A. afarnesis hint at the possibility a pair-bonding reproductive strategy. An analogue of this principal in the modern world would be the obligate monogamous species of marmosets, which are also monomorphic. It has also been discovered in studies on prairie voles that no single gene determines monogamous behavior in mammals. Instead, monogamous reproductive strategies emerge as a response to environmental pressures, genetic influence, physiological adaptations, and models of social behavior. The interplay of these factors determines a species own unique reproductive strategy. So, by placing A. afarensis within a model of a pair-bonding species which engaged in at least habitual monogamy, the data uncovered by Reno et al seems to support the idea that monomorphism within A. afarensis was a result of a shift in breeding strategy diverging from the more chimp-like life histories of early Austalopithecines. This would explain the reduction of canine size in A. afarensis as being correlated with a reduction of male display behavior and male/male competition for mates. If A. afarensis did in fact use a pair-bonding reproductive strategy, it would have far-reaching implications in the development of our own uniquely human reproductive strategy as well our most basic social institution (the family).
