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Reexamining Human Origins in Light of Ardipithecus ramidus

C. O. Lovejoy

Chimpanzees, bonobos, and gorillas are our closest living relatives. The most popular reconstructions of human evolution during the past century rested on the presumption that the behaviors of the earliest hominids were related to (or even natural amplifications of) behaviors observed in these living great apes. One effect of chimpanzee-centric models of human evolution has been a tendency to view Australopithecus as transitional between an ape-like ancestor and early Homo.

Ardipithecus ramidus nullifies these presumptions, as it shows that the anatomy of living African apes is not primitive but instead has evolved specifically within extant ape lineages. The anatomy and behavior of early hominids are therefore unlikely to represent simple amplifications of those shared with modern apes. Instead, Ar. ramidus preserves some of the ancestral characteristics of the last common ancestor with much greater fidelity than do living African apes. Two obvious exceptions are its ability to walk upright and the absence of the large projecting canine tooth in males, derived features that Ardipithecus shares with all later hominids.

Ar. ramidus illuminates our own origins because it clarifies our relationship to Australopithecus. For example, the enlarged rear teeth of Australopithecus have long been viewed as adaptations to a rough, abrasive diet. This has led to speculation that canine teeth might have become smaller simply to accommodate the emergence of these other enlarged teeth, or that the importance of canine teeth in displays of male-to-male aggression waned with the development of weapons. Ar. ramidus negates such hypotheses because it demonstrates that small canines occurred in hominids long before any of the dental modifications of Australopithecus or the use of stone tools. The loss of large canine teeth in males must have occurred within the context of a generalized, nonspecialized diet. Comparisons of the Ar. ramidus dentition with those of all other higher primates indicate that the species retained virtually no anatomical correlates of male-to-male conflict. Consistent with a diminished role of such agonism, the body size of Ar. ramidus males was only slightly larger than that of females.

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Breakthrough adaptations can transform life-history by deviating from typical reproductive strategy. Early hominids show feminized male canines (left) and primitive bipedality (right). These suggest that females preferred nonaggressive males who gained reproductive success by obtaining copulation in exchange for valuable foods (vested provisioning). Success would depend on copulatory frequency with mates whose fertility remained cryptic (e.g., absence of cycling in mammary size). The result would be reduced agonism in unrelated females, and cooperative expansion of day ranges among equally cooperative males, eventually leading to exploitation of new habitats. Credit: Illustration of Ar. ramidus: copyright J. H. Matternes.

The discovery of Ar. ramidus also requires rejection of theories that presume a chimpanzee- or gorilla-like ancestor to explain habitual upright walking. Ar. ramidus was fully capable of bipedality and had evolved a substantially modified pelvis and foot with which to walk upright. At the same time, it preserved the ability to maneuver in trees, because it maintained a grasping big toe and a powerful hip and thigh musculature. Because upright walking provided no energy advantage for Ar. ramidus (it lacked many of the adaptations evolved in later hominids such as Australopithecus), reproductive success must have been central to its evolution in early hominids.

Loss of the projecting canine raises other vexing questions because this tooth is so fundamental to reproductive success in higher primates. What could cause males to forfeit their ability to aggressively compete with other males? What changes paved the way for the later emergence of the energy-thirsty brain of Homo? Such questions can no longer be addressed by simply comparing humans to extant apes, because no ape exhibits an even remotely similar evolutionary trajectory to that revealed by Ardipithecus.

When the likely adaptations of early hominids are viewed generally rather than with specific reference to living chimpanzees, answers to such questions arise naturally. Many odd hominid characteristics become transformed from peculiar to commonplace. Combining our knowledge of mammalian reproductive physiology and the hominid fossil record suggests that a major shift in life-history strategy transformed the social structure of early hominids. That shift probably reduced male-to-male conflict and combined three previously unseen behaviors associated with their ability to exploit both trees and the land surface: (i) regular food-carrying, (ii) pair-bonding, and (iii) reproductive crypsis (in which females did not advertise ovulation, unlike the case in chimpanzees). Together, these behaviors would have substantially intensified male parental investment—a breakthrough adaptation with anatomical, behavioral, and physiological consequences for early hominids and for all of their descendants, including ourselves.

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