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Macrovertebrate Paleontology and the Pliocene Habitat of Ardipithecus ramidus

Tim D. White et al.


Ever since Darwin, scholars have speculated about the role that environment may have played in human origins, evolution, and adaptation. Given that all living great apes live and feed in trees, it has been assumed that the last common ancestor we shared with these forms was also a forest-dweller. In 1925, Raymond Dart described the first Australopithecus, a child’s skull, at Taung, South Africa. Its occurrence among other fossils indicative of a grassland environment prompted speculation that the open grasslands of Africa were exploited by early hominids and were therefore somehow integrally involved with the origins of upright walking.

The Ardipithecus-bearing sediments at Aramis now provide fresh evidence that Ar. ramidus lived in a predominantly woodland setting. This and corroborative evidence from fossil assemblages of avian and small mammals imply that a grassland environment was not a major force driving evolution of the earliest hominids. A diverse assemblage of large mammals (>5 kg body weight) collected alongside Ardipithecus provides further support for this conclusion. Carbon isotopes from tooth enamel yield dietary information because different isotope signatures reflect different photosynthetic pathways of plants consumed during enamel development. Therefore, animals that feed on tropical open-environment grasses (or on grass-eating animals) have different isotopic compositions from those feeding on browse, seeds, or fruit from shrubs or trees. Moreover, oxygen isotopes help deduce relative humidity and evaporation in the environment.

The larger-mammal assemblage associated with Ardipithecus was systematically collected across a ~9-km transect of eroding sediments sandwiched between two volcanic horizons each dated to 4.4 million years ago. It consists of ~4000 cataloged specimens assigned to ~40 species in 34 genera of 16 families.

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Carbon and oxygen isotope analyses of teeth from the Ar. ramidus localities. Species are listed in order of abundance, and isotopic data separate species by what they ate and their environment.

There are only three primates in this assemblage, and the rarest is Ardipithecus, represented by 110 specimens (a minimum of 36 individuals). Conversely, colobine monkeys and a small baboon-like monkey (red crosses in figure) account for nearly a third of the entire large mammal collection. Leaf-eating colobines today exhibit strong preferences for arboreal habitats, and the carbon isotope compositions of the fossil teeth are consistent with dense to open forest arboreal feeding (see figure).

The other dominant large mammal associated with Ar. ramidus is the spiral-horned antelope, Tragelaphus (the kudu, green circle). Today, these antelopes are browsers (eating mostly leaves), and they prefer bushy to wooded habitats. The dental morphology, wear, and enamel isotopic composition of the Aramis kudu species are all consistent with such placement. In contrast, grazing antelopes (which eat mostly grass) are rare in the Aramis assemblage.

The large-mammal assemblage shows a preponderance of browsers and fruit eaters. This evidence is consistent with indications from birds, small mammals, soil isotopes, plants, and invertebrate remains. The emergent picture of the Aramis landscape during Ar. ramidus times is one of a woodland setting with small forest patches. This woodland graded into nearby habitats that were more open and are devoid of fossils of Ardipithecus and other forest to woodland community mammals. Finally, the carbon isotopic composition of Ar. ramidus teeth is similar to that of the predominantly arboreal, small, baboon-like Pliopapio and the woodland browser Tragelaphus, indicating little dietary intake of grass or grass-eating animals. It is therefore unlikely that Ar. ramidus was feeding much in open grasslands.

These data suggest that the anatomy and behavior of early hominids did not evolve in response to open savanna or mosaic settings. Rather, hominids appear to have originated and persisted within more closed, wooded habitats until the emergence of more ecologically aggressive Australopithecus.

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