River-margin habitat of Ardipithecus ramidus at Aramis, Ethiopia 4.4 million years ago

Abstract

The nature and type of landscape that hominins (early humans) frequented has been of considerable interest. The recent works on Ardipithecus ramidus, a 4.4 million years old hominin found at middle Awash, Ethiopia, provided critical information about the early part of human evolution. However, habitat characterization of this basal hominin has been highly contested. Here we present new sedimentological and stable isotopic (carbon and oxygen) data from Aramis, where the in situ, partial skeleton of Ar. ramidus (nicknamed 'Ardi') was excavated. These data are interpreted to indicate the presence of major rivers and associated mixed vegetations (grasses and trees) in adjacent floodplains. our finding suggests that, in contrast to a woodland habitat far from a river, Ar. ramidus lived in a river-margin forest in an otherwise savanna (wooded grassland) landscape at Aramis, Ethiopia. Correct interpretation of habitat of Ar. ramidus is crucial for proper assessment of causes and mechanisms of early hominin evolution, including the development of bipedalism.

On the Environment of Aramis

“…Within this succession, the Ar. ramidus-bearing rock unit comprises silt and clay beds deposited on a floodplain," an interpretation also preferred independently by Gani and Gani (2011), who also found strong evidence of fluvial sedimentation and suggested the presence of riparian environments. This agrees with Cerling et al (2010), who suggested "a riparian corridor bordered by mixed and more open environments, including woody grasslands with !…”

Parasites and human evolution

“…Since the lice life cycle does not include a free‐living phase, the inferred Pthirus host species transfer would imply spatiotemporal overlap and physical contact between hominins and ancestral gorillas, probably between 4 and 3 MYA. A dearth of fossils makes it impossible to confidently reconstruct Pliocene gorilla habitats, and reconstructions of hominin paleoenvironments have been controversial . Nevertheless, given the absence of hominin and gorilla fossil co‐occurrence at sites in eastern or southern Africa and the tropical forest habitats of modern gorillas, the Pthirus lice speciation pattern suggests that our hominin ancestors may have inhabited a more diverse range of environments than typically is assumed.…”

“…A dearth of fossils makes it impossible to confidently reconstruct Pliocene gorilla habitats, and reconstructions of hominin paleoenvironments have been controversial. [34][35][36] Nevertheless, given the absence of hominin and gorilla fossil co-occurrence at sites in eastern or southern Africa and the tropical forest habitats of modern gorillas, the Pthirus lice speciation pattern suggests that our hominin ancestors may have inhabited a more diverse range of environments than typically is assumed.…”

Hominin taxic diversity: Fact or fantasy?

“…Some specimens found above the DABT are overlain by the Wodara Basaltic Tuff (WOBT) ). WOBT, which is not dated, lies underneath the 4.317 6 0.055 Ma Kullunta Basaltic Tuff (KUBT), and Gani and Gani (2011) argue that an average sedimentation rate of 0.5 mm yr 21 between DABT and KUBT indicates that WOBT was likely deposited within 5 ky of the deposition of the DABT. Until an age for WOBT is determined, we use 4.262 Ma for the minimum LAD of Ar.…”