Wild chimpanzee behavior suggests that a savanna-mosaic habitat did not support the emergence of hominin terrestrial bipedalism

Abstract

Bipedalism, a defining feature of the human lineage, is thought to have evolved as forests retreated in the late Miocene-Pliocene. Chimpanzees living in analogous habitats to early hominins offer a unique opportunity to investigate the ecological drivers of bipedalism that cannot be addressed via the fossil record alone. We investigated positional behavior and terrestriality in a savanna-mosaic community of chimpanzees ( Pan troglodytes schweinfurthii ) in the Issa Valley, Tanzania as the first test in a living ape of the hypothesis that wooded, savanna habitats were a catalyst for terrestrial bipedalism. Contrary to widely accepted hypotheses of increased terrestriality selecting for habitual bipedalism, results indicate that trees remained an essential component of the hominin adaptive niche, with bipedalism evolving in an arboreal context, likely driven by foraging strategy.

The influence of multiple variables on bipedal context in wild chimpanzees: implications for the evolution of bipedality in hominins

“…As in studies of great ape locomotion using behavioral frequency data, we found that arboreal bipedalism occurred chiefly in a feeding/ foraging context (Hunt, 1996(Hunt, , 1998Thorpe et al, 2007;Drummond-Clarke et al, 2022). Bipedality during feeding/foraging was principally by adult individuals and was also associated with hand usage on arboreal substrates, as in studies of both chimpanzees (Hunt, 1996(Hunt, , 1998 and orangutans (Thorpe et al, 2007).…”

“…As noted above, previous behavioral frequency studies in wild chimpanzees (Hunt, 1996(Hunt, , 1998Drummond-Clarke et al, 2022) found that foraging was the primary behavioral context eliciting bipedalism. However, most of the bipedalism documented in these studies was postural rather than locomotor (ibid.).…”

“…In a study by Drummond-Clarke et al (2022), 109 bipedal observations were made, and as in Hunt's work, were predominantly postural (79% postural vs. 21% locomotor). Also as in Hunt's work, bipedality occurred in a foraging context, and was overwhelmingly arboreal (86% of observations), despite trees being less available than in Hunt's studies (Drummond-Clarke et al, 2022). These findings were thus viewed as strengthening hypotheses associating bipedal origins with forested habitats (Drummond-Clarke et al, 2022).…”

African apes and the evolutionary history of orthogrady and bipedalism

“…Many have imagined arboreal bipedalism in hylobatids and great apes as "preadaptive" for hominin terrestrial bipedalism (Almécija et al, 2021;Böhme et al, 2019;Crompton et al, 2008;Crompton, 2016;DeSilva, 2021;Drummond-Clarke et al, 2022;Filler, 2007a;Gregory, 1928;Keith, 1923;Kelley, 1986;Morton, 1926;Tuttle, 1974;Rosen et al, 2022;Senut et al, 2018;Thorpe et al, 2007Thorpe et al, , 2014. However, in neglecting the numerous morphological and behavioral synapomorphies of African apes, including some features that are retained in hominins, these models favor parallel and unparsimonious evolutionary transformations for the separate hominine (African ape and human) lineages.…”

A review of the distal femur in Australopithecus

“…This in turn will have different effects on the transmission of forces through the knee joint in these groups. Furthermore, Drummond‐Clark et al 36 have shown that bipedal posture in chimpanzees occurs more frequently in forest conditions than in the savanna making it unclear what the relationship is between bipedalism and bicondylar angle expression in chimpanzees.…”