Bipedalism — walking upright on two legs — us a defining feature of the human lineage. It is thought to have evolved as forests retreated in the Late Miocene to Pliocene period. 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. In new research, scientists focused on a community of eastern 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 human bipedalism.
Drummond-Clarke et al. show that trees were an essential component of the hominin adaptive niche, with bipedalism evolving in an arboreal context, likely driven by foraging strategy. Illustration by Arturo Asensio, via Quo.es.
“Terrestrial (land-based) bipedalism is a defining feature of modern humans, and its morphological adaptations are critical to distinguishing fossils that fall within the human clade (hominins) from those of other apes (hominoids) over the past 7 million years,” said University of Kent researcher Rhianna Drummond-Clarke and colleagues.
“The shift to more arid and open environments in the Late Miocene-Pliocene (10 to 2.5 million years ago) has played a central role in hypotheses about hominin evolution.”
“In particular, the emergence and evolution of bipedalism is often considered to be a key adaptation to more open, dry habitats — termed ‘savanna,’ which includes wooded habitats with a grassy understory rather than only treeless grassland assumed in traditional ‘savanna hypotheses’ — in which hominins reduced the time spent in trees and increased terrestrial foraging and traveling as forests retreated.”
“Paleoenvironmental reconstructions indicate that early hominins were not living in tropical forests common to most extant apes today,” they said.
“Instead, the earliest fossil hominins, including Orrorin, Ardipithecus, and early Australopithecus, would have moved and foraged in mosaic savanna habitats dominated by woodland with strips of riparian forest vegetation, often termed ‘savanna-woodland’ or ‘savanna-mosaic’.”
“Compared to tropical forest, these savanna-mosaic habitats would have elicited different selective pressures associated with reduced tree density and increased seasonality.”
In their research, Drummond-Clarke and co-authors explored the behaviors of wild eastern chimpanzees living in the Issa Valley of western Tanzania, within the region of the East African Rift Valley.
“Issa Valley is characterized as a savanna-mosaic similar to the paleoenvironments reconstructed for the early hominins Orrorin, Ardipithecus ramidus, and Australopithecus afarensis and hosts a recently habituated chimpanzee community,” they explained.
“Issa chimpanzees are well situated for testing the savanna effect on chimpanzee positional behavior, not only through comparison to forest-dwelling communities but also by comparing how individuals adjust their positional behavior across vegetation types within a savanna-mosaic habitat.”
The researchers recorded more than 13,700 instantaneous observations of positional behavior from 13 chimpanzee adults (six females and seven males), including almost 2,850 observations of individual locomotor events (e.g., climbing, walking, hanging, etc.), over the course of the 15-month study.
They then used the relationship between tree/land-based behavior and vegetation (forest vs woodland) to investigate patterns of association.
Similarly, they noted each instance of bipedalism and whether it was associated with being on the ground or in the trees.
“We found that the Issa chimpanzees spent as much time in the trees as other chimpanzees living in dense forests, despite their more open habitat, and were not more terrestrial as expected,” they said.
“Furthermore, although we expected the Issa chimpanzees to walk upright more in open savanna vegetation, where they cannot easily travel via the tree canopy, more than 85% of occurrences of bipedalism took place in the trees.”
Despite these findings, why humans alone amongst the apes first began to walk on two feet still remains a mystery.
“To date, the numerous hypotheses for the evolution of bipedalism share the idea that hominins came down from the trees and walked upright on the ground, especially in more arid, open habitats that lacked tree cover. Our data do not support that at all,” said Dr. Fiona Stewart, a researcher at University College London.
“Unfortunately, the traditional idea of fewer trees equals more terrestriality just isn’t borne out with the Issa data.”
“What we need to focus on now is how and why these chimpanzees spend so much time in the trees — and that is what we’ll focus on next on our way to piecing together this complex evolutionary puzzle.”
The results were published in the journal Science Advances.
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Rhianna C. Drummond-Clarke et al. 2022. Wild chimpanzee behavior suggests that a savanna-mosaic habitat did not support the emergence of hominin terrestrial bipedalism. Science Advances 8 (50); doi: 10.1126/sciadv.add9752
