In an interesting example of how science constantly revises what it thinks it knows and changes its mind when the information changes, and quite incidentally, routinely refutes Creationists claims, a team of researchers from UCL, the University of Kent, and Duke University, USA, has found what they believe is evidence that bipedalism evolved in remote human ancestors because they walked upright in trees, rather than because they abandoned the trees and took to living in open, relatively treeless savanna. This is the reverse of that is commonly accepted.
Briefly, the scientific consensus was that climate change led to a reduction of the African forests and left the ancestors of the hominins stranded in open savanna and isolated from their forest-dwelling cousin, the ancestral chimpanzee, where they evolved upright walking and the hominin foot, while retaining much of the upper body of their chimpanzee cousins with whom they shared a common ancestor. This suggested that they brachiated in trees using their arms an upper body, while they walked upright on the ground.
However, by observing a troupe of modern chimpanzees living in the Issa Valley of western Tanzania, within the region of the East African Rift Valley, the scientists noted that 85% of the time they spent walking upright was in trees, rather than on the ground. The Issa Valley is what is known as 'savanna-mosaic’ – a mix of dry open land with few trees and patches of dense forest, much like the environment it is believed the earliest hominins inhabited and in which bipedalism evolved.
As explained in UCL News:
The study is the first of its kind to explore if savanna-mosaic habitats would account for increased time spent on the ground by the Issa chimpanzees, and compares their behaviour to other studies on their solely forest-dwelling cousins in other parts of Africa.
Overall, the study 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 (land-based) as expected.
Furthermore, although the researchers 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.
The authors say that their findings contradict widely accepted theories that suggest that it was an open, dry savanna environment that encouraged our prehistoric human relatives to walk upright – and instead suggests that they may have evolved to walk on two feet to move around the trees.We naturally assumed that because Issa has fewer trees than typical tropical forests, where most chimpanzees live, we would see individuals more often on the ground than in the trees. Moreover, because so many of the traditional drivers of bipedalism (such as carrying objects or seeing over tall grass, for example) are associated with being on the ground, we thought we’d naturally see more bipedalism here as well. However, this is not what we found.
Our study suggests that the retreat of forests in the late Miocene-Pliocene era around five million years ago and the more open savanna habitats were in fact not a catalyst for the evolution of bipedalism. Instead, trees probably remained essential to its evolution – with the search for food-producing trees a likely a driver of this trait.
Dr Alex K. Piel, corresponding-author
Department of Anthropology
University College London, London, UK.
To date, the numerous hypotheses for the evolution of bipedalism share the idea that hominins (human ancestors) 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.
Unfortunately, the traditional idea of fewer trees equals more terrestriality (land dwelling) 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.
Dr Fiona Stewart, co-author
Department of Anthropology
University College London, London, UK. And School of Biological and Environmental Sciences
Liverpool John Moores University
Liverpool, UK.To establish their findings, the researchers recorded more than 13,700 instantaneous observations of positional behaviour 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 behaviour 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.Many fossil human relatives have ape-like forelimbs – with long arms, more mobile shoulders and curved fingers – that are useful for swinging and climbing in the trees. While some have argued that these ‘tree adaptations’ are remnants from a more ape-like, arboreal past and no longer functionally important, our study suggests that trees remained an important part of their daily lives.
Rhianna Drummond-Clarke. lead author
School of Anthropology and Conservation
University of Kent
Canterbury, UK.
The authors note that walking on two feet is a defining feature of humans when compared to other great apes, who “knuckle walk”. Yet, despite their study, researchers say why humans alone amongst the apes first began to walk on two feet still remains a mystery.
(A) Issa Valley’s location in western Tanzania relative to Taï (North Group, Ivory Coast), Kibale (Ngogo, Uganda), Bwindi (Uganda), Mahale (M-Group, Tanzania), and Gombe (Kasekela, Tanzania). For comparative purposes, sites are grouped into three categories reflecting the percent forest cover and dryness [following (35)]: dense forest (dark green circles; Taï, Kibale, and Bwindi), forest-mosaic (light green circles; Mahale and Gombe), and savanna (orange rectangle; Issa). Forest sites are considered as closed and savanna as open habitat. The Issa study area is a savanna-mosaic habitat with a long dry season that is dominated by miombo woodland, represented in highlighted habitat map and view of site (B). Issa’s deciduous miombo woodland (C) is classed as open vegetation [grassy understory, broken canopy, low tree density (0.02 trees per square meter), and majority of trees <15 m high], while the evergreen riparian forest (D) is classed as closed vegetation, with vine-dense understory, twice the tree density, taller trees, and a more connected canopy than the woodland (table S4). Bwindi is only included in the intersite comparison of bipedal behaviors as no positional behavior frequency data were collected at this site (38).
Copyright: © 2022 The authors.Published by American Association for the Advancement of Science. Open access. (CC BY 4.0)
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.
Drummond-Clarke, Rhianna C.; Kivell, Tracy L.; Sarringhaus, Lauren; Stewart, Fiona A.; Humle, Tatyana; Piel, Alex K.
Wild chimpanzee behavior suggests that a savanna-mosaic habitat did not support the emergence of hominin terrestrial bipedalism
Science Advances; 8(50), eadd9752. DOI: 10.1126/sciadv.add9752
Published by American Association for the Advancement of Science. Open access
Reprinted under a Creative Commons Attribution 4.0 International license (CC BY 4.0)
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