A new fossil find in Ethiopia adds interesting confirmation that our last common ancestor with chimpanzees did not look like a modern chimpanzee, according to Scott Simpson and colleagues who reported their findings in the April Journal of Human Evolution. The fossils, excavated in Ethiopia’s Gona Project area, are the first found from this hominid species since 110 Ardipithecus ramidus fossils, including a partial skeleton of an individual nicknamed Ardi, were found about 100 kilometers to the south of this new find, and they seem to confirm that many of the features found in modern chimpanzees were never a part of the human heritage at all.
The discovery in 1990s of ‘Ardi’ forced a re-think about why our ancestors became upright walkers, the favoured theory previously being that they had learned how to walk on two legs after their forest habitats changed to grasslands. But Ardi changed all that as it became increasingly clear that upright, straight-legged walking actually evolved while our hominin ancestors still lived for the most part in the forests. In other words they were part-time bipeds long before they abandoned the trees because Ardi’s bones indicated that although she travelled over large tree branches on four limbs, on smaller branches she walked on straight legs using her arms for balance.
This lead to the idea that certain physical features and behaviours found in modern apes are relatively recently evolved and are not ancestral to humans. For example, modern great apes have feet which are very similar to their hands in terms of grasping and flexibility, while modern humans have a wholly rigid foot. But Ardi’s semi-rigid foot and relatively long lower limbs, both similar to those of the gibbon, look like pre-existing adaptations for tree-dwelling which were later modified by hominins for life on the ground[1]. Another feature of ‘Ardi’s anatomy suggested she did not use knuckle-walking like modern chimps, but placed her hands flat when walking on ‘all fours’.
The latest Ardipithecus ramidus fossils leave little doubt that Ardi’s skeleton was not a ‘one off’ adaptation, and that these rather surprising features were common within the Ardipithicus ramidus species generally. And while there were some differences between Ardi’s posture and that of other ramidus populations, they confirm that modern African apes followed a different evolutionary adaptive history to modern humans, and their current anatomy is not indicative of our shared ancestor.
Citation: S. Simpson et al. Ardipithecus ramidus postcrania from the Gona Project area, Afar regional state, Ethiopia. Journal of Human Evolution. Vol 129, April 2019, p.1. doi:10.1016/j.jhevol. 2018.12.005
Image taken from cited study.