The human family tree is surprisingly bushy. Only a few thousand years ago we had Homo sapiens, H. neanderthalensis, H. floresiensis and the Denisovans all living side by side. And only a few million years before that we had several species of Australopithecus, Paranthropus and early Homo also sharing an environment.
However, the earlier in hominin history one travels the less diverse it gets. There is a fairly clear “trunk” to the bushy tree, with one species appearing at a time and gradually being replaced by the next. This is likely an artefact of preservation bias – older specimens are less likely to survive and so we get a distorted picture of the more distant past. As such, most evolutionary anthropologists fully expect that we will find older species to flesh out the early part of human history.
And now newly discovered Ethiopian fossils seem to do just that! A site in the Afar region of the country has yielded an incomplete foot fossil, consisting of 8 well preserved phalanges and metatarsals from a single foot – bones often missed during excavation as they are very small.
Isotopic analysis of the surrounding soil, as well as a study of the animal remains from the site, has revealed that the environment the foot owner would’ve lived in was a flood plain or river delta. This area would’ve originally been forested but over the years this gave way to open grassland. The surrounding rocks also allowed the foot to be dated. The fossils were found a few metres above volcanic tuff, created during an eruption. As such they are younger than this tuff, which was dated to 3.469 million years old (+/- 0.8 my). The researchers attempted to provide a more specific age for the foot by looking at how long it would’ve taken the sediment between the tuff and the fossil to accumulate. This provides an age range of between 3.2 and 3.4 million years ago, although this method is obviously a lot less accurate than the dating of the tuff – hence why it comes with a larger margin of error.
During this time period the only currently known contemporary hominin species was Australopithecus afarensis (Lucy). However, the morphology of the foot is significantly to that of Au. afarensis‘. Most notably, the foot of Au. afarensis has a permanently adducted hallux (its big toe always points forwards), like we have. However the joints of the hallux are more consistent with of an opposable toe, like that modern chimps have. In particular its hallux lacks the ability to tilt upwards as much as an obligate bipedal, like us or Au. afarensis, can. This is important in “toeing off” when, just before we lift our rear foot of the ground we push off with our tipey-toes.
Further, the hallux is also relativley small in comparison to the other toes. If used for toeing off it must be fairly hefty to withstand the pressure associated with pushing off from the ground. As such, it would seem this species used it’s hallux for grasping but not walking. As such it could probably climb quite well, consistent with being found in a wooded environment.
However, whilst their big toe could not have “toed off” the rest of them did have that ability. They had joints that could tilt upwards and were also fairly strong. So this creature was able to walk upright fairly well but the big toe did not play an especially important role in this, being reserved for climbing. Such a foot is dissimilar to that of Au. afarensis but is rather similar to the ancestral species, Ardipithecus ramidus. Living ~4.4 million years ago, Ar. ramidus also had feet that had an opposable hallux as well as various bipedal adaptations. Yet it also lacks some of the more derived bipedal features Ardipithecus has, such as ligament attachments that make the foot more rigid and thus stronger.
Given all these archaic traits it would seem that this foot belongs to a new hominin species that was actually morphologically rather old, yet survived until relatively recently. Since it’s similar to, yet still more archaic than, Ardipithecus it may well be a remnant of their ancestor (or a remnant of a close relative of their ancestor) which means it can potentially offer valuable insights into how bipedalism evolved.
However, placing it more definitely in our family tree would require more of the skeleton than we have at the moment. In the mean time all we can say for sure is that Lucy was not alone.
|Haile-Selassie, Y., Saylor, B., Deino, A., Levin, N., Alene, M., & Latimer, B. (2012). A new hominin foot from Ethiopia shows multiple Pliocene bipedal adaptations Nature, 483 (7391), 565-569 DOI: 10.1038/nature10922|