Hominin Evolutionary Patterns in the Early to Middle Pliocene: How Many Forms Are There?

It has been traditionally recognized in palaeoanthropological circles that the early to middle Pliocene (5.5 to 3.5 million years ago) is the time when the majority of human characteristics such as bipedalism (walking upright), modern-looking teeth, and (comparatively) expanded braincase first evolved. What has also been tacitly assumed is that an massive expansion of early human-like australopithecine taxa followed this time period and bridged the Plio-Pleistocene boundary (3.2 to 1.5 million years ago). In tandem with this thought was the belief that the early to middle Pliocene was characterized by very few hominin taxa.

The early Pliocene is a very murky time period in early human evolution. Scattered remains exist that hint at bipedalism and early human-like adaptations but it is not until approximately 4.4 million years ago that we have the first concrete evidence of some these behaviors, in the form of Ardipithecus ramidus. Ardipithecus, while having adaptations for bipedalism, did not have to walk upright all the time, however. It possessed long arms relative to its legs and its big toe was opposable, or grasping. Clearly, this form was just as happy in the trees as on the ground.

It is not until we reach Australopithecus anamensis (c. 4.0 million years) that we get the first evidence of obligate, or habitual, bipedalism (Ward, Leakey, & Walker, 2001), the traditional hallmark of humanity. Indeed, so convinced were many that there were very few species in the mid-Pliocene that Kimbel and colleagues argued that Au. anamensis and its direct successor in time Au. afarensis represented an evolutionary species, or one continuous lineage, from 4 million years down to 3.3 million (Kimbel et al., 2006).

Even so, these forms were still considered to be generalized and it was considered that Au. afarensis (the taxon that gave us the famous “Lucy” fossil) could serve as a potential progenitor for the forms that followed. This idea began to be challenged in the late 1990s, with the discoveries of several morphologically different forms that were thought to be broadly contemporaneous with Au. afarensis, with the finding of Kenyanthropus platyops andAu. bahrelghazali. The problem was that few concrete interpretations could be attached to these finds because of their fragmentary condition (See The Human Fossil Record, Part 6, for a description of these finds). Thus, the ancestral position of Lucy remained largely unchallenged. With one swift stroke, that has changed.

As noted on Science Saturday Links, a new fossil has been found, dated to between 3.3 and 3.5 million years that displays morphology that cannot be assimilated within Au. afarensis (Haile-Selassie et al., 2015). Found at the site of Burtele, in the Ethiopian Afar Triangle, this new find represents a different hominin taxon at the same time period as Au. afarensis. Further, its proximity to that form geographically suggests that it adapted differently than Au. afarensis to the same environmental conditions. This new find has been given its own taxonomic designation: Au. deyiremeda. While it is similar in size to Au. afarensis, it differs from Au. afarensis in the broadness of its jaw, its forward-facing cheeks and the small size of the back teeth.

Palaeoanthropologist Fred Spoor suggests that this find shares some characteristics with the facial portions of K. platyops that are usable, such as the forward-facing cheek bones and the small back teeth, but that the front of the face lacks the flatness present in K. platyops, and is more similar to Au. afarensis, having a pulled-out region just below the nose (Spoor, 2015). This particular characteristic is primitive, relative to later hominins. Interestingly, however, the front part of the mandible (where the chin would be in us) is similar to early Homoalthough the rest of the remains lack the derived traits found in early Homo, such as the thinning of the jaw, the facial flattening and the enlarged front teeth.

What this find suggests is that our view of hominin radiation in the Pliocene has reflected an incomplete understanding of the variability present at that time. It now appears that the early radiation of australopithecines matches that found in the Pleistocene and that this variability characterizes both epochs. How far back this morphological variability extends is not clear, since, as we travel back in time, our fossil resolution deteriorates.

We know that Ardipithecus expressed some bipedalism but, beyond this, the certainty diminishes. While it is possible that Orrorin tugenensis, found near Lukeino, in Kenya and dated to around 6 million years, possesses some of the anatomical modifications that indicate bipedalism (Pickford, Senut, Gommery, & Treil, 2002), the post cranial remains are fragmentary and much is inferred. The 7 million year old skull from the Sahel region of Chad, while possibly possessing facial characteristics that lean toward the hominin condition, was badly crushed and may be a surface find (Beauvilain, 2008).

As noted above, this find is contemporaneous with Au. afarensis. Further, the region of Burtele is only thirty miles from Hadar, the site where Lucy was discovered. Assessment of the paleoenvironments of these areas indicates that they were broadly similar. In addition to the cranial fragments, postcranial remains recently found in the same region indicate a very different form of bipedalism that that practiced by Au. afarensis, being more of a facultative biped, similar to Ar. ramidus which is further evidence that there were multiple ecological adaptations going on in the middle Pliocene.

These discoveries elicit a host of questions about how we interpret palaeoenvironments and hominin diversity. Given that Lucy practiced, for all intents and purposes, a modern form of walking, why would an Ardipithecus-like gait, with its opposable, grasping big toe and ape-like body, have persisted in this region? Should we dismiss Kenyanthropus platyops out of hand as most palaeoanthropologists have done, or does it also represent another taxon at this time? Further, at this point, we do not have definitive evidence that the Burtele foot remains are the same taxon as that represented by the Burtele cranial remains. If they are not, then we may have three to five contemporaneous but very different hominins running around at the same time in the mid-to-late Pliocene.

This, then raises the possibility that our understanding of early Pliocene hominin diversity may also be suspect. Given that evolutionary changes such as bipedal and dental adaptations are conservative (whether or not you can get around and eat efficiently is pretty important) it may very well be that other, more derived hominin forms existed at the same time as Ardipithecus, from which some (or none?) of the middle Pliocene forms sprang.

These finds open up a new horizon of research into the earliest hominins and how they lived. They also provide us with even more clues in our ongoing search for the last common ancestor.