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

Intro by Brad Kramer, Content Editor: When I was learning how to ski in middle school, I was very grateful for the icons at the top of each slope informing me of what skill level was needed to get down the mountain while staying upright. Green Circles and Blue Squares were perfect for me, and my adventurous attempt at a Black Diamond ranks among the most terrifying experiences of my childhood (I rolled down the hill more than I skied). These simple symbols were very helpful in finding the right slopes for my ability level.

Similarly, while much of our content is “Green Circles” or “Blue Squares”—meant to be understood by those without an advanced knowledge of theology or science—we do want to highlight professional work done by Christian scholars and scientists attempting to understand God’s Word and God’s world. Paleoanthropologist James Kidder is one of these professionals, and below is his explanation of the importance of a recent hominin (human ancestor) fossil discovery. This article is a “Black Diamond”, meaning that readers should expect to find technical terms that require background knowledge in paleontology and paleoanthropology. But don’t let this scare you away! For those non-specialists who want to dive into the exciting world of paleoanthropological research, I recommend browsing Kidder’s excellent BioLogos series on the Human Fossil Record for an introduction to the field, and perhaps also having an online dictionary handy as you read this article (or ask the author a question in the comment section). One of the wonders of the internet age is that anybody with a computer can stay connected to the latest discoveries about God’s world, almost as soon as they happen. BioLogos is proud to help connect people to the world of science.

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 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 and Au. 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 Homo although 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.

This is a companion discussion topic for the original entry at https://biologos.org/blog/hominin-evolutionary-patterns-in-the-early-to-middle-pliocene-how-many-form

The Human Fossil Record, the story of how evolution produced the species Homo sapiens from the proto-primates of the Miocene, is so fascinating that it is well worth the reader’s effort to brave the “Black Diamond” warning Brad gives to Dr. Kidder’s blog. But the BioLogos community should not lose sight of the fact that, while certain features, such as an opposable thumb for tool making and bipedalism for more efficient traversing the open savannahs, contributed to the success of the Homo sapiens species, these physical attributes were not what made us Human. Ian Tattersall, curator emeritus of the American Museum of Natural History and its Spitzer Hall of Human Origins, emphasizes this in his books, ‘Becoming Human’ and ‘Masters of the Planet’. In the Prologue of the latter he states: “…if there is one single thing that above all else unites all human beings today, it is our symbolic capacity: our common ability organize the world around us into a vocabulary of mental representations that we can recombine in our minds*…” and “…the acquisition of this uniquely modern sensibility was an abrupt and recent event. Indeed, it was an event that took place within the tenure on Earth of humans who looked exactly like us.” In other words, Darwinian evolution seems adequate to explain the physical appearance of Homo sapiens, but something unusual turned organic Brain into symbolic Mind. Unfortunately the fossil record will not inform us as to how this came about.
Al Leo


You are quite correct. I debated about whether or not to include something about symbolic expression but, owing to space, elected not to.

With regard to your last point, however, there have been seminal studies by a number of neuroanatomists that shed light on changes in the structure of the hominin brain over time that suggest increasing cognitive capabilities and that these really ramp up beginning around Homo ergaster and Homo erectus.

Indeed, there is some controversy amongst experts on the topic of when the species, Homo, attained the level of symbolic thought. The early evidence of flower pollen from the Neanterthal burial of Shanidar 4 indicated that these folks may have reached that level. But considerable doubt has since been raised that this was evidence of an actual ‘burial bouquet’. Ian Tattersall (in refs. mentioned earlier) presents indisputable evidence that modern Homo sapiens were the first to supply the deceased with elaborate gifts and tools for an afterlife, and to craft exquisite statues and paintings that are the envy of today’s artists.

Whoops! I forgot to mention that the impression that the soft brain makes on the hard fossil skulls (indicating the presence of Broca’s area for example) is extremely weak, and I know of no neuroanatomist that would be brave enough to conclude that it indicated ‘increasing cognitive capabilities’. I am convinced that the increase in total brain size from A. afarensis to H. sapiens is, in reality, an exaptiation and is misleading. Look on the web under “hydrocephalus, French” and see the CT scan of a French civil servant living in modern society with half the brain capacity utilized by Lucy, the famous A. afarensis female. Once the Homo sapiens brain has been programmed to operate in the Noosphere, much of it proves to be redundant.

Randall White, who has extensively examined the Middle-to-Upper Palaeolithic transition in Europe indicates that it is a matter of degrees, not a sharp break of any kind. Further, recent examinations of the Chatelperronian tool industries of western Europe, which bridge the Neandertal/early modern transition, indicate a very high level of cognition, on a par with modern humans.

What is generally lost in the discussion is that even after modern humans first appear in the fossil record—close to 160-190 kya—tool types don’t become more complex until roughly 100 thousand years later. Further, tool type complexity really ramps up in the Upper Palaeolithic and technology turn-over becomes quicker and quicker.

As I indicated, there is lively discussion concerning the time it took to make the transition from the early, non-symbolic Homo sapiens (160-190 kya) to the modern (Cro-Magnon, ~50 kya). I feel that the evidence presented by Tattersall, Morris and Diamond is more convincing than that offered by White and others. But anthropology is merely a hobby with me, and so I readily admit that my acceptance of the Great Leap Forward may be influenced by how well it fits into my beliefs as a Christian. I was impressed by the fact that Richard Dawkins, in his book “The Ancestor’s Tales”, accepted the evidence for the GLF as it applied to humanity, although it seemed to defy the rules that Darwinian evolution applied to all other life forms (i.e. tiny steps taken in no particular direction). When Dawkins and Pope John Paul II agree on such an important matter, one must pay attention.
Al Leo

The problem I have, once again, with the GLF is that it simply is absent of evidence in East Africa, where you have the origins of modern humans. If you could demonstrate that, in some way, shape or form, these finds from Omo and Herto were not modern, then we might have something but it appears that modern humans really did arise then. What were they doing for 100 thousand years. Worse, the moderns seem to have gone for rolls in the hay with archaic Homo sapiens in the area.

Then you have the case in the Near East where the early moderns of the area, from Skhul and Qafzeh (which are dated to around 100 kya) seem to be using exactly the same stone tools that the Neandertals in the area, the Levantine Mousterian. There are some slightly shifted ratios in terms of blade production but, as I mentioned in a post, either the Neandertals were every bit as smart as the Modern humans in this region, or the modern humans were every bit as dumb as the Neandertals.

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Your quotation “What were they doing for 100,000 yrs.” is exactly the words used by Simon Conway Morris (Life’s Solutions) in proposing GLF. Of course you are correct that Omo & Herto show evidence of Homo sapiens at ~190,000 yrs. BP, but for 100,000 yrs they used the same tools and lived like their cousins, the Neanderthals. To me that is sufficient evidence that they were not US. There is almost 100% agreement that Cro-Magnon culture appeared quite suddenly with an abrupt advance in tool making, but the most significant point to note for BioLogos discussions is that at this point in time Homo sapiens became capable of symbolic thought. They buried their dead with tools and gifts for an afterlife and invented art and music. At this point they probably sought out a relationship with their Creator, were aware of right and wrong, and could, therefore, sin. Tattersall is probably the most experienced anthropologist in the area of human origins, and so, if you haven’t already done so, you owe it to yourself to read his "Becoming Human’ & ‘Masters of the Planet’.

As to the “rolls in the hay with archaic Homo sapiens”, isn’t this where Cain found a mate with the ‘people of Nod’? Anyone who really wants to retain a literal Adam & Eve could postulate the they were the first to (somehow) have their Brains programmed to operate as Minds, and could pass that on to the archaic Homo sapiens through language (which first had to be taught).
Al Leo

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