Recent Fossil Find and Human Evolution
Many people are convinced that human evolution is a fact. Often they will cite the existence of hominids in the fossil record as evidence for their conviction. These creatures presumably represent evolutionary intermediates between an ape-like creature and modern humans.
The standard evolutionary model for human origins views Homo habilis as the first member of our genus (Homo). This hominid initially appears in the fossil record about 2.6 million years ago (mya) and seemingly gives rise directly to Homo erectus around 1.9 mya.
The direct transformation of H. habilis into H. erectus appeared to gain support from the recovery of hominid remains from Dmanisi, Georgia, in 1999 and 2001. Dating at about 1.8 million years in age, these hominids appear to be intermediate in morphology between H. habilis and H. erectus.
The recent analysis of H. habilis and H. erectus fossils recovered near Lake Turkana, Kenya, in 2000 muddies the place of H. habilis in human evolutionary scenarios and widens the gap between H. erectus and modern humans. The fossil specimens consisted of a faceless H. erectus skull, dated at 1.55 million years in age, and a H. habilis jawbone dated to be 1.44 million years old. Paleoanthropologists find each of these fossils surprising for different reasons.
The 1.44 million-year-old date for H. habilis means that this hominid must have coexisted with H. erectus. This finding contradicts the standard model. If H. habilis gave way to H. erectus it should not have coexisted with this hominid. The fossils indicate that H. habilis and H. erectus were morphologically distinct species that exploited different ecological niches within the same geographical region, reinforcing the conclusion that H. habilis didn't directly transform into H. erectus.
This conclusion spawned headlines in a number of popular media outlets that challenged the validity of human evolution. (For example see here and here.) As is often the case, the headlines exaggerated and sensationalized the implications of these fossil finds.
The coexistence of H. habilis and H. erectus doesn't directly challenge human evolution. It simply means that H. habilis didn't evolve into H. erectus through a process known as anagenesis. (According to this idea, one species transforms into another with the original species disappearing.) Instead, H. habilis could have given rise to H. erectus through branching from the original population via a process called cladogenesis. Alternatively, H. habilis and H. erectus could have shared a common ancestor, though no evidence currently exists to substantiate this proposal.
While the coexistence of H. habilis and H. erectus doesn't invalidate human evolution, this discovery highlights a couple reasons why it's premature to claim that the hominid fossil record substantiates human evolution.
Human evolutionary models, even the ones that appear to be the best-established, are highly speculative and, at best, have tenuous support from the fossil record. Time and time again a single fossil find overturns a "well-established" idea in human evolution. It's hard to know what other entrenched ideas will soon be abandoned as new hominid specimens are unearthed and studied. It's hard to accept human evolution as a "fact" given the actual level of uncertainty about the relationships among the hominids in the fossil record and the constant flux within the discipline.
It is hard to know which hominid fossils are transitional intermediates and which ones are not. Prior to this most recent discovery, the hominids recovered in Dmanisi, Georgia, were considered important transitional intermediates between H. habilis and H. erectus that supported an anagenetic transformation. The coexistence of these two hominids means that the Dmanisi hominids can't be transitional forms. This raises questions such as, "How many other transitional intermediates in the hominid fossil record have been misinterpreted?" and "Could it be that other key transitional fossils have been misclassified?"
For human evolution to be declared a fact, anthropologists must define the evolutionary route that transformed an ape-like creature into modern humans—replete with a progression of intermediate forms. The insight gained from this recent work highlights how far evolutionary biologists are from establishing this requisite understanding.
The H. erectus find holds surprises too. Next week I'll describe what those surprises are and how they impact human evolutionary models.
For more information on the relationship between the hominid fossil record and human evolution, see Who Was Adam?