One of the posts that I have had in draft form for several months, waiting for the proper motivation and approach to finalize, is titled, “lineages and species in the fossil record.” This week, perhaps, the time and motivation has come to finish the post, prompted this morning by Ken Weiss’s piece at The Mermaid’s Tale on revisiting the single species hypothesis. Ken’s piece is prompted by a series of review articles in last week’s Nature that deal with recent transitions in thinking about recent human evolution, the evolutionary forces at play, and the overall pattern of evolutionary changes represented by the available fossil, archaeological and genetic evidence.
Ken, like me, is a graduate of the University of Michigan department of Anthropology (though we are 34 years removed from one another in this accomplishment). This is important, as Ken places his review of the old “single-species hypothesis” — the notion that fossil hominins occupied a single evolutionary lineage owing to the expansive niche resulting from human-like behavioral tendencies and the ecological challenge of hosting multiple hominin species within that niche — within the context of Michigan Biological Anthropology:
A more serious argument for the single-species view was that possession of culture was our defining trait and signs of it could be seen even in ancient sites. It was our ‘ecological niche’, and by the ‘competitive exclusion principle’ of population ecology at the time, only one species could occupy a given niche: other interlopers would be driven out, and to extinction. The single-species view could be called the Michigan school of paleoanthropology, because it was the view at the University of Michigan (where I was studying) by major protagonists Loring Brace and Milford Wolpoff. The view was also influenced by the sophisticated population thinking of geneticist Frank Livingstone, and that was how I was trained.
As it turns out, despite our temporal displacement from one another, I share many of the same Michigan influences as Ken. Loring Brace taught my graduate seminar on the history of Physical Anthropology. I met Frank Livingstone while at Michigan, though he was retired by the time I arrived. More significantly, when I taught at Michigan for two years I revived an old Frank course titled, “Biology, Society and Culture.” Given the opportunity to teach the course, I asked what, given the title, I should teach about (the course had been dormant in the curriculum since prior to my arrival as a graduate student in 2001). I was told, and I don’t know how apocryphal this story might be, that every morning Frank would read the newspaper over breakfast, then walk from his home to campus. During that walk he would usually work up some kind of ire over what he had read, and organize his lecture for “Biology, Society and Culture” around that complaint. I have also heard that Frank was a brilliant lecturer and his legacy within Anthropology and human population genetics is huge. And Milford Wolpoff was and remains my advisor, though I am now long past the days of being his graduate student, as well as the most significant influence on my own anthropological views.
What all three of these Michigan anthropologists share in common, I believe, and what helped shape Ken’s understanding of “the Michigan School of Paleoanthropology” is the view that evolution is a population-level process and that humans, rather than serving as a very poor model for the study of human evolution (we live too long and do way too many strange things), offer an intriguingly unique perspective into the way that evolutionary dynamics can operate in a truly complex behavioral organism. The tendency of Michigan to produce paleoanthropological “lumpers,” those who tend to identify fewer species in the (especially more recent) hominin fossil record, has its origins from a focus on the intricate ways in which biological variation gets distributed and parsed within living humans coupled with the intellectual exercise of applying the knowledge gained from this view backwards into our evolutionary past.
For me, the dividing line of this approach, the horizon beyond which I lose confidence in that perspective, is the origin of our genus, Homo. The evolutionary transition from our Australopithecine pre-Homo ancestor to Homo is without a doubt of the one three most significant evolutionary transitions in the hominin story (the other two being the origin of bipedality at the beginning of the story and the demographic explosion and massive ecological shift that occurs at the end of the Pleistocene when populations begin becoming sedentary farmers), and a current topic of considerable investigation. Most of the debate about the “single-species hypothesis” that is referenced by Weiss’s piece was focused on the Australopiths. The Australopithecines were and remain a real challenge for paleoanthropological inquiry because they are a true transitional evolutionary lineage (a real “missing link”), distinguished from all of their hominoid predecessors by a bipedal lifestyle but not yet possessing the key attribute of humanity, an expanded brain and all that goes with encephalization. Homo, on the other hand, even in its earliest agreed upon fossil representatives, is more readily comparable to living humans. The post-cranial skeleton coupled with a technologically-mediated and cooperative ecology make them modern humans in the making.
But what does that mean? Ken’s piece is just part 1, and I assume in the subsequent pieces he will discuss our changing view of the evidence for modern human origins. Whereas 15 years ago (even 10 years ago) there was a consensus view, though not unanimous, that the genetic evidence showed clear signals of widespread population (species) replacement by expanding modern human populations over the past 100,000 years, we now have strong genetic evidence that view is false. Neandertals and Denisovans (and probably other as-yet identified extinct lineages) are part of the genetic ancestry of modern humans. Not unlike the complex ways in which academic ideas percolate down and find themselves simultaneously realized, in slightly different forms, in the thoughts of Ken Weiss and my own, modern humans share a complex and diverse ancestry. These new data post interesting challenges for how we think about and use concepts like “species” and “lineage.”
The literature on what is a species is vast, vastly interesting, theoretically compelling, but also something I cannot fully review here. I will recommend Ernst Mayr’s, “The Growth of Biological Thought” (1982) as a starting point of any adventure down that path, but otherwise I will simply say that the concept, though central to evolution, is theoretically and operationally challenging for human evolutionary studies. The notion of genetic isolation (the Biological Species Concept) is appealing, and I think fundamentally correct, but impossible to apply to fossils. So we make due with work-arounds like the Evolutionary Species Concept or the Phylogenetic Species Concept (or others). The use of evolutionary lineage, in contrast to a species, relaxes the constraints on the model. Now, instances of infrequent but significant genetic exchange, something we know occurs in the evolution not just of modern humans but also different species of gorillas and a host of other organisms, is possible. It also allows for conceptual realizations that speciation is typically not an instantaneous process and divergent lineages do not necessarily evolve into divergent species. So while Neandertals are certainly a distinct evolutionary lineage, it is less clear whether they are truly a different species.
An image I often go back to in thinking about this in the context of early Homo is one taken from the evolutionary biologist John Endler (1977), a mock-up of which I have below:
One of the major questions I seek to address in my research is how the evolutionary transition that led to the emergence and subsequent dispersal of Homo early in the Pleistocene impacted processes of population differentiation, and subsequently evolutionary patterns in Homo. The infusion of contemporary and ancient genomic data into questions of modern human origins has given us a vastly better view of these processes in the late Pleistocene, something last week’s Nature makes clear. But it also gives us a new way of approaching the early evolution of Homo. The most comparable think to Homo at the beginning of the Pleistocene is Homo at the end of the Pleistocene. The challenge in putting the two together is figuring the right way to test hypotheses about how evolutionary forces change as human behavior becomes more complex throughout the Pleistocene.
Obviously I will continue to have more to say on these issues…and I will look forward to reading what Ken says.
*****
1. Mayr, E. (1982). The growth of biological thought: Diversity, evolution, and inheritance. Cambridge, Belknap Press.
2. Endler, John A. (1977). Geographic variation, speciation, and clines. Princeton, NJ: Princeton University Press.
