Spend time reading about human evolution and it will not be long until you come across a discussion of “bushy” vs. “linear” evolutionary scenarios or arguments between taxonomic “splitters” (favoring more species) and “lumpers” (favoring fewer species). I will, up front, admit my bias towards being a strong “lumper,” at least when it comes to consideration of relationships within the genus Homo (pre-Homo hominins may be a very different story). This is an issue I spend quite a bit of time thinking about, particularly when it comes to the early members of the genus Homo. What kind of evolutionary pattern was established in the transition from our Pliocene, presumably Australopithecine, ancestor to Homo?
One of the issues I come up against at times is what I view as a conflation between taxonomic diversity, meaning lots of species, and evolutionary complexity. The flip side to this is the assumption that a linear model of evolution, one without a large number of species, must reflect a “simple” evolutionary scenario. For early Homo, this issue relates to whether or not there is an adaptive radiation of early members of the genus (e.g. Homo habilis, Homo rudolfensis, Homo ergaster, Homo erectus, Homo georgicus…) or a less speciose pattern. One thing that is certain is that the fossils generally categorized within this/these group(s) encompass a broad range of variation, perhaps providing support for greater taxonomic diversity. But I would suggest that the variation we see in the fossils is more parsimoniously assigned to greater evolutionary complexity – complexity that may come from the rapid development of differentiated niche structures and reproductive barriers in early Homo, but that also might come from the development of a highly structured, geographically dispersed, behaviorally flexible, polytypic lineage. Indeed, most of the changes we observe in early Homo can be interpreted as changes towards a broader, more generalized and flexible ecology.
I think this issue is becoming increasingly relevant for our understandings of earlier hominins, as well. At the recent AAPA meetings presentations highlighted the potential diversity of functional morphology associated with locomotion in hominin specimens older than 3 million years. A. afarensis is doing one thing, Ar. ramidus may be doing another, the Burtele foot may be doing yet another thing, and whatever gave rise to A. sediba may have been doing some combination of all of the above. It is another scenario where we see evidence of variation in the fossils that cries out for a complex evolutionary explanation. But again, complex need not imply isolation and speciation processes. Complexity can come from ecological variation, development, behavioral adaptation or a host of other “complex” evolutionary factors.
Related to this issue is a recently released paper in the journal Evolution by Folmer Bokma, Valentijn van den Brink and Tanja Stadler, titled, “Unexpectedly many extinct hominins.” The paper is essentially a mathematical exercise that attempts to derive a probability distribution for the number of extinct hominins we might reasonably expect. There are a number of assumptions they make that are critical to the results they come to, of course. I will just highlight one they are quite explicit about:
…we assume that the stochastic rates of speciation and extinction (λ and μ, respectively) that gave rise to that phylogeny have been constant across the DLMH [direct line to modern humans], and equal in all species that descended from it.
The evolutionary line leading to living humans has undergone at least two major ecological changes over that time – the transition from Australopiths to Homo at the beginning of the Pleistocene, and the demographpic expansion and transition to domestic food production at the end of the Pleistocene – both of which likely have enormous consequences for the evolutionary ecology, and hence speciation processes in the human lineage.
Despite this, the paper is an interesting and thought-provoking exercise as are its conclusions.
These estimates contrast sharply with some fossil interpretations: Where we estimated that the total number of extinct species is 7.7, fossil studies recognize a median of 14 species with a lower limit of 5, and some taxonomies recognize up to 25 species (Curnoe and Thorne 2003; Foley 2005).
…
Compared to the expectations calculated here, in particular the number of species in the very recent past is improbably high (Fig. 2). Analyses of nuclear genomes revealed gene flow between H. sapiens, Neanderthal, and Denisova hominins (Reich et al. 2010; Green et al. 2010; bi-Rached et al. 2011), which may indicate that these populations did not constitute biological species.
Evolution produces variation through a diversity of processes, not all of them dependent on reproductive isolation and speciation.
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1. Bokma, F., V. van den Brink, et al. (2012). “UNEXPECTEDLY MANY EXTINCT HOMININS.” Evolution (early view). DOI: 10.1111/j.1558-5646.2012.01660.x
