This is a new paper in the Journal of Human Evolution. It makes the argument that Pleistocene Homo could not have speciated into several species because we are like canids in many ways and canids have not speciated very much:
In this paper, I use an analogy to the wolf-like canids (Canis lupus, C. rufus, and C. latrans) to ask the question, How many Homo species should there be, given their likely behavioral profile(s)? (cf. Foley, 1991). The wolf-like canids are behaviorally similar to Pleistocene hominids in three key ways: (1) they are adapted for endurance locomotion, (2) they have a diverse diet, and (3) they are socially flexible. These characteristics lead to high habitat tolerance, which appears to have made the wolf-like canids resistant to allopatric speciation. I argue that analogous habitat tolerance would have likewise made isolation and allopatric speciation among Pleistocene Homo unlikely, especially in Africa. In contrast to an earlier single-species hypothesis which posited competitive exclusion between sympatric hominid species (Wolpoff, 1971), this paper explores behavioral constraints on the process of speciation itself under conditions of temporary allopatry.
This paper cites the endurance running model of humans (D.M. Bramble and D.E. Lieberman, Endurance running and the evolution of Homo, Nature 432 (2004), pp. 345–352). I'm surprised how often that paper gets cited.
I think the paper is an important contribution, and I personally have always liked the comparison between humans and canines (cooperative hunting, highly social, cooperative child care, fission-fusion groups, etc...), and I think the issue of whether several Homo species lived together at the same time is important and fascinating, but there are still many questions that remain, as discussed by the authors in the conclusion. Here is the last paragraph:
Finally, a “species resilience” perspective on Pleistocene Homo evolution offers the potential for new comparative analyses of modern species to explore constraints on the speciation process. It has long been recognized that eurytopic mammals tend to be more widely distributed and less speciose than stenotopic ones. However, detailed quantitative analyses of mobility, dietary breadth, and genetic variation within and between eurytopic species, in conjunction with fossil evidence, are needed to resolve more precisely the determinants of relative species resilience. Especially interesting would be comparative analyses of intercontinentally distributed carnivores such as ursids, felids, and canids, as well as of widely distributed primates. Coyne and Orr (2004: 425) note that relatively little research has been devoted to factors that prevent speciation. Comparative analyses of species resilience in Homo and other eurytopes, therefore, offer the possibility of contributing both to debates about hominid taxonomy and to our understanding of mammalian speciation in general (cf. Vrba, 1992).
Species resilience in Pleistocene hominids that traveled far and ate widely: An analogy to the wolf-like canids
A. Clark Arcadia
Journal of Human Evolution Vo. 51, Issue 4 October, 2006: 383-394
Abstract: Morphological and genetic analyses have yet to resolve the question of whether more than one species of Homo existed contemporaneously in the Pleistocene. In an effort to contribute a process-related perspective to hominid phylogenetic reconstruction, this paper uses an analogy to the northern wolf-like canids (the wolves and coyotes) to ask the question, How many Homo species should there be, given their likely behavioral profile(s)? In contrast to earlier comparisons to social carnivores which sought to illuminate specific aspects of hominid behavioral ecology, this paper explores behavioral constraints on the process of speciation itself. The analogy suggests that because Pleistocene Homo probably exhibited high habitat tolerance, they would not have had the opportunity to speciate, especially in Africa. In contrast to an earlier single-species hypothesis based on competitive exclusion between sympatric hominid species, this paper explores constraints on the process of speciation under conditions of temporary allopatry.