Afarensis

Evolgen has some interesting things to say on hybrid speciation. The introgression of Neanderthal genes into Homo sapiens has been in the news a lot. Then there is H. floresiensis. What do hybrid speciation and introgression have to do with the Hobbit? Wait and see.

Back in July an interesting paper on primate hybrids was published in the Journal of Human Evolution. The paper, Identifying the morphological signatures of hybridization in primate and human evolution, concerned research on baboon hybrids. The paper argues that baboons make a good model for considering hybridization in human evolution for two reasons. First, they - probably- resemble hominins in population structure and diversity. Second, extent baboons began diverging from their most recent common ancestor circa 1.8 million years ago. This is approximately the time of the emergence of Homo - excluding H. habilis/rudolfensis. With that in mind the paper examined the skulls of yellow baboons, olive baboons, F₁ crosses of olive and yellow, and B₁ backcrosses of the F₁ generation with olives. The study examined 39 measurements on the skull, as well as five non-metric traits. The results were interesting. One of the aims of the study was to detect heterosis and dysgenesis. Heterosis was found in 28 traits in the F₁ generation and 19 traits in the B₁ generation. Non-metric traits also displayed differences, some of which seem to be based on sex. For example, ten hybrids from the F₁ generation had supernumerary teeth (mainly permanent molars, although there were some extra canines as well). Nine of those were male. Females had a higher incidence of unusual or extra zygomaxillary sutures. The researchers draw several interesting conclusions from this.
First, based on the metric data, heterosis does not affect a majority of traits. Second, based on the non-metric data, primate hybrids display higher incidences of developmental anomalies largely attributable to mixxing two separately coadapted genomes. So can we go out and apply this to the fossil record? Trinkaus, for example, has argued that the Pestera Muierii remains display traits representing both modern humans and Neanderthals and consequently indicates admixture. Advocates of multiregional continuity have made similar claims about other fossils. As the above discussion shows, however, detecting admixture - i.e. hybrids - is rather difficult and we probably have been looking at the wrong traits. Over and above that, the hominin fossil record samples, mainly, individuals and does not cross boundries of biological populations. As the authors of the above paper point out, heterosis is a population-level phenomenon. The authors recommend that we look for traits related to the breakdown of development, such as the above mentioned supernumerary teeth and sutural anomalies.
Which brings us to H. floresiensis. One of the interesting features of the Hobbit lies in the maxillary premolars. Both are rotated. In the purebreed baboons mentioned above, supernumerary teeth occured at a low frequency, in the hybrids the frequency was much higher and the supernumerary teeth were in different spots (maxillary incisors vs maxillary molars). Since we only have one H. floresiensis we can't really tell what the rotated pre-molars may mean in terms of hybridization. As the authors of the paper point out:

The paucity of evidence may also result from the sparse fossil record; it is possible that because hybrid zones are narrow... they simply have not yet been well sampled. It is also possible that individual hybrids within such zones have not yet been sampled, as hybrid zones can comprise a complex mixture of first-generation hybrids, backcrossed hybrids, and parental populations... Finally, even in situations where hybrid zones (and the hybrids within them) are adequately sampled, there remains the possibility that those hybridizing hominin populations that were exchanging migrants were doing so at such a high frequency, or were separated for such a short time, that they should not be considered evolutionarily distinct.

Finally, I began this post by linking to RPM's post on hybrid speciation. As RPM notes:

Hybrid speciation occurs when two species produce hybrids that are both fit and capable of becoming reproductively isolated from the two parental species. The new species will often exploit a niche unique from the parent species as well. This differs from a hybrid zone, which often exists between closely related species in close proximity. Hybrid zones tend to have an excess of F1 hybrids, whereas hybrid species are in Hardy-Weinberg equilibrium and are genetically and ecologically unique from the two parental species.

The above paper dealt, mainly, with F₁ hybrids and showed the difficulty of detecting them in the fossil record. Detecting a true hybrid species of hominin in the fossil record would be even more difficult...

Here is a picture of some of the baboon skulls used on the study:

From left to right: F₁ hybrid, F₁ hybrid, yellow purebreed, olive purebreed.

Literature Cited:

Rebecca Rogers Ackermann, et al., Identifying the morphological signatures of hybridization in primate and human evolution, Journal of Human Evolution (2006), doi:10.1016/j.jhevol.2006.07.009