Hybrids, hominoids and hominins

Is the Eastern lowland gorilla (Gorilla beringei graueri) a hybrid (sub)species? A recent study by RR Ackermann and JM Bishop suggests this scenario.

Their study used morphological, genetic and geographic information to analyze variation in extant gorilla species and subspecies. A previous study by Ackermann and colleagues (2006) on baboons found that non-metric traits–namely pairs of extra teeth and unusual sutures between some facial bones–have freakishly high frequencies in known hybrids compared to their parents of different species. Well wouldn’t you know it: G. b. graueri had a significantly higher frequency of such traits than the other gorilla species/subspecies (the putative ‘parental’ species, the Eastern mountain gorilla G. b. beringei and the Western lowland gorilla G. gorilla gorilla).

Additionally, for a number of cranial metric traits, graueri had significantly higher values than the other gorilla species’ sample averages. “Heterosis” refers to a condition wherein a hybrid phenotype exceeds the combined parental mean–it appears that if this is truly a hybrid population, graueri displays heterosis for a number of cranial features. Finally, it is notable that graueri has been described as more like Western gorillas in some respects, but more like Eastern mountain gorillas in others, and then totally unique in some aspects. This is arguably a result of graueri possessing genes from two other distinct species.

Oh, and the mtDNA evidence suggests fairly recent gene flow from Western lowland gorillas eastward. Because mtDNA is maternally inherited, this implies that females have been involved in this west-to-east gene flow. However, it is unclear the extent to which there was east-to-west gene flow, or the potential involvement of male gorillas here.

Why is graueri likely a largely hybrid sample, and not just part of a morphological and genetic cline conecting Western and Eastern gorilla populations (i.e. making gorillas a single, polymorphic, geographically broad species)? The hybrid morphologies above are believed to indicate complications that arise in development, due to the union of two species’ distinct sets of genes. Such signatures of hybridizaiton would not be expected to appear in a regularly panmictic species. It also seems that the separation of Western and Eastern gorilla species occurred during the Pleistocene, which means that the two sides have been diverging for quite a long time and have recently come back into contact.

I think the authors make a very good case for the importance of hybridization in the evoluton of gorillas, at least as we know them today. I like their use of both morphological and genetic data, which complement one another nicely in support of a hybrid-type nature of Gorilla beringei graueri. In addition, the implicaitons of the study are fantastic! Even though the authors did not know for sure whether individual specimens were hybrids, they were able to use the results of previous work to make a convicning case that a number of their specimens were very likely to be hybrids. This is a good sign for persons like myself who are interested in the detection of hybrids in skeletal/fossil samples. Another great implication is that hybridization indeed has a place in hominoid evolution–it awaits to be seen what role hybridization may have played in the course of human evolution.

References
Ackermann RR, Rogers J and Cheverud JM. 2006. Identifying the morphological signatures of hybridization in primate and human evolution. Journal of Human Evolution 51: 621-645.

Ackermann RR and JM Bishop. Morphological and molecular evidence reveals recent hybridization between gorilla taxa. Evolution: in press.

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More convergence and arboreality: Knuckle-walking and the African apes

As long as we’re on the topic of homoplasy, a recent study suggests that knuckle-walking evolved independently in chimpanzees (Pan) and gorillas (Gorilla). If true, this suggests that hominins did not evolve from a knuckle-walking ancestor. Interesting.

Take-home points from the paper include:

  • Many purported ‘knuckle-walking’ features of the hominoid wrist might rather indicate arboreal wrist postures
  • Knuckle-walking in Pan and Gorilla are biomechanically distinct, and may thus have evolved independently in each lineage
  • More tentatively: Humans may not have evolved from a knuckle-walking ancestor, lending further credence to the idea that Pan is not a great model for the Pan-human common ancestor
  • This may be another example of one of Futuyma’s Principles of Evolution: HOMOPLASY IS COMMON IN EVOLUTION

“Knuckle-walking” refers to the mode of locomotion employed by most Pan and Gorilla when on the ground. Whereas most terrestrially quadrupdal primates use either the palmar surfaces of their ‘fingers’ or their palms to contact the ground, Pan and Gorilla‘s hands contact the ground with the back surface of the middle of their fingers (their intermediate phalanges, in technical terms). It is a very unusual posture–so far as I know, among all animals it is unique to these apes. So, it is perfectly sensible to assume that that knuckle-walking in chimpanzees and gorillas is homologous, represents the ancestral posture in African apes, and that humans evolved from a knuckle-walking ancestor.

But Tracey Kivell and Dan Schmitt present evidence from the wrist that suggests knuckle-walking in Pan and Gorilla are biomechanically and developmentally distinct. They point to several features of the wrist bones (carpals) that have traditionally been assumed to reflect knuckle-walking behavior. The expression of these features does not fit expectations given size and maturation differences between the two African apes. In fact, most of the features are more common/pronounced in Pan, and sometimes even other primates, more so than in Gorilla. The authors thus posit that many of the hitherto-knuckle-walking features of the wrist are actually indicative of arboreal wrist postures, and not knuckle-walking.

That authors acknowledge that it is possible that the wrist differences between Pan and do not necessarily preclude the possibility that knuckle-walking in the two apes has a common, ancestral origin, and that the differences accumulated after the evolutionary split between Gorilla on the one hand and Pan-humans on the other. That is to say, the behavior in the apes is homologous (as in common ancestry) but non-identical. Another possibility, which would also indicate that humans did evolve from a knuckle-walking ancestor, is that the behavior evolved separately in the Gorilla lineage, and in the Pan-hominin lineage before the split between Pan on the one hand and hominins on the other. The only way to test such a hypothesis is with fossils, fossils which so far as I know we do not have (yet).

Reference
Kivell T and Schmitt D. Independent evolution of knuckle-walking in African apes shows that humans did not evolve from a knuckle-walking ancestor. Proceedings of the National Academy of Sciences, in press.