How human is handedness?
A study published in the latest issue of the Journal of Human Evolution (citation at the bottom of this post) suggests that handedness is not as unique to humans as some have previously thought. An analysis of more than seven hundred individuals across extant great ape species was performed and it was found that population level handedness was observable. The extant African great apes (gorillas, chimpanzees and bonobos) showed population level right handedness (like humans) while orangutans showed population level left handedness.
From an evolutionary perspective this tell us something about our past. It is widely accepted in world of evolutionary anthropology that all African ape species (for the sake of this post I will ignore the taxonomic/semantic arguments surrounding whether humans should or should not be considered great apes) share a common ancestor. If all extant species show a tendency toward right handedness, it is reasonable to assume that our common ancestor may also have been predominantly right handed. There are of course other factors; there could have been some environmental pressure that selected for this particular pattern of handedness and thus caused it to evolve separately in each species but the Occam’s razor within finds this explanation at best unlikely.
This leaves the suspicious left handedness of the orangutans without an explanation. I’ll offer my thoughts here and post later if I find anything that counteracts or compliments my thinking. Firstly, of all great apes, the orangutan is our most distant relative. Its branch of the family tree split from ours the longest ago and much of the evolution of the unique characteristics of the orangutan most likely occurred in Asia. A few things could have happened with regard to the left handedness of orangutans. The common ancestor of all great apes (further back than the one for all African apes) may have been left handed, in which case the orangutan has maintained the ancestral tendency toward this trait. If this were the case the shift toward right handedness may have occured later in an ancestor of the African apes as a result of natural selection (I find this unlikley as stated) or as a result of a population bottleneck that by chance saw an increase in genes related to right handedness accumulate in the gene pool (genetic drift). Alternatively, the ancestor of all great apes may have been predominantly right handed, in which case all extant African great apes (us included) have retained the ancestral characteristic. In this case a reduction of population in the orangutan ancestor in the distant past (founder effect?) may have yielded lefthandedness.
Of the two situations outlined above I find the idea that the ancestor of all great apes was right handed and that organutan left handedness occured independently the most parsimonius. It involves the least variables in terms of population bottlenecks etc.
I’m neither a professional primatologist nor paleontologist, most of my education being based in general principles of evolution, so if anyone would like to comment or add to this please feel free to do so!
“Hand preferences for coordinated bimanual actions in 777 great apes: Implications for the evolution of handedness in Hominins ”
William D. Hopkins, Kimberley A. Phillips, Amanda Bania, Sarah E. Calcutt, Molly Gardner, Jamie Russell, Jennifer Schaeffer, Elizabeth V. Lonsdorf, Stephen R. Ross and Steven J. Schapiro.
Journal of Human Evolution
Volume 60, Issue 5, May 2011, Pages 605-611