Jim Thomeson – who gets a brofist for being one of the most frequent commenter here – asks
I wonder about handedness. i am right handed, and my right hand can do things for which my left hand has no clue. On the other hand, both hands attended typing class at the same time, and type equally well; maybe the left is a hair better.
Ok fine, he isn’t really asking anything. There’s no question here but I felt like writing something and this is as good a starting point as any. What about handedness? Is it something we evolved because it confers an advantage or is it actually neutral and just got carried along with a beneficial gene? Or is there no genetic influence at all?
For the longest time these questions were made all the more fascinating by the fact it was believed that a right-handed majority was a uniquely human trait. Early studies suggested there was an even split, with half of primate populations favouring each hand. This raises the interesting question of why so many humans are right-handed given it clearly wasn’t the ancestral condition.
Except it kind of was. Later research found that there was indeed a population wide preference for a particular hand, although curiously only for certain tasks. Most primates favoured the left hand for reaching whilst the right hand was preferred for manipulative tasks.
So where does that leave us? On the one hand (if you’ll pardon the pun) there is no primate population that is as right handed dominant as humans. Even for manipulative tasks only 60-70% of the group preferred their right hand whilst 90% of humans like their right hands.
On the other hand humans do engage in more manipulation than other primate species which might explain the overall dominance of that hand. After all, are not most tests of handedness (i.e. writing) manipulative tasks? However, there is no way of knowing whether this increase in manipulation is sufficient to account for the differences between humans and primates so there is still the possibility that some recent mutation is responsible for this dominance.
according to wikipdia to my knowledge only one gene associated with handedness (in particular left-handedness) has been identified yet it is not responsible for every case of left-handedness. Since it is also associated with schizophrenia (and that is associated with left-handedness) it may well be that this particular gene is just linked to schizophrenia and not involved with determining hand preference at all.
So if we don’t know the ultimate cause of handedness, can we still work out its evolutionary history? Pretty much. The muscles on a creature’s preferred hand are used more often and thus bigger than their other hand. Such an increase in size typically leaves a bigger muscle scar on the bone allowing handedness to be identified in the skeletal remains of our ancestors.
The results are that pretty much every specimen we can identify handedness on shows a bias towards the right hand. However, a lack of well preserved, articulated hands for early hominin species means that all of these right handed specimens come from the genus Homo. This only emerged 2.6-2 million years ago, meaning there are still several million preceding years of unknown preference.
You can also work out handedness by looking at stone tools. However, since Homo is the only genus known to manufacture stone tools this research doesn’t really add anything new to that picture.
So we don’t know how it evolved nor do we know when. Could we work out why? Err…no. Whilst some studies proclaiming an advantage to one handedness (normally left) do occasionally appear there are also many studies – including reviews of the literature – finding no such effect.
Many have suggested that it is linked to hemisphere dominance in the brain with most humans, for one reason or another, favouring their left hemisphere (linked to the right side) resulting in most humans using their right hand. So right handedness itself might not offer an advantage but is just a by-product of another beneficial trait.
However, if using the left hemisphere is so great then why do left-handed people still exist? One favoured explanation for their persistence is frequency-dependent selection. This is when the advantages of one trait are dependent upon how frequent another trait is. For example, in an environment where everyone fights if they see someone else an individual runs away when they see someone will be at a large advantage as they are less likely to be injured in a fight. Conversely, in an environment where everyone runs away someone who fights will do well as nobody fights back.
There is a ratio of fighters to runners where both are similarly advantageous. This ratio will be stable and persist through time, with any influx of runners giving an advantage to the fighters thus restoring the balance. Perhaps left handed individuals do well in a right handed world and the 90/10 ratio of right/left hands we see is the “evolutionary stable strategy” for handedness.
Yet worldwide studies of handedness show that this “stable” ratio can vary by as much as 10%! This isn’t what we would expect if it were an evolutionary stable strategy. This might have something to do with different cultural stigmas against a particular hand preference artificially altering the ratio. We’ve all heard tales of left-handed people not being allowed to become knights.
Whilst this explanation means left-handedness could still be the product of frequency-dependent selection it also means that we would have no way of determining if it actually is. Any divergence from predictions could just be explained away as cultural influence. Whilst a very detailed study could control for such influences, to my knowledge none have been done.
In conclusion, we know a gene causes handedness, although we don’t know which, and we know a large right-handed dominance emerged in Homo although we don’t know when or why. Kind of a non-answer to the question really, but I think that’s allowed since it was a non-question to begin with.
|Cashmore L, Uomini N, & Chapelain A (2008). The evolution of handedness in humans and great apes: a review and current issues. Journal of anthropological sciences = Rivista di antropologia : JASS / Istituto italiano di antropologia, 86, 7-35 PMID: 19934467|
|FINCH, G. (1941). CHIMPANZEE HANDEDNESS Science, 94 (2431), 117-118 DOI: 10.1126/science.94.2431.117|
|Francks C, Maegawa S, Laurén J, Abrahams BS, Velayos-Baeza A, Medland SE, Colella S, Groszer M, McAuley EZ, Caffrey TM, Timmusk T, Pruunsild P, Koppel I, Lind PA, Matsumoto-Itaba N, Nicod J, Xiong L, Joober R, Enard W, Krinsky B, Nanba E, Richardson AJ, Riley BP, Martin NG, Strittmatter SM, Möller HJ, Rujescu D, St Clair D, Muglia P, Roos JL, Fisher SE, Wade-Martins R, Rouleau GA, Stein JF, Karayiorgou M, Geschwind DH, Ragoussis J, Kendler KS, Airaksinen MS, Oshimura M, DeLisi LE, & Monaco AP (2007). LRRTM1 on chromosome 2p12 is a maternally suppressed gene that is associated paternally with handedness and schizophrenia. Molecular psychiatry, 12 (12) PMID: 17667961|
|Halpern DF, Haviland MG, & Killian CD (1998). Handedness and sex differences in intelligence: evidence from the medical college admission test. Brain and cognition, 38 (1), 87-101 PMID: 9735180|
|Hopkins, W. (1996). Chimpanzee handedness revisited: 55 years since Finch (1941) Psychonomic Bulletin & Review, 3 (4), 449-457 DOI: 10.3758/BF03214548|
|Keller JF, Croake JW, & Riesenman C (1973). Relationships among handedness, intelligence, sex, and reading achievement of school age children. Perceptual and motor skills, 37 (1), 159-62 PMID: 4727991|
|Perelle, I., & Ehrman, L. (1994). An international study of human handedness: The data Behavior Genetics, 24 (3), 217-227 DOI: 10.1007/BF01067189|
|Toth, N. (1985). Archaeological evidence for preferential right-handedness in the lower and middle pleistocene, and its possible implications Journal of Human Evolution, 14 (6), 607-614 DOI: 10.1016/S0047-2484(85)80087-7|