A couple of weeks ago, I wrote about handedness and pondered whether the phenomenon occurred in other animals or if it was a purely human condition. I am always delighted when I receive follow up to a posting and, on this occasion, Charles Manning emailed me. As he is located in New Zealand, he is, I suppose, one of my most distant readers [unless there is anyone on the ISS tuning in].
Charles made a couple of interesting points …
First was a question:
“I think you will find that non-humans are as handed as humans. We are made of the same basic ingredients so why would you think otherwise?”
This is, of course, quite true. We are made of the same “stuff”. At the lowest level, the organic chemistry of all life on Earth is much the same. Similarly, some more complex aspects of life are consistent – I am thinking of DNA etc. Amongst the animal kingdom, many organs and systems are identical or very similar across a wide range of species. But I am not sure that this has a direct bearing on handedness, which, as far as I can see, is a manifestation of brain function or structure. It is our large, complex brain that makes humans stand apart from most other animals. It would seem likely [to me] that handedness is an artifact or side-effect of the evolution of our brains. Perhaps it is somehow related to the separate functionality of the right and left lobes of the brain. [I have no idea whether any other animal’s brain is like this.]
Charles makes a further point:
“There are of course also some obviously asymmetric animals. Here in NZ we have the wrybill with the beak bent sideways (always to the right). Elsewhere there are fiddler crabs which have one claw bigger than the other. Fiddler crabs are both leftpaws and rightpaws. While I did live near a fiddler crab population years ago, I no longer do. IIRC, most, if not all, the crabs I saw as a child had enlarged left claws. I suspect there is some genetic factor such that certain populations are dominantly L or R.”
Although I wrote about bodily asymmetry – or, rather, the lack thereof in humans – when I was talking about handedness before, I do not feel that such asymmetry is really handedness. Human handedness is invisible and not apparent until someone performs an action which shows a preference for right or left. The appearance of asymmetry in the bodies of other animals is interesting. I cannot decide whether this goes against the theory that we interpret symmetry as a sign of “healthy genes” and breed selectively on this basis.