Are other animals right handed like humans?

Something that has baffled biologists for decades is the powerful preponderance among the human race - and even our cave-dwelling ancestors tens of thousands of years ago - to prefer to use our right hands in the majority of cases. But are we alone in doing this, or do any of our close evolutionary relatives show signs of a hand bias too? Speaking with Chris Smith, Kai Caspar…

Kai - We wanted to learn how handedness evolved within the primates, right? Because we as humans have quite distinct handedness patterns and no one really knows how those emerged. And so by looking at different primate species, we hope that we could get hints on how the specific handedness of our species came from.

Chris - When we look at the human race, it's not just that a few people are handed, it's really, really skewed, isn't it? Like 90% of the population are right-handed. So how does that compare with our next nearest relatives in evolutionary terms? Mm-Hmm. <Affirmative>, what, what does it look like in that perspective?

Kai - It's actually very different. So as you said, we have as humans for quite a long time, this extreme right handedness propensity; it's present in all of our different populations. So you might look at hunter gatherers at urbanised societies, and even our closest living relatives to chimpanzees and bonobos. If you look at those, we have only about 50% right-handers. The right handers are still the largest fraction, very few left-handers and quite a number of ambiguously handed individuals that do not have a strong hand preference whatsoever. This is something that appears rather alien to us humans, but is rather common in many of the non-human primates.

Chris - How did you come up with this conclusion? How did you actually gather the data and then produce this very comprehensive look at our close relatives when you were doing this work?

Kai - We were trying to find a test. So a certain task that a lot of different species of apes and monkeys could do to have a standard surveying literature. We came across this so-called tube task, which is very simple. You just have small tube that you fill with some food incentive that the primates like. And then you will see of course that the animal will grasp the tube of one hand while the other one will retrieve the food. And if you observe it, you can do statistics and find out whether there is a significant bias towards right or left-handedness at the individual level. And when you test a number of individuals, of course you can do this for the population and by going through various zoos, we compiled quite a nice sample of different species and could then use these data to really open the phylogenetic perspective onto this. So we go through the primate family tree and see, well, species A has a certain hand preference pattern, species B has another one. And within this compilation we can also place humans of course, and view humans within this evolutionary framework.

Chris - You present the data as a big sort of graph where you've got a line representing how handed the population is and then other boxes for how big their brain is and so on it. It's a very nice way of displaying it, but it does show humans are way out there on their own with the vast majority of the population skewed one way and everybody else in all these different species, it seems to be roughly down the middle. So this argues then that whatever happened to us happened after we split away from all these other groups.

Kai - Yes, this is one of the main conclusions of the paper that we, humans are really unique, not so much in the strength of hand preference. This means if we are looking at an individual, be it an individual human or individual monkey. So if a human is a right-hander or lefthander, we are very consistent in using this one hand. The same might be true for various monkeys and also some apes, but what is really unusual is this population level bias that if we look at humans, we have this over-representation of right-handers. This really seems to be unique, has to be something that is unique to our lineage that led to the emergence of these hand preference patterns.

Chris - When you ask researchers over the years, why do you think that there is this hand preference, they, they kind of wave their hands about a bit and excuse the pun, and they say it's because the left side of the human brain is dominant, that's where language is. Well that to me just sort of kicks the can down the road because it then makes the question, well why is language on that side of the brain? Why should that hemisphere be dominant? What do you think based now on what you can see from across the animal spectrum and there isn't this relationship, what do you personally think is going on that that means we have this population level bias towards being right hand dominant 90% of the time.

Kai - So honestly I don't know. And what makes this study important is that we challenge many of the ideas prevailing in the research community, right? For instance, that the lifestyle of a species, whether it lives in trees or it lives on the ground, has something to say about these hand preference patterns. So that whether it uses tools or not, there's quite an influential hypothesis that claims that tool use is the main driver of hand preferences. But we looked at different tool using species and others that do not habitually use tools. And we found no differences whatsoever. But what our data do not allow us to do is come up with an alternative hypothesis. So I think you can criticise all the different approaches that have been proposed in the past, but we still miss the key to find out what really drove the evolution of this righthandedness in our species.

Chris - Often when we want to know the answer to why we do a certain thing or look a certain way or a certain thing works a certain way, we look to evolution because we can see how incrementally it's happened or it gives us clues. This lifestyle factor goes with this behavior, which is what you were just saying about things like tree living or ground living, isn't it? Yeah. So if we can't delve into the primate history to find the answer, if we look outside primates, if we look at other animals mm-hmm. <Affirmative>, now I know that's outside the scope of what you were considering here, but are there any animals that have the same population level handedness bias that we do that might give us clues as to why we are like this?

Kai - There are certainly non primate animals, also non primate mammals that have strong hand preferences and also significant population level hand preferences. So ground living kangaroos, there is strong bias in these animals towards right handedness, curiously. And it has been hypothesised that, again, terrestrial lifestyle has led to this and walking on two legs. And we also have evidence from certain groups of birds parrots for instance, that they have very strong limb preferences. Of course we're not talking about hands here but about feet. But yes, in principle maybe these animal groups that are more distant from us might give us some hints in the future.