How Humans Became (Mostly) Right-Handed

Thanks to Great Courses Plus for supporting PBS! Once there was a Neanderthal who was cleaning an animal's skin. They were holding the skin between their teeth while pulling it tight and scraping it with a stone tool. And every now and then the scraper would slip and accidentally scratch their front teeth. This individual lived about 130,000 years ago in what's now Croatia. And the scratches that these random accidents left on their teeth reveal important clues about the hands that made them. Based on the orientation of these scratches and those found on teeth from other sites, anthropologists have figured out that most Neanderthals were right-handed, just like most of us Homo sapiens. 70 to 95% of us, to be exact, including me. But today, no other placental mammal that we know of prefers one side of the body so consistently, not even our closest primate relatives. And not only that, but no human population has ever been recorded as being mostly left-handed. It turns out that our preference for one hand over another might be tangled up with some of the other unique traits that we inherited from our ancestors after our lineage split from the chimpanzees, namely walking upright and making stone tools. In fact, being right-handed may have deep evolutionary roots in our lineage. And yet, being a lefty does sometimes come with some unexpected advantages. Now, we generally think of being right or left-handed to mean preferring to use one hand over the other. But it's actually more complicated than that. Handedness, if you can call it that, seems to be more of a spectrum, with some people being strongly right or left-handed and others being somewhere between the two. And we could also do a lot of different things with our hands, from simply holding an object all the way to precise, delicate manipulation like threading a needle. People who generally prefer their right hand for tasks that require fine motor skills could be said to be right-handed, but they still use their left hands a lot in their daily lives. Take something like cutting up a carrot. I'd hold the knife in my right hand and do most of the work with it, but I'd still stabilize the carrot on the cutting board with my left. Now, like in all vertebrates, each hand is controlled by a different side of our brain. The left hemisphere controls the right side of our bodies, and vice versa. Also, the two hemispheres of our brain aren't perfectly symmetrical, which leads to different cognitive processes taking place in different parts of the brain. This separation is known as lateralization, and it's found in all vertebrates and some invertebrates too, and allows us to simultaneously process different types of information. But like many other traits, asymmetry and lateralization are pretty extreme in us humans, especially compared to other primates. And this may be part of the reason that we eventually came to prefer one hand over another. 99% of people have a dominant hand, and there's lots of evidence that it's been this way for a long time. Cave paintings all over the world from the late Pleistocene depict wild animals, hunting events, and notably, a ton of human hands. To make these hands, an artist probably placed one hand on the rock and then sprayed pigment over it by blowing into a straw-like tube, leaving an outline of the hand. And interestingly enough, the vast majority of the hands on the walls are left hands. So experiments that recreated this method have shown that these artists were predominantly right-handed. Hand preference in our species is often so strong that you could even see it in the skeleton, especially among athletes like tennis players that use one arm a lot more than the other. The bone of their dominant upper arm becomes thicker in certain places compared to the bone of their non-dominant arm, because they're subjecting it to more force more often. In fact, although we're the only species on Earth today that's so strongly handed, there's a lot of evidence to suggest that our extinct hominin relatives, that is, the members of our lineage after the split with chimpanzees, were mostly right-handed, too. And, weirdly enough, some of the oldest unambiguous evidence comes from teeth. Scientists had already suspected that Neanderthals were mostly right-handed based on their upper arm bones. Like a right-handed tennis player, Neanderthals had thicker upper arm bones in their right arms. But it's rare to find fossils of earlier hominins with bones from both arms preserved in order to compare them. So when scientists found microscopic scratches on Neanderthal teeth that were caused by tools, like those found on that Croatian specimen, they began to wonder about two things. First, how far back in the fossil record this evidence for handedness could be recognized. And second, what other behaviors handedness could be associated with. As it turns out, hominins have been using their teeth basically as a third hand for quite a while. So pretend you're pulling a piece of animal hide tight between your front teeth and left hand, stretching it out in front of you and holding a sharp stone tool in your right hand. To clean the hide, you scrape the tool across it from left to right. Now, if you slip and scratch your teeth, those scratches go from the upper left corner to the lower right corner of your incisors. If you were holding the stone tool in your left hand, they would go the opposite direction, from upper right to lower left. Similar scratches were found on 500,000-year-old teeth from Spain belonging to a large group of Homo heidelbergensis, the species that might be our last common ancestor with Neanderthals. And those scratches have even been detected on the teeth of a 1.8 million-year-old specimen of Homo habilis from Tanzania. While one right-handed Homo habilis doesn't mean the whole species was right-handed, it's clear that handedness itself is pretty old. And because no other primate species has extreme hand dominance, this trait must have emerged after our split from chimpanzees. But why did more than one hominin species start preferring one hand in the first place? And what's so special about the right hand anyway? Many studies have turned to genetics to try to find the elusive handedness gene. Observations of families and genetic studies have shown that handedness does appear to be somewhat heritable, and that men are left-handed more often than women. But many searches through our genome haven't found the gene that's responsible for left or right handedness. Instead, it seems like several genes may have some minor effects, and that other factors might also be at play. So other scientists have focused on the importance of brain lateralization and tool use. Brain scans of people performing a variety of tasks have shown that a specific region of our left hemisphere, called BA44, plays an important role in manipulating objects, including making and using tools. Since the left hemisphere controls the right hand, it's possible that the development of tools millions of years ago led to this hand eventually being favored across hominin species. And having a species-wide hand preference at all may be linked to an even older trait, bipedalism. Some other bipedal mammals, like kangaroos, seem to have a hand preference, which suggests that not moving on all fours may have something to do with it. What's interesting is that these kangaroos are mostly left-handed, and they don't use tools like we do. So if us being right-handed is somehow related to tool use in our left hemisphere, then why are there any lefties today at all? While we still don't know for sure, it's possible that at some point after the development of stone tools, everyone became right-handed. In this scenario, left-handedness may have emerged later, as a result of one, or several, genetic mutations. And since lefties make up a pretty consistently small portion of the population in our largely right-handed world, there must be some kind of evolutionary advantage. And this, too, might all go back to lateralization. Experiments and brain scan studies have shown that lefties tend to have less lateralized brains than righties. This means that they process information more evenly across their brains, and this may be associated with better coordination, memory, and verbal skills. Plus, according to several studies, it also gives them an unexpected edge in physical combat. Now, imagine you're in a boxing match. If you're right-handed and you've only encountered right-handed opponents, you'll probably be expecting hits to come from your opponent's right hand. But if your opponent is left-handed, they'll have the advantage of surprise, because they'll be striking from an unexpected angle. This left-handed advantage is seen in a range of interactive sports today, and is absent in non-interactive sports like darts. And it seems to be stronger in men than in women. One study from 2019 even found that left-handed boxers and MMA fighters win matches significantly more often than their right-handed opponents. This benefit could have directly led to increased survival among left-handed people. And it might also explain why the frequency of left-handers is so low. If too many people are left-handed, the advantage disappears. So, like tool use, bipedalism, and being relatively hairless and sweaty, being right-handed seems to have a deep evolutionary history in our lineage. Thanks to our highly lateralized brains, many of the mental processes that we use to make tools are concentrated in one area, which in most of us happens to be in the left hemisphere. And, since this hemisphere also controls the right side of our bodies, we tend to favor this hand more often. While having hand dominance is found in a few other bipedal mammals, no other primate shows the degree of favoritism for one hand that we do. And even though the exact origin of right hand preference isn't yet clear, lefties might have enough important evolutionary advantages for them to still be around. It just goes to show how variable we are as a species, and that there's no right way to be human. Thanks to Great Courses Plus for supporting PBS. 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