The short answer

While we don't know for sure, it's theorized that humans predominantly became right-handed due to the development of the left brain hemisphere associated with language and tool use. Additionally, right-handedness may have offered advantages in fighting and soothing infants.

The long answer

From scissors to can openers, and school desks to pants zippers, ​we live in a right-handed world​.

Roughly 90% of the human population prefer using their right hands. While some animals do show a preference for using their left or right hands (or paws), humans' overwhelming right-handedness sets us apart. In fact, no human population has ever reported a majority of left-handed individuals.

Our right-handedness is not a recent evolutionary development. The earliest evidence of right-handedness dates back as far as 1.8 million years ago, from fossil records of Homo habilis, an early human species.

Researchers discovered microscopic marks on the teeth of these early humans. The hypothesis went that these marks were made when early humans held materials, like animal hides, in one hand and used their teeth to secure the other side, freeing a hand to use stone tools, which sometimes hit their teeth (ouch). The angle of the marks indicates that these early humans were predominantly right-handed.

So we've been mostly using our right hands for a long time. It raises the question: why did the right hand become dominant and not the left?

While this question has been the subject for plenty of scientific debate, the fact is that we don't have a definitive answer as to why humans are mostly right-handed. But here are three leading theories.

Reason #1: Humans evolved to use language.

You've probably heard before that the left side of your brain controls the right side of your body, and vice versa.

But you know what else is on the left side of your brain? Language.

This hypothesis states that as humans developed more complex spoken language, the left brain hemisphere evolved. The preference for using the right hand, controlled by the left brain, may have intensified as linguistic processing advanced.

Reason #2: Humans evolved to use tools.

Another thing that sets humans apart from many other animals is our ability to create and use tools.

Brain scans have shown that a specific region in our left brain hemisphere, called Brodmann area 44 (BA44), fires up when we use tools for various tasks.

Similar to the language hypothesis, the idea is that increased specialization in tool use led to more activity in the left brain hemisphere. Since the left side of our brain controls the right side of our body, this tool-making behavior may have contributed to the preference for using the right hand.

Reason #3: There are some behavioral advantages to being right-handed.

Finally, there are some advantages associated with right-handedness. The fighting hypothesis suggests that a fighter who held his spear in his right hand improved his chances for survival by better protecting the heart, located on the left side of the body. This would lead the right hand to be used for more skilled movement.

Another hypothesis suggests that mothers holding infants in their left arms could better soothe them with their heartbeat, freeing the right hand for complex tasks.

Of course it is possible that the predominance of right-handedness happened before these behaviors.

I guess we're left to wonder, which one of these theories is right.

Check out some other curious questions:

Sources

Frayer, D. W., Clarke, R. J., Fiore, I., Blumenschine, R. J., Pérez-Pérez, A., Martinez, L. M., Estebaranz, F., Holloway, R., & Bondioli, L. (2016). OH-65: The earliest evidence for right-handedness in the fossil record. Journal of Human Evolution, 100, 65–72. https://doi.org/10.1016/j.jhevol.2016.07.002

Goldman, J. (2014, December 14). Evolution: Why are most of us right-handed?. BBC News. https://www.bbc.com/future/article/20141215-why-are-most-of-us-right-handed

Kolb, B., & Whishaw, I. Q. (2009). Variations in Cerebral Asymmetry. In Fundamentals of Human Neuropsychology (6th ed., pp. 309–313). Worth Publishers. https://students.aiu.edu/submissions/profiles/resources/onlineBook/d7t3G4_0716795868_Fundamentals.pdf

Larsson, M., Schepman, A., & Rodway, P. (2023). Why Are Most Humans Right-Handed? The Modified Fighting Hypothesis. Symmetry, 15(4), 940. https://doi.org/10.3390/sym15040940

PBS Eons. (2021b, March 24). How humans became (mostly) right-handed. YouTube. https://www.youtube.com/watch?v=vb11oOHYNXM

Price, M. (2009, January). The left brain knows what the right hand is doing. Monitor on Psychology. https://www.apa.org/monitor/2009/01/brain

Spry, C. (2017, January 10). Why are most people right handed? The answer may be in the mouths of our ancestors. The Conversation. https://theconversation.com/why-are-most-people-right-handed-the-answer-may-be-in-the-mouths-of-our-ancestors-69712

Tarlach, G. (2018, May 31). Right or Left: Human Handedness Is An Ancient Trait. Discover Magazine. https://www.discovermagazine.com/the-sciences/right-or-left-human-handedness-is-an-ancient-trait