Ancient Asymmetry

New study: 289 million years ago, animals already showed brain asymmetry.

Posted May 21, 2020

According to the world’s largest study on left-handedness, about 10.6% of people are left-handed, while 89.4% are right-handed (Papadatou-Pastou et al., 2020). While researchers initially thought the handedness is something uniquely human, we now know that many animal species show preferences to use one paw or limb more often than the other—including cats and dogs.

Handedness (or pawedness) is caused by the brain and represents one form of so-called brain asymmetries—e.g., left-right differences in the brain. Specifically, in left-handers, the motor cortex in the right side of the brain (the left side of the body is controlled by the right side of the brain, and vice versa) is dominant for fine motor behavior. In contrast, in right-handers, the left motor cortex is better at fine motor tasks such as writing or drawing. In addition to handedness, there are many other forms of brain asymmetries, e.g. in many humans, language is mostly processed by the left half of the brain or faces are mostly processed by the right side of the brain.

To understand, how and why these brain asymmetries developed, it is crucial to understand their evolution. One long-standing question has been, how far back in time handedness and brain asymmetries go back in time. However, it is not easy to answer this question scientifically based on the fossil record. Since the brain is a soft tissue, it does not fossilize and it is almost impossible to find evidence for “left-handedness” or “right-handedness” of ancient organisms based on fossilized bones.

A multinational research team from Canada, Germany, China, and the USA now found a creative way to investigate asymmetries in fossils (Reisz et al., 2020). The scientists investigated almost one hundred fossilized jaws from an extinct reptile species called Captorhinus aguti. Specifically, they looked at tooth wear on the left and the right sides of the jaws.

The results were pretty clear: On average there were more worn teeth on the right side (almost 40%) of the fossilized jaws than on their left side (a little bit over 20%). This indicates that this ancient reptile preferred to process food on the right side of the jaw and used a stronger bite force on the right side of the jaw. Since the right side of the body is controlled by the left side of the brain, this suggests that the left side of the brain of Captorhinus aguti was dominant for feeding behavior.

This finding has important implications for understanding brain asymmetries in humans. Captorhinus aguti lived 289 million years ago and the findings by Reisz et al. (2020) suggest that brain asymmetries are at least that old if not older. This suggests that the many asymmetries in our own brains have an ancient origin. Fascinatedly, humans today still show a strong rightward preference for chewing food. A 2004 study from Israel found that 78.3% of people preferred to chew food on the right side (Nissan et al., 2004), indicating that our brains are asymmetrically organized when it comes to chewing food—just like that of a reptile that lived 289 million years ago. Some things never change, I guess.

References

Nissan J, Gross MD, Shifman A, Tzadok L, Assif D. (2004) Chewing side preference as a type of hemispheric laterality. J Oral Rehabil. 31, 412‐416. 

Papadatou-Pastou M, Ntolka E, Schmitz J, Martin M, Munafò MR, Ocklenburg S, Paracchini S. (2020). Human handedness: A meta-analysis. Psychol Bull, 146, 481-524

Reisz RR, MacDougall MJ, LeBlanc ARH, Scott D, Nagesan RS. (2020). Lateralized Feeding Behavior in a Paleozoic Reptile. Curr Biol, in press.