The emergence of laterality—the tendency of the body or brain to favour one side over the other—long predates handwriting, tools, or surgery. It predates humanity itself by hundreds of millions of years.

Laterality likely emerged around 550–600 million years ago with the appearance of bilaterians, the first animals with a defined front, back, left, and right. Before this transition, life was largely radial: organisms such as jellyfish were symmetrical and unconcerned with directionality. Once animals began moving deliberately through their environment, however, orientation became essential. Coordinated movement, predator avoidance, and goal-directed behavior required a clear left–right distinction.

Gastropods provide one of the earliest and most striking examples of stable left–right asymmetry. Their spiral shell coiling, present since early Cambrian forms, established a fixed internal chirality akin to situs inversus. The predominance of dextral coiling (Fig. 3) reflects evolutionary canalization rather than functional necessity: sinistral forms have existed for more than 500 million years and persist today, either as rare variants or species-specific traits. Importantly, sinistrality does not confer reduced biological performance.

Dextral is not defined by how the spiral looks, but by the position of the aperture

For much of modern history, brain laterality was thought to be uniquely human, with language confined to the left hemisphere and right-handedness as its consequence. This view is now obsolete. Evidence of cerebral specialization [15] extends back more than 500 million years. Many fish, birds, and reptiles display functional hemispheric asymmetry, typically with one hemisphere monitoring threats while the other supports foraging or routine tasks. Such division of labour enhances efficiency. Preference for one limb over another is ancient and widespread: invertebrates such as octopuses show side preferences. Dinosaurs and early mammals seem to favor one limb or claw, based on fossil wear patterns. great apes demonstrate task-dependent laterality—especially for complex actions like tool use. laterality, therefore, is not an afterthought. it is deeply embedded in motor control and problem-solving across species.

In humans, laterality shifted significantly with the development of language and complex tool-making around two to three million years ago. Stone tools attributed to Homo habilis display apparent asymmetries consistent with right-handed use. As language networks became mainly localized in the left hemisphere, which controls the right hand, precision tasks similarly adapted. The result? A reinforcing cycle: language in the left hemisphere, fine motor control in the right hand, and cultural transmission driven by consistent dominance. Laterality transitioned from being simply an individual trait to a collective one.

Humans are unusual. About 90% of us are right-handed, a ratio rarely seen in the animal kingdom. Sometimes, what is considered “unique to humans” turns out to be an “evolutionary anomaly.” Our species is the only primate with such a strong preference for the right hand (Fig. 4). Only one in ten humans is left-handed, and around one in a hundred is ambidextrous. The Javan langur (Trachypithecus auratus) is the only other species showing a stronger individual manual preference: regardless of side (right or left), it always uses the same hand, for example, to grasp a branch—a trait also seen in other arboreal monkeys, like spider monkeys, but to a lesser degree.

In most humans, language areas are located in the left hemisphere. Since this hemisphere controls the right side of the body, grouping language and fine motor skills together likely improved speed and efficiency

While genes play a role, it isn't a simple "left-hand gene" (Fig. 5). Identical twins may differ in handedness. This proves that environment and developmental chance in the womb are just as important as DNA. There is no “single left-handed gene.” Instead, many genes influence brain asymmetry, including LRRRTM1, PCSK6, MAP2, and several others identified in extensive genome-wide studies. Their effects are modest but cumulative. This suggests a genetic predisposition influenced by developmental and environmental factors.

Two right-handed parents have a 90% chance of having a right-handed child. Two left-handed parents still only have about a 26% chance of having a left-handed child

Culture tried (and failed) to eliminate it: For centuries, left-handedness was discouraged, corrected, or punished. As these pressures eased, the reported prevalence increased—then stabilized around 10–12%, indicating a biological baseline. The message is clear: culture can suppress expression but cannot erase biology.

This left-hemisphere specialization for language areas, present in 90% of humans, is also seen at birth in baboons and chimpanzees—species that do not speak. It predicts right-hand preference only for communicative gestures. But the picture is not simple: among humans, left-handers are not just the opposite of right-handers. How do left-handed people point? Mostly with the right hand. Why? Because pointing isn't just a motor action — it’s mainly a communicative gesture closely tied to language. In about 90% of humans, including around 70% of left-handers, language is controlled by the left hemisphere, which governs the right hand. As a result, when writing, manipulating objects, or pointing, left-handers usually use their left hand; but for indicating or signaling, they often use the right hand. These links between manual control and gestural signaling suggest that human language specialization might have origins “at least 25 million years ago.”