ABSTRACT
The primary motor cortex originated no later than 160 million years ago with mammals. The cerebellum also has motor function and dates to 400-500 million years ago. Other motor areas date roughly from the origin of primates 83 million years ago.
An important monkey development, extending down to apes and humans, was precise premotor cortex control over the lower limbs. Mirror neurons in its lower portion rapidly fire when action is observed and copied, playing a large role in social learning and mimicry.
Population handedness, the tendency of a population to be either right- or left-handed, is a behavioral marker for aspects of cognition. Evidence suggests the existence of three handedness components. Judging from living primate species, the unskilled component, involved in reaching and pointing, became predominantly right-handed by 32 million years ago in monkeys. A slightly right-handed skilled component, used in hammering and food extraction, evolved in apes about 13 million years ago and became more right-handed in hominins. An insertion-turning component, initially slightly left-handed, became right-handed in hominins after 7.5 million years ago.
In humans, population right-handedness likely reflects the unification of movement timing and sequencing in the brain's left hemisphere. A major cognitive advance, it allowed fast, smooth control over movement, improving tool skills.
But why has left-handedness continued in 10% of the population? A good guess is that it affords an advantage in face-to-face confrontations. Interactive sports like martial arts and hockey have more left-handers than noninteractive sports like swimming and archery. Also, traditional societies with higher homicide rates have more left-handedness. These observations support an "unorthodox style" hypothesis: Right-handers are confused by left-handed defensive stances, while left-handers are familiar with much more common right-handed stances.
