As has been found in some bird and primate species, human eye and visual system sizes vary with the levels of ambient light in a given environment, according to a study published yesterday (July 26) in Biology Letters. Near the Earth’s poles, where average day length and light levels decrease, people have evolved larger orbital volumes, which are known to correspond to larger eyeballs and visual cortex volumes. Visual acuity, however, does not vary among human populations by latitude, suggesting that the changes in ocular size exactly compensate for the lower light levels to maintain an average visual performance.
While a similar relationship between low light levels and visual system size is well documented in birds and non-human primates, which are active at dawn and at night, respectively, this is the first evidence that such variation occurs among human visual systems. Larger eyes, with larger corneas and pupils and more rods and cones, allow for enhanced sensitivity and acuity (e.g., night vision).