ABSTRACT
Judging location, depth, motion, and quantity are important spatial perception abilities. Initially, location sensing used the simple optical properties of the eye, with every point in space mapped to a specific retinal location. Beginning with early primates (83 million years ago), the superior parietal lobe of the brain began judging location relative to the body, an egocentric frame of reference. In contrast the temporal lobe judges location relative to the environment, an allocentric frame of reference that is also available to bats and rats, suggesting an origin at least 97 million years ago.
Depth provides many cues, with binocular disparity a particularly powerful one. Primates have always had forward-facing eyes, producing a large binocular field of view from at least 55 million years ago. It has increased in the descent to humans.
Motion detection partly depends on optic flow, in which objects rush by in the periphery while changes in central vision are slower. Area MT/V5 in the brain's temporal lobe processes it, while the parietal lobe computes heading. MT/V5 was a primate innovation, likely emerging 73-83 million years ago.
The ability to judge quantity can help determine which tree has more fruit, or which way to run when escaping groups of predators. Old World monkeys and apes, as well as humans, show evidence of "subitizing", a spatial method of fast counting small numbers of objects. It therefore appears to have originated at least 32 million years ago.
Orienting, reaching, grasping, and navigation are other important spatial abilities that in monkeys and humans involve parietal lobe mechanisms. Expansion of the lobe is visible in Australopithecus afarensis, and again in Homo habilis, suggesting a general enlargement relative to apes beginning at least 3.5 million years ago. Presumably this aided the numerous spatial processes served by our parietal lobes.
