Our unique primate eyes

Humans and other anthropoid primates (monkeys and apes) are the only groups of day-active mammals who have not retained the anatomical signs of nocturnal life in their eye structures.

The new study, led Dr. Chris Kirk, from the The University of Texas at Austin, compared the eye morphology of 266 mammal species in order to understand the relationship between eye shape and activity patterns in mammals.

Most vertebrate groups exhibit eye shapes that vary predictably with their activity pattern. Nocturnal vertebrates typically have large corneas relative to eye size for increased visual sensitivity, and diurnal (active during day) vertebrates have small corneas relative to eye size for increased visual acuity. However, the research team found that nearly all living mammals have eye-shapes that appear nocturnal, regardless of their activity. They compared eye morphologies of mammals, birds and lizards using the ratio of cornea size and eye length - measures of the eye's ability to let in light and form sharp images. They found that diurnal and cathemeral (active during day and/or night) mammals were both similar to nocturnal birds and lizards. In fact, the only group to diverge from this pattern are anthropoids (higher primates: monkeys and apes, including humans) which have small corneas relative to eye length for increased visual acuity - most similar to those of diurnal birds and lizards.

So what does this tell us of our evolutionary past? It provides evidence for the 'nocturnal bottleneck' theory which suggests that mammalian sensory traits were shaped by a period of nocturnality during the Mesozoic Era (250 million years ago to 65 million years ago). During this time, mammals developed various adjustments such as high-frequency hearing and excellent olfactory sensitivity to survive at night and avoid predation by day-active dinosaurs. Adaptations for photopic (well lit) vision such as high acuity and good colour vision were lost as they passed through this 'nocturnal bottleneck'. When predatory dinosaurs died out, many mammals became diurnal but there was a lack of pressure to re-evolve daytime vision. Anthropoids however, relied on high acuity when predating small prey and so re-evolved this capacity. "Humans and other anthropoid primates are so dependent on vision for everything that they do," said Dr.Kirk. "We found that the distinctive eye shapes that set humans apart from most other mammals evolved a long time ago - way back with the origin of anthropoid primates."

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http://rspb.royalsocietypublishing.org/content/early/2012/10/18/rspb.2012.2258.abstract.