Since the age of dinosaurs, most species of day-active mammals have retained the imprint of nocturnal life in their eye structures. Humans and other anthropoid primates, however, are the only groups that deviate from this pattern, according to a new study from Midwestern University and The University of Texas at Austin.
The findings, published in a forthcoming issue of Proceedings of the Royal Society, is the first to provide a large-scale body of evidence for the "nocturnal bottleneck theory," which suggests mammals evolved a variety of characteristic sensory traits along the evolutionary trek.
"The divergent patterns are probably a result of 100 million years of nocturnality when our distant mammalian ancestors were active at night and running from dinosaurs," says Chris Kirk, associate professor of anthropology at The University of Texas at Austin. "For the first time, we provide a really good test for the bottleneck hypothesis instead of just constructing a story to fit the facts."
The research team, led by Margaret Hall, an evolutionary biologist at Midwestern University's Arizona College of Osteopathic Medicine (AZCOM), analyzed one of the largest datasets on eye morphology ever assembled. Using a sample of eyeballs from 266 mammal species, the researchers used a multivariate statistical method to show that mammals active by day or night show only minor differences in eye morphology.
The researchers then compared the eyes of mammals, birds and lizards using the ratio of cornea size and eye length - two functionally important measures of the eye's ability to admit light and form sharp images. These analyses showed that diurnal (only active by day) and cathemeral (active by both day and night) mammals don't differ in their eye shapes. At the same time, diurnal and cathemeral mammals have eye shapes that are very similar to those of nocturnal birds and lizards. These results reveal that most day-active mammals have eye shapes that appear "nocturnal" when compared with other vertebrates.
One likely reason for these findings, Kirk says, is because mammals evolved other compensating senses (smell, touch, hearing) to adjust to nocturnal life. As a result, when some mammals became day-active after the extinction of non-avian dinosaurs, there was less pressure to evolve eye shapes for acute diurnal vision like those of other day-active vertebrates. Anthropoid primates (the group including monkeys, apes, and humans), are the only mammalian species that re-evolved eye shape for fine detailed daytime vision.
"Humans and other anthropoid primates are so dependent on vision for everything that they do," Kirk says. "In this case, we are radically different than other mammals. 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 more than 45 million years ago."