Evidence Found To Support Nocturnal Bottleneck Theory
November 1, 2012

The Effect Of Prehistoric Nocturnal Life On Mammalian Vision

April Flowers for redOrbit.com - Your Universe Online

Most species of diurnal mammals have retained the imprint of nocturnal life in their eye structures since the age of dinosaurs. According to a new study from The University of Texas at Austin and Midwestern University, anthropoid primates - including humans, monkeys and apes - are the only groups that deviate from this pattern.

This study is the first to provide a large-scale body of evidence for the "nocturnal bottleneck theory," suggesting that mammalian sensory traits have been profoundly influenced by an extended period of adaptation to nocturnality during the Mesozoic Era. The Mesozoic lasted from 250 million years ago to 65 million years ago.

The study, published in an upcoming issues of Proceedings of the Royal Society B, explains that  to survive in the night, mammals had a host of visual capabilities which were lost as they passed through the nocturnal bottleneck. Some of these visual traits include good color vision and high acuity.

“The fact that nearly all living mammals have eye shapes that appear ℠nocturnal´ by comparison with other amniotes [mammals, reptiles and birds] is a testament to the strong influence that evolutionary history can have on modern anatomy,” says Chris Kirk, associate professor of anthropology at The University of Texas at Austin.

Early mammals were predominantly nocturnal during the Mesozoic, according to Kirk, partly as a strategy for avoiding predation by day-active dinosaurs.

“It´s a bit surprising to still see the effects of this long period of nocturnality on living mammals more than 65 million years after non-avian dinosaurs went extinct, but that´s exactly what we found,” Kirk says.

Margaret Hall, an evolutionary biologist at Midwestern University´s Arizona College of Osteopathic Medicine, led the team as they analyzed one of the largest datasets on eye morphology ever assembled. With eyeball samples from 266 mammalian species, the team showed that mammals active by day or night show only minor differences in eye morphology using a multivariate statistical method.

Then, the team compared the eyes of mammals, birds and lizards by using the ratio of cornea size and eye length. These are two functionally important measures of the eye's ability to admit light and form sharp images. Diurnal — only active during the day — and cathemeral — active during both day and night — animals don't differ in their eye shapes, the study found. Both groups also have eye shapes very similar to those of nocturnal birds and lizards. However, when compared with other vertebrates, the study found that most day-active mammals have eye shapes that appear nocturnal.

Kirk says that one possible reason for these findings is that after the extinction of non-avian dinosaurs, some nocturnal mammals became day-active. This put less pressure on them to evolve eye shapes for acute diurnal vision such as other day-active vertebrates.

The only mammalian group that re-evolved eye shape for fine detailed daytime vision, anthropoid primates have small corneas relative to eye length. Like in diurnal birds and lizards, this is an adaptation for enhanced visual acuity.

The findings of this study provides a deeper understanding of human sensory systems and our connection with monkeys and apes, our closest living primate relatives.

“Humans and other anthropoid primates are so dependent on vision for everything that they do,” Kirk says. “In this case, we are radically different from 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.”