Evolution

A new study has concluded that most species of mammal still possess nocturnal traits in their eyes structures. This provides substantial evidence for the "nocturnal bottleneck" theory, which suggests that mammalian evolution has been strongly influenced by nocturnal adaptation in the past.

Using one of the largest data sets on eye morphology ever collected - with eyeballs from over 260 mammal sp

ecies - researchers found that eyes of diurnal (day-active) mammals only showed very slight differences in appearance to the eyes of nocturnal mammals. Analyses were then conducted on mammalian, reptilian and avian eyes regarding the ratio of cornea size to eye length. These results revealed that aside from anthropoid primates (humans, apes and monkeys), most diurnal mammals have "nocturnal" eyes when compared to other vertebrates.

The explanation for this lies in the Mesozoic Era, when mammals shared the world with dinosaurs. Due to the threat of predation from day-active dinosaurs, mammals became more active at night and nocturnal adaptations evolved. When the non-avian dinosaurs died out some mammals became diurnal, although due to lack of selection pressure they did not develop strong diurnal traits.

So why are us primates so different? It seems there was more selection pressure on our ancestors to develop acute daytime vision, perhaps to compensate for other weaker senses. "Humans and other anthropoid primates are so dependent on vision for everything that they do," says Chris Kirk (University of Texas at Austin), a member of the research team. "In this case, we are radically different from other mammals".

Photo credit: Josh Bishop.

http://www.utexas.edu/news/2012/10/29/effects-prehistoric-nocturnal-life-mammalian-vision/