For decades paleontologists have assumed that early primates rested by day and fed by night. But a new study of light-sensing proteins suggests that the first primates' eyes were better suited for daytime and that only later did some shift their activities to the night. If that's the case, then primates have always been adapted to looking for food and shelter during the day and didn't have to develop this capability as they diversified.
Modern primates can be either diurnal or nocturnal, but the most primitive ones--such as bushbabies--are night owls. They have a special lining behind their retina for concentrating light, which is useful for night foraging. Some species that are active in the day also have this lining, suggesting that ancestral primates were night-dwellers, with some primates later evolving daytime routines.
Wen-Hsuing Li, a molecular evolutionary biologist at the University of Chicago, and colleagues examined the molecular evidence for this scenario. They compared the DNA sequence of genes for light-sensitive proteins--called opsins--from species widely distributed on the primate tree. For one analysis, they looked at DNA from 25 species, concentrating on the genes for pigments sensitive to green or red wavelengths, which are not useful at night. If ancestral primates were nocturnal, then these genes should vary among living species, because mutations wouldn't have compromised survival.
They found the opposite: The red and green pigment genes were quite similar across species. That means "they came from a common ancestral gene," says Li. The same was true for genes for blue-purple vision, also not essential for night vision. The exception was bushbabies and related primates that had switched lifestyles early enough in their history to allow these genes to mutate. Taken together, the data indicate that the common ancestor of primates was active in daytime, says Li, whose group publishes its findings online this week in Proceedings of the National Academy of Sciences.
"The results are very, very surprising," says John Fleagle a paleontologist at Stony Brook University in New York. "It contradicts everything we have thought for a century about the evolution of primate vision." But Christopher Heesy at the New York College of Osteopathic Medicine in Old Westbury thinks the work needs to be expanded to a broader range of primates. "I don't think the question is closed," he says.