The Development of Color Vision

Last year we looked at the research suggesting that the reason why primates developed color visions was so that they could pick up color changes on the faces and posteriors of other apes.

Now researchers at Ohio University have found that after primates evolved the ability to see red, they began to develop red and orange skin and hair.

As we discussed last year, humans, apes and Old World monkeys, such as macaques and leaf monkeys, all have trichromatic vision, allowing them to distinguish between blue, green and red colors. Primatologists have disagreed about whether this type of color vision initially evolved to help early primates forage for ripe fruit and young, to be able to red leaves among green foliage, or to tell the state of anger or arousal in other apes or to help them select mates.

Now a new study published online this week in American Naturalist by Ohio University researchers Andre´Fernandez and Molly Morris suggests that red-color vision evolved for non-social purposes, possibly foraging. But once developed, trichromaticism – i.e. three color vision – drove the evolution of red skin and hair through sexual selection.

Andre´Fernandez first began to question the strict correlation of food choice and color vision while studying howler monkeys in Costa Rica. He recently compiled data on the color vision, social and sexual habits and red skin and pelage (hair, fur and wool) of 203 different primate species.

The researchers then used a phylogenetic tree representing the evolutionary relationships among all the primate species under study to test hypotheses about the order in which the traits of red color vision, gregariousness (highly social behavior) and red coloring evolved. By comparing the traits of individual species in this evolutionary context, Fernandez and Morris could use statistical methods to deduce the probability of their ancestors having the same traits. They could also see if any of the traits were correlated with one another.

They found that the species that could discern red and orange hues were more likely to develop red and orange skin and hair, as well as highly social habits that make it easier to visually compare mates. In fact, the more social the three-color vision apes are, the more red coloring they show.

Fernandez said,

“Neuroscience research has found some evidence of a perceptual bias for more brilliant colors, so it is reasonable for primates with trichromatic color vision to respond more when they see bright colors.”

Molly Morris, the co-author on the paper is a fish biologist who studies how physical traits such as coloring evolve through sexual selection.

She said,

“It looks like red skin and hair became a sexual preference, so while the benefits in terms of eating may not apply anymore, the (red-color) vision in some groups is now relevant in social terms.”

So while foraging may have initially sparked red color vision, the new ability was likely “recruited” for social purposes.

This is not just an interesting little evolutionary tidbit. There are plenty of systems in the human brain that evolved to do one thing, and are now used for something quite different. And sometimes these new uses conflict with some of the other programs running in the brain. This is likely to be one of the causes for some types of psychological problems. when the brain is trying to do too many things at once, and there is an imbalance between the older limbic regions of the brain and the cerebral cortex.

Why Aren’t We Furry?

For years now, people have wondered why most humans have very little in the way of fur. I have just read about an interesting idea that I hadn’t seen before. The suggestion is that humans are hairless because Stone-Age mothers regarded furry babies as unattractive.

The article was written by written by Judith Rich Harris, the author of two controversial, but challenging and absorbing books: The Nurture Assumption and No Two Alike. The article is entitled “Parental selection: a third selection process in the evolution of human hairlessness and skin color” and was published in the journal Medical Hypotheses. The article just won the 2006 David Horrobin Prize for medical theory, which has been awarded annually and named in honor of Dr. David Horrobin, who was a renowned researcher, expert in biotechnology and founder of Medical Hypotheses, who died in 2003.

In the paper Rich Harris speculates that in Stone Age societies the mother of a newborn had to decide whether she had the resources to nurture her baby. The newborn’s appearance probably influenced whether the mother kept or abandoned it and therefore an attractive baby was more likely to be kept and reared.

Harris’ theory is that this kind of parental selection may have been an important force in evolution. If Stone Age people believed that hairless babies were more attractive than hairy ones, this could explain why humans are the only apes who don’t have a natural fur coat. She goes on to suggest that Neanderthals must have been furry in order to survive the Ice Age in more Northern Regions than the Cro-Magnons from whom we are descended. Our species would have seen them as “animals” and potential prey. So her final hypothesis is that Neanderthals went extinct because our human ancestors ate them.

It is certainly an interesting idea, but I need a bit more convincing. Humans, like most other mammalian species are hardwired to protect the young at all costs, and it difficult to imagine how many Stone Age Sophie’s Choices would have had to have been made to create a hairless species.

We may learn more from continuing research into the fate of the Neanderthals: did they die out, or, as the psychologist and writer Stan Gooch suggested, did we mate with them? Are modern humans not simply the children of Cro-Magnons, but hybrids of Cro-Magnons and Neanderthals?