Primate roots of red-green vision

A chimpanzee reflects on lunch. Image: National Geographic

By BBC News Online's Helen Briggs

New research sheds light on how primates, including man, evolved the ability to see red and green.

Scientists believe this more advanced form of colour vision emerged because it helped our ancestors to forage for food.

Leaves must have been important as a critical, if small, part of the diet of our ancestors

Peter Lucas, University of Hong Kong

According to a study of chimps and monkeys in the forests of Uganda, red and green vision allows primates to select tasty young red leaves among the green hue of the trees.

The work overturns a long-held theory that the ability to see red, green and yellow/blue light (trichromatic colour vision) arose to help primates pick ripe fruits to eat.

A few animals can see colour, including man and the other primates, fish, amphibians, and some birds. But most of these animals have a relatively primitive form of colour vision limited to blue and yellow light.

It is only a small group of primates, including humans, that can see the full range of colours ranging from red to green as well as blue-yellow.

Forest fruits

Peter Lucas and Nathaniel Dominy of the University of Hong Kong, China, monitored the eating habits of primates in the Kibale National Park, Uganda, to see how their ability to see colours influenced the fruits or leaves they ate.

The black-and-white Colobus monkey, one of four primates studied. Image: National Geographic

They found that apes and monkeys can choose fruit using only yellow/blue vision but the animals had to see red and green to find the most nutritious young leaves which often have a tinge of red that sets them apart from the green forest.

"There is a very definite reward for being able to see red and green - that is higher protein food that is easier to digest," said Professor Lucas.

"Most people in the past have thought that red-green vision was important for finding fruits," he told BBC News Online. "We think it is more to do with finding leaves for food which often start out as red when young then change to green, in the Tropics.

"If animals feeding on these leaves can see red and green, they would be able to choose a young, red leaf rather than a tough, old one."

Colour blindness

The scientists believe that the development of red-green vision gave our ancestors a survival advantage over other primates.

Intriguingly, more humans suffer from colour blindness than monkeys, so we may be losing our ability to distinguish red from green.

"In some ethnic groups, particularly Caucasians, up to 8% of males have red-green colour blindness," said Professor Lucas.

"But this is almost unknown in the small group of primates that can routinely see red and green.

"Leaves must have been important as a critical, if small, part of the diet of our ancestors," he added.

The research is reported in the scientific journal Nature and supported by the Research Grants Council of Hong Kong and the National Geographic Society.

Young leaves are tinged with red. Image: National Geographic