Scientists have traced all of the evolutionary pathways, going back 90 million years, that led to human colour vision.
"We've clarified these molecular pathways at the chemical level, the genetic level and the functional level," said lead author Shozo Yokoyama, a biologist at Emory University.
Around 90 million years ago, our primitive mammalian ancestors were nocturnal and had UV-sensitive and red-sensitive colour, giving them a bi-chromatic view of the world.
By around 30 million years ago, our ancestors had evolved four classes of opsin genes, giving them the ability to see the full-colour spectrum of visible light, except for UV.
"Gorillas and chimpanzees have human colour vision. Or perhaps we should say that humans have gorilla and chimpanzee vision," Yokoyama said.
The researchers focused on seven genetic mutations involved in losing UV vision and achieving the current function of a blue-sensitive pigment. They traced this progression from 90-to-30 million years ago.
The researchers identified 5,040 possible pathways for the amino acid changes required to bring about the genetic changes.
"We did experiments for every one of these 5,040 possibilities. We found that of the seven genetic changes required, each of them individually has no effect," Yokoyama said.
"It is only when several of the changes combine in a particular order that the evolutionary pathway can be completed," Yokoyama said.
About 80 per cent of the 5,040 pathways the researchers traced stopped in the middle, because a protein became non-functional.
Chemist Ahmet Altun, former post-doc at Emory who is now at Fatih University in Istanbul, solved the mystery of why the protein got knocked out.
It needs water to function, and if one mutation occurs before the other, it blocks the two water channels extending through the vision pigment's membrane.
"The remaining 20 per cent of the pathways remained possible pathways, but our ancestors used only one. We identified that path," Yokoyama said.
The study is published in the PLOS Genetics paper.
