After two decades of work, an international team of researchers has discovered how humans evolved to see all the colours of the rainbow. By figuring out how our ancestors swapped ultraviolet (UV) vision for blue-light (or violet) vision, they have finally pieced together a timeline for one of our species' most defining features.
"We have now traced all of the evolutionary pathways, going back 90 million years, that led to human colour vision," lead author and biologist, Shozo Yokoyama from Emory University in the US, said in a press release. "We've clarified these molecular pathways at the chemical level, the genetic level and the functional level."
In previous studies, Yokoyama and his colleagues showed that our early ancestors added green-sensitivity to their pre-existing red-sensitivity between 45 and 30 million years ago. To pinpoint how we ended up with the tricolour vision that allows us to see all the colours of the visible spectrum today, he needed to figure out when the eyes of our ancestors achieved blue-light vision.
In 2008, Yokoyama and his team showed how the deep sea scabbardfish switched from UV vision to blue-light vision thanks to a single genetic mutation. Our ancestors, of the other hand, needed to undergo many genetic mutations over several million years to achieve the same. "The evolution for our ancestors' vision was very slow, compared to this fish, probably because their environment changed much more slowly," says Yokoyama.
This time, the team looked deeper into this process by analysing ancestral molecules - proteins and pigments once present in our earlier ancestors that can now be synthesised in the lab. They discovered that five classes of opsin genes, which are found in the photoreceptor cells of the mammalian retina, are needed to encode visual pigments for dim-light and colour vision. As the environment changes around a species over tens of millions of years, they found, little bits of these opsin genes change with it to allow their vision to adapt.
Publishing in the journal PLOS Genetics, Yokoyama's team found that about 90 million years ago, our early mammalian ancestors were nocturnal creatures with UV-sensitive and red-sensitive vision. This means they basically saw the world in just two colours. Then, by around 30 million years ago, these mammals had evolved into primates with four different classes of opsin genes, which allowed them to see entire spectrum of visible light - everything except UV light.
"Gorillas and chimpanzees have human colour vision," said Yokoyama in the press release. "Or perhaps we should say that humans have gorilla and chimpanzee vision."
They found that seven genetic mutations and 5,040 possible pathways for the amino acid changes were required to bring about the evolution of human tri-colour vision. "We did experiments for every one of these 5,040 possibilities," Yokoyama says. "We found that of the seven genetic changes required, each of them individually has no effect. It is only when several of the changes combine in a particular order that the evolutionary pathway can be completed."
According to the press release, this means environmental influences were not enough to drive the evolution of colour vision - the change also required shifts in our ancestors' molecular environment.
The team found that about 80 percent of the 5,040 pathways stopped in the middle because a protein had finally been rendered useless by a new mutation that preceded it in the pathway. "The remaining 20 percent of the pathways remained possible pathways, but our ancestors used only one," Yokoyama says. "We identified that path."
It's pretty incredible to consider how such a drawn-out and complex series of changes could come together to result in something that most of us now take for granted.
"We have no more ambiguities," Yokoyama says. "Down to the level of the expression of amino acids, for the mechanisms involved in this evolutionary pathway."