Wednesday, 31 December 2014

Glorious Technicolour Part Two:

Opsin proteins are vital in allowing animals to see in colour
In the previous post I detailed the discovery of 300 million year old photo-receptors in the eyes of a fossil fish. Believed to be the common ancestor of most groups of living vertebrates, this makes it the oldest known instance of colour vision in the fossil record.

Yet colour vision across the vertebrate family varies massively. While the urban legend that dogs can only see in black and white is false, their vision is different to ours. Similarly, birds have the ability to see UV while we do not. Since colour vision evolved and was passed on to our ancestors, mutations have altered the proteins and pigments involved in the detection of colour to produce the variety of results today.

A genomic study of primates has revealed the precise origins of human colour vision and its evolutionary story. Five classes of genes which encode for pigment proteins known as opsins are involved in dim light and colour vision. 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, with the ability to see the full-colour spectrum of visible light, except for UV which shifted to allow for the detection of blue light.

5,040 mutation pathways leading to changes in the amino acid sequences responsible for the shift from UV to blue sensitivity were identified by the researchers at Emory University, Atlanta, Georgia. In total this involved seven genetic mutations, but the combination of changes was important. 'We have now traced all of the evolutionary pathways, going back 90 million years, that led to human colour vision,' said lead author Shozo Yokoyama. 'We've clarified these molecular pathways at the chemical level, the genetic level and the functional level.'

Gorillas, chimpanzees and humans share the same type of colour vision
80% of pathways rendered the protein non-functional as they prevented water from entering the pigment complex - a fundamental error as water is needed for the pigment to function. Out of the remaining 20% the precise pathway was identified. Alongside the identification of the three amino acid changes responsible for allowing us to see green, this has given us the precise mutational pathway leading to human colour vision. 'Gorillas and chimpanzees [our closest relatives] have human colour vision,' said Yokoyama. 'Or perhaps we should say that humans have gorilla and chimpanzee vision.'

In previous research, Yokoyama showed how the scabbardfish, which today spends much of its life at depths of 25 to 100 metres, needed just one genetic mutation to switch from UV to blue-light vision. Human ancestors, however, needed seven changes and these changes were spread over millions of years. 'The evolution for our ancestors' vision was very slow, compared to this fish, probably because their environment changed much more slowly,' added Yokoyama. Fossils give us an understanding of what happens at the level of individual organisms - a very Darwinian approach. Studies which put their focus at the level of the gene offer us fresh insight from the gene-centred view of evolution into how life on Earth has evolved. By taking both viewpoints and correlating them with the geological timescale we gain a deeper understanding of why organisms have evolved in the way they have.