Each little flower that opens,
Each little bird that sings,
He made their glowing colours,
He made their tiny wings.
All of us have been moved by the riot of colours in a spring meadow, the majesty of a sunset or the beauty of a bird in breeding plumage. What greater evidence could there be of a loving creator than our colourful world and the human eye that is equipped to appreciate it?
This is the sort of simplistic argument that convinces a lot of creationists about their beliefs.
It's time to "unweave the rainbow".
Light is detected by a proteins called opsins. We have four types of opsin in our eyes. One called rhodopsin is extremely sensitive to light but is no use in determining colour. The other three opsins are very similar to each other but are tuned slightly differently to detect different frequencies of light - approximately red, green and blue.
In order to distinguish a colour, it is necessary for at least two opsins to be excited. Rather like the way our brain detects depth by comparing the differences in the picture between our eyes, it sees colour by detecting the different response between two or more opsins.
In having three types of colour receptor, humans, apes and New World monkeys are different from all other mammals who have only two. But before we get too smug about our abilities, consider that we are practically colour-blind compared to most fish, reptiles and birds who have four or more colour opsins.
The rise - demise - and further rise - of our colour vision is a classic story of evolution.
Our ancestors of modern mammals went through a period of living quietly and coming out to feed after dark. Those who were foolish enough to go out in daylight mostly became dino-snacks and left no offspring. We saw in #4 of this series that we still have 400 broken genes for olfactory receptors that were vital to our nocturnal ancestors. During this time there was no selective pressure to retain colour vision and mutations left us with just two working colour opsins.
Following the sudden demise of the dinosaurs 65 million years ago our ancestors began to thrive. Lots of vacant niches were available to be exploited including the opportunity to feed in daylight. But having lost one or more of our opsins how was it possible to replace it?
It turns out that it really wasn't too difficult.
In mammals the gene for the opsin that reacts to blue light (SWS opsin) is located on chromosome 7. It is sensitive to light of a wavelength of 430 nm (nanometres) Mammals then have a second gene on the x chromosome that has a maximal absorbance between 510-540 nm.
Humans and our closest primate cousins have two genes at this location on the x chromosome. They lie head-to-tail as a tandem pair and are 98% identical. They are one of many examples of gene duplication - copying errors.
The proof that many of our genes arose in this way is found in the identical non-coding DNA that gets duplicated along with the gene.
These duplication events increase the "information" that is available for natural selection to go work on.
Small mutations resulting in trivial differences in amino acid sequences have fine-tuned the pair of opsins so that one has a sensitivity to green light at 530 nm (MSW opsin) and the other to red at 560 nm (LSW opsin)
There are just 15 amino acid differences between the green and red opsins. Experiments have discovered that three of these differences are crucial - at positions 180, 277 and 285. At each of these places a single letter of DNA has been substituted resulting in different amino acids at those locations.
This also explains why colour-blindness is mostly a male problem. Men don't have a spare x chromosome with a backup copy of MSW and LSW if they are unlucky enough to inherit a faulty one.
There is so much more that can be said about is fascinating topic but that is probably enough for starters. Next time you wonder at a beautiful array of colours in the natural world remember to thank the series of mutations that restored our trichromatic vision.
Evolution is a Fact - Index #1-20
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Evolution is a Fact #21 - Footprints in the Sand
Footprints at Laetoli show our Australopithecus afarensis ancestors were bipedal 3.6 million years ago.
Evolution is a Fact #22 - The Hillocks of Hiss
A vestigial feature if the human ear shared by 10% of the population demonstrates our evolutionary history.
Evolution is a Fact #23 - Faunal Succession
The consistent sequence of fossils found in the rocks can only be explained by evolution.
Evolution is a Fact #24 - The Origin of Your Inner Ear
How the bones that reptiles eat with became the bones that we hear with.
Evolution is a Fact #25 - Deep Time
Scottish geologist Andrew Hutton discovered the proof of earth's great antiquity.