Evolution mostly involves small, gradual changes, and for good reason - we might expect that large changes to an animal's genetic code, and therefore to its body plan, simply wouldn't work. It would be like shoving an extra cog into a finely-tuned machine and expecting it to fit in - the more likely outcome is a malfunctioning mess.
But that's not always the case, at least not for the evolution of the human eye. New research shows that the eye and its connections to the brain are surprisingly flexible, and can incorporate major evolutionary changes with ease.
In our retinas, cone cells are responsible for giving us colour vision. Most mammals have just two types, one that is sensitive to short violet-ish wavelengths of light (S cones), and another that responds strongly to medium greenish-blue wavelengths (M cones).
But somewhere in our history, humans and many other primates picked up a third cone sensitive to longer wavelengths (L cones), that allows us to see colours near the red end of the spectrum.
You might expect that adding another type of cone cell into the eye would be a very large step, requiring substantial (and gradual) changes in the wiring of both the retina and the brain. But Gerald Jacobs from the University of California has shown that it's as easy as installing new software into your computer. Together with Jeremy Nathans from Johns Hopkins Medical School, Jacobs genetically engineered a strain of mice that had human L cones in addition to their medium- and short-wavelength ones.
Using a technique called electroretinography, which measures the electrical responses of cells in the retina, they confirmed that these added cones were in full working order and were sending electrical outputs to the brain. They were clearly providing visual signals, but did this translate into any meaningful visual information?
Nathans and Jacobs set the mice a challenge to test their new retinal powers. They were shown three panels lit with coloured lights and had to pick out the one that was lit differently. The normal mice failed to tell the difference between greenish-blue and yellowy-orange lights. They only chose correctly about a third of the time - the success rate you'd expect from random guesswork.
But the triple-coned mice passed with flying colours, so to speak. After lengthy training, they picked the odd panel out up to 80% of the time. Their genetic change had clearly been smoothly slotted in to their nervous system. They were seeing in combinations of three basic colours.
The success of Nathans's experiment is testament to the tremendous flexibility of a mammal's nervous system. And it gives us a tantalising glimpse into how modern primate vision evolved. At some point in our evolutionary past, one of our ancestors was born with a mutation that gave it a third and slightly different type of cone cell. This change would have brought it an instantaneous competitive edge over its two-coned peers.
Having three types of cones, rather than two, greatly expands the range of light that an animal can detect, and gives it a much broader colour palette. Such an animal would have gained a deeper appreciation of its surroundings than its peers - its eyes would quite literally have been opened to new possibilities.
Some scientists believes that the key advantage lay in being able to discern unripe green fruit from ripe ones that are typically red or orange. The bright colours of fruit may even have co-evolved with the advent of three-coned primate vision, to take advantage of these new seed-dispersing agents.
Over future generations, these new visual powers would have been honed by further genetic changes, but it is highly likely that the initial genetic jump-start would have spread like evolutionary wildfire. The same may even apply to other senses, like taste and smell, with new genetic changes caused profound effects by adding new receptors and expanding an animal's sensory range.
Reference: G. H. Jacobs, G. A. Williams, H. Cahill, J. Nathans (2007). Emergence of Novel Color Vision in Mice Engineered to Express a Human Cone Photopigment Science, 315 (5819), 1723-1725 DOI: 10.1126/science.1138838
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So, my kids can have UV vision (from birds), but I can't, right?
Man, I want infrared and ultra violet vison so bad, so very very bad.
There's eye transplants, no?
That said I'll wait until the the technique is well established before I go under the knife/virus.
Interestingly, there's some evidence that some women have *four* color cones, which gives them even better ability to distinguish colors. http://www.post-gazette.com/pg/06256/721190-114.stm
*I* have UV vision.
It's not an advantage. For all intents and purposes, I'm blue/green colorblind. (Why? Because I can distinguish between shades that the rest of the world think are identical. How can I tell which color is different when they all are?).
Roses are striped. Tide soap is bright purple, and so are clothes washed in it. Moonlight is almost as bright as sunlight. Sunlight is excruciating.
I can also see the flicker in fluorescent light. Not sure if that is related, but the switchover from incandescents is already causing me problems.
I wonder if its more important to see clearly or to see colors, or phrased differently, what's worse: to be near sighted or to be color blind. I've also heard that there is a certain advantage to not seeing clearly, ... near sighted people tend to see the world differently that our sharped eyed breathren, much like lefties and righties often have different brain hardware.
Harlan,
Cool article! I've always wondered, as sometime's colors are different shades than someone else is telling they are, if we all saw color differently. For instance, that I would see a particular yellow more sun like and you would see it as mustard color... I am near sighted and also notice a particular clarity when I do not use my contacts and read or look at things close (where it's not blurry for me) I can see far more detail, like for eyebrow plucking ;) But, anyways. Great thought provoking post and thank you Harlan for the link!
Robert,
As a pretty blind far sighted person (-5.5 in both eyes; contacts ~ I walk into things) I can say that I would much rather see blurry color's than clear black and white. These studies are wonderful for the blind... Blindness is my second worse affliction fear (first is Alzheimer's)
Another great post Ed ~ Thanks!
KAS
Ringo, how could you have UV vision?
Have you heard of anyone else who can?
I would like very much to understand why all squirrel males are color blind or in other words why they don't seem to inherit the 3 opsin genes from their mother