human evolution continues -- 6/27/18
Today's selection -- from A Brief History of Everyone Who Ever Lived by Adam Rutherford. Subtle evolutionary changes in humans are occurring continuously:
"Our genomes are where evolution takes place. Our DNA changes over time, every generation. Most of these changes are subtle, many trivial. Some are teasingly interesting. We humans are trichromatic -- we see in three colors. In the back of our eyes we have photoreceptors, highly specialized cells whose purpose is to literally capture the photons of light that flood through our pupils. There are two classes commonly known as rods and cones: The rods are attuned to pick up movement and low lighting conditions, and they sit in the periphery of the retina, which is why we see indistinct but moving things out of the corners of our eyes. The cones are central, which is why your sharpest color vision is right in front of your eye. If you wave something in a hand far outstretched to your side and look straight ahead, you can see it move, but not what color it is.
"Then there are three types of cone, each further attuned to a specific wavelength of light, which determines what colors we see. Broadly, they are short, medium, and long wave, but roughly correspond to blue, green, and red, though they overlap in their range, and are subtly variable between people. The difference between each of these cones is down to a single protein called an opsin. The photon passes through your clear cornea and the nucleus-free cells of the lens, through the jelly aqueous, then vitreous humors, through three layers of brain cells, nerves, and blood vessels, and into the very back of the eye where the opsins sit bound into the pointy tips of the cones. There, the photons are captured
by the opsin molecules, which physically jiggle their shape in response, and that molecular shrug triggers an electrical impulse, which shoots out of the other end of the photoreceptor and through the several layers of nerve cells, which collectively bundle their nerve fibers into the optic nerve, into the visual cortex of the brain, and this is how you see.
"Many mammals have only two cone opsins, and so see color with less acuity than us. Most apes have three, as do the Old World monkeys that are indigenous to Africa and Asia. Cats have many more rods and so see in the dark much better than us, but not color. Certain species in the family of the mantis shrimp have at least sixteen opsins, fine-tuned to see red, blue, and green, as well as polarized light, ultraviolet, and a host of light unseen by us that we can only dream about.
"The mutations that gave rise to three colors in us (and the many in the shrimp), were not initially the single letter changes that make up most genomic change, but large duplications of whole sections of DNA, and subsequent typos. Colors are determined by the wavelength of the light we see, and the gene for the Shortwave opsin is on chromosome 7, whereas the Medium and Long are on the X. This is why men are more prone to color blindness than women: A faulty opsin on one X can be compensated for by a woman's second; men have no such insurance. The duplication of one opsin on the X to two at some point in our primate evolution allowed one of them to mutate freely without a loss of function, and thus we were free to acquire a new color sensitivity. That all happened tens of millions of years ago, long before humans, but something similar might be happening now in us -- in fact, some of half of us. Some women might be tetrachromatic. They, through another random chance duplication, have acquired a fourth opsin on one of their X chromosomes. Around one in eight women are estimated to have this extra gene variant, but whether that bestows tetrachromacy is not yet known. The ones who do have this power see colors where we see monotones. It's a new area of research, and the condition appears to be rare, and poorly accounted for. A few women have been studied, and they seem to see clear differences in colors that are merely shades to normal trichromats. When examining red-green color blindness, the Ishihara test presents a circle containing circles in different hues. Hidden in plain sight (to those with typical vision) is a number, but due to the design of the shades that pick out the number, it is invisible to color-blind people. The tetrachromat tests also rest on the ability to discriminate distinct hues of green where we only see olive.
"The theories behind why we evolved three-color vision are wide and varied. Many of them suppose that the ability to discriminate the redness of berries in a busy green forest canopy would be of great advantage to our foraging simian ancestors swinging in the trees.
"The advantage of the ability to discriminate four colors is a mystery. While many animals have more than our three, tetrachromacy in humans is likely to be recent and random -- chance plus time -- but not a mutation that has been negatively selected as it is unlikely to cause any phenotypic problem. It simply is -- another example of our infinite variation. It's not likely to spread far and wide, but who knows? Ask me again in 5,000 years."