Your Inner Fish

Neil Shubin

            Neil Shubin is a paleontologist whose team discovered fossils in 2004 of an animal which is a transition between a fish and a land-living animal. How we have evolved is fascinating and much more than a curiosity. He uses examples from living animals, fossils, and DNA to show how we are all related and how that information can be used to help understand our health problems and is being used to develop new treatments. The links are fascinating. How can it not be fascinating when we realize that all of our cells have the same genetic code and our cells differentiate and develop into thousands of kinds of cells which link to become our organs through communicating with each other and turning genes on and off. It is, as Neil Shubin states, like a pile of bricks being able to build a house by themselves. Mutations have enabled evolution but are also the cause of cancers. Did you know…

            Our sense of smell allows us to discriminate among five thousand to ten thousand odors. Some people can detect the odor molecules in a green bell pepper at a concentration of less that one part per trillion. That is like picking out one grain of sand from a mile-long beach…. Fully three percent of our entire genome is devoted to genes for detecting different odors…. Our large number of odor genes makes sense – mammals are highly specialized smelling animals….  But when geneticists looked at the structure of human genes in more detail, they found a big surprise: fully three hundred of these three thousand genes are rendered completely functionless by mutations…. Why have so many of our odor genes been knocked out? The answer was found by comparing genes among different primates. Primates that developed color vision tend to have large numbers of knocked-out smell genes. The conclusion is clear. We humans are part of a lineage that has traded smell for sight. We now rely on vision more than smell, and this is reflected in our genome.

            If we look around the animal world, we can assess whether animals are specialized for daylight or night by looking at the percentage of each type of light-sensing cell in their eyes. In humans these cells make up about 70 percent of all the sensory cells in our body. That is a clear statement about how important vision is to us.

            The whole system we use to perceive position and acceleration (our vestibular system) is connected to our eye muscles via connections in our brain. The motion of our eyes is controlled by six small muscles attached to the side walls of the eyeball. The muscles contract to move the eye up, down, left, and right. We can move our eyes voluntarily by contracting these muscles each time we decide to look in a new direction; but some of the most fascinating properties of these muscles is related to their involuntary action. They move our eyes all the time, without our even thinking about it. To appreciate the sensitivity of this eye-muscle link, move your head back and forth while looking at the page. Keep your eye fixed in one place as you move your head. What happened during this experiment? Your eyes stayed fixed on a single point while your head moved. This motion is so commonplace that we take it for granted, but it is incredibly complex. Each of the six muscles in both eyes is responding to the movement of the head. Sensors in your head record the direction and velocity of your head’s movement. These signals are carried to the brain, which then sends out signals telling your eye muscles to fire. This system can misfire, and misfires have much to tell us about our general well-being. This is one of the many things that our ambient visual system does subconsciously. This system can be disrupted from drinking too much alcohol. It can also be disrupted when the connections and timing are disturbed by a head injury and is often the most debilitating problem in post-concussion syndrome. So few things in life are simple.

For More:

Plasticity in Sensory Systems

The Body: A Guide for Occupants