Monday, May 05, 2008

* Your Inner Fish by Neil Shubin

Turns out you really are just a big fish out of water after all. Most of the things that make you who you are, turn out to be things that your fishy ancestors had nearly 400 million years ago (MYA). Using this scaly past, it can help us understand why we have certain quirks in our body plans, senses, and behaviors. Here are a few memorable highlights that stood out.

Keep moving or else

Our circulatory system works superbly in an active animal, which uses its legs to walk, run, and jump… If the legs are not used much, the muscles will not pump the blood up the veins. Problems will develop if blood pools in the veins, because that pooling can cause the [one way] valves [in the veins] to fail. This is exactly what happens with varicose veins... This arrangement of veins can also be a real pain in the behind… During long hours of sitting, blood pools in the veins and spaces around the rectum. As the blood pools, hemorrhoids form – an unpleasant reminder that we were not built to sit for too long, particularly on soft surfaces. P188

Silence is not always golden

Sleep apnea is a potentially dangerous trade off for the ability to talk… The flexibility of our throat, so useful in our ability to talk, makes us susceptible to a form of sleep apnea that results from obstruction of the airway… During sleep, the muscles of the throat relax. In most people this doesn’t present a problem, but in some the passage can collapse, blocking breathing. P189

Your inner tadpole and hiccups

It turns out that the [brain stem] pattern generator responsible for hiccups is virtually identical to the one in amphibians. And not just in any amphibians – in tadpoles, which use both lungs and gills. When gill breathing, tadpoles want to pump water into their mouth and across their gills, but they don’t want water to enter the lungs. To prevent drowning, they close their glottis, the flap that covers the breathing tube. The brain stem pattern generator signals that an inspiration is followed immediately by a closing glottis (a hiccup in us)… The parallels between our hiccups and gill breathing are so extensive that many have proposed that the 2 are 1 in the same. Gill breathing in tadpoles can be blocked by carbon dioxide or by stretching the wall of the chest, just like our hiccups [think breathing into a paper bag and holding our breath to keep our chest expanded]… The longest uninterrupted hiccups in a person lasted from 1922 to 1990 (68 years!) P192

Why a man’s testes are to blame for his hernia

Our gonads begin their development in much the same place as a shark’s – near the heart and liver. While in utero as a fetus, our gonads grow and descend. In females the ovaries descend to lie near the uterus. In males, the testes must descend all the way outside the body cavity to the scrotum sac. This descent creates a weak spot in the body wall… When males contract their abdominal muscles, our guts push against the body wall. A weakness in the body wall means that guts can escape, creating a hernia… Men’s tendency to develop is a trade off between our fish ancestry and our mammal present. P196

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