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SCIENCE & DISCOVERY
THE FISH INSIDE US ALL
Tiktaalik: The missing link between fish and land-living amphibians and reptiles
Gill arches can be seen early on in human embryos
Back in Earth`s dim and distant past, a strange creature stuck its head out of the murky depths, gazed upon the land and thought to itself: “I`ll have some of that.” Tiktaalik, as it will come to be known, was a fish but it was a fish unlike any that had come before –its fins were functioning legs, complete with knees and wrist joints, and it could walk on land.
Some 380 million years of evolution later and its descendants can walk on two legs, need never visit water to breed and can operate a mobile phone. Its descendants are you and me.
But what price do we pay for our fishy ancestry? Neil Shubin, a palaeontologist and professor of anatomy, believes we pay a high one. In a new book, Your Inner Fish, he explains that, from the legs we walk with to the spine that holds us upright and the eyes we see the world with, we owe it all to fish. We also owe them for many of the ailments we suffer from.
Dr Shubin –who discovered tiktaalik in 2004 – describes humans as being like an over –tuned, over-modified hotrod – “great to drive but a little fragile”.
He states: “Take the body plan of a fish, reconfigure it to be a mammal. Then tweak and twist until it walks on two legs, talks and has superfine control of its fingers – and you have a recipe for trouble.”
In what form does that trouble manifest itself? Well, he argues that everything from hiccups, snoring and a bad back to hernias, haemorrhoids and weak knees can all be traced back to our fishy ancestry. In fact, virtually every illness we suffer from has its roots in the mammals, amphibians and fish that came before us.
Don`t be too hard on our scaly forebears, however – we can also thank them for pretty much everything else we have that does work.
BLAME IT ON THE FISH
Hiccups
Hiccuping has it roots in our fish and amphibian ancestry. It occurs when a primitive area of our brains prompts a reflexive firing of nerves that cause muscles in our chest, diaphragm, neck and throat to contract.
In fish, this area of the brain prompts the muscles of the throat and gills to contract and allow the fish to breath. For a fish, whose gills and throat surround the brain area, the nerves don`t have to travel far to reach them. As humans have evolved, however the nerves have had to travel much farther to reach the chest which leaves them open to interference – causing them to misfire which results in hiccuping.
Hiccups also have their roots in our amphibian past, specifically our time as tadpoles. Tadpoles use both lungs and gills to breath meaning that they must pump water into the mouth and throat and out across their gills. To prevent water entering their lungs as they do so, they seal the breathing tube with a flap called the glottis.
It is the rapid closing of the glottis in the human throat that causes the “hic” noise of a hiccup.
A fun piece of evidence of the link between gill breathing in tadpoles and hiccups: Gill breathing can be blocked by carbon dioxide just as hiccups can – try breathing into a paper bag.
Weak knees
The first evidence of knees appear in the fossils of fish pelvic fins about 380 million years ago. These early knees belonged to creatures that still spent most of their time in water – so didn`t have to carry much weight. As such, they are a
pretty simple hinge that is flat and paddle-shaped. Human legs look the same in the womb until about eight weeks when the knee rotates to point forward.
The human knee is still based on this basic hinge design which must carry all our weight when walking, running and jumping with very little lateral support or shock absorption. As a result it is particularly vulnerable to damage, especially later in life.
Snoring and sleep apnea
Humans produce speech with movements of the larynx, back of the throat and tongue –all of which are modifications of the basic design of any mammal or reptile which, in turn, finds its roots in fish. The larynx (voice box), for example, is made up of cartilages that originated as gill arches in fish.
To accommodate speech, the basic design has been modified to include a wider and more flexible throat. It is the flexibility of the throat that, in some cases, causes snoring or, more seriously, sleep apnea. The flexible throat muscles relax during sleep which can cause the passage to collapse and the sleeper to cease breathing temporarily (apnea) or, the passageway to vibrate (snoring).
BY BEN GILLILAND
METROCOSM |
