Saturday, 15 February 2014

Transitional Fish-Face Fossils

Ancient fish was pivotal in evolution of face researchers find | euronews

This week sees the publication of yet another rebuttal of the creationist faith-based, evidence-denying mantra, "There are no transitional fossils". Swedish and French researches have reconstructed in three dimensions the evolving head parts of a small, primitive armoured early fish called Romundina which lived 415 million years ago and whose fossil remains were found in Arctic Canada. In doing so they have revealed the step by step evolutionary process which led to the evolution of the face.

We take faces for granted. Just about every animal more advanced that a worm has a face of some sort. Biologically it makes sense for all the senses and organs associated with finding and eating food to be collected together at the front end and near the mouth. It makes less sense for the respiratory organs to be there though, unless you are breathing water, not air, then it helps to point the intake hole at where the water (and the food) are coming from.

In fact, so inevitable was it's evolution that by a parallel process, even arthropods and some molluscs have evolved a broadly similar 'face', at least so far as the general arrangement of eyes and mouth-parts are concerned, though there may well be several more eyes which work by a different mechanism.

So, pretty much every animal has a 'face' and vegetarians can use 'never eat anything with a face' as a good rule of thumb which practically restricts their diet to vegetation, eggs and dairy products.

But it was not always that way. Like everything else, the face had to evolve because there was nothing to say where the face had to be or what it should look like, apart from the pressure of natural selection giving differential success to anything with an advantage over the others. How it evolved in this primitive fish known as a placoderm is significant in that this was right at the beginning of the evolution of jawed vertebrates and so in the order which was ancestral to all vertebrates, including all birds, reptiles, amphibians and mammals, and it's where we get our basic face from.

Using high-energy x-rays produced by the European Synchrotron in France the team found the evolution from jawless to jawed with a face occurred in three stages and it was the evolution of the jaw which led to the evolution of the face. The only remaining jawless vertebrates found today are the lampreys and the hagfish.

  • The jaw evolved and a single nostril running under the brain was replaced by a solid floor and two symmetrical openings above the jaw to form the 'nose' between the eyes. The forebrain remained short and the skull extended below the eyes and nose, over the lower jaw.
  • The skull then receded back leaving the nose in front of the eyes whilst the forebrain remained short.

  • In the final phase the forebrain enlarged and the face lengthened.

When you look at Romundina, it’s like looking at yourself in the mirror, but with a 415 million-year-old image. It’s like in a science-fiction movie. You look at the mirror, but it’s not you. It’s your ancestor.

Vincent Dupret, Uppsala University, Sweden
And so this pattern was inherited by all the descendants of this primitive placoderm fish to give us the readily recognisable face we see on all vertebrates but it contained a fundamental flaw which, at the time of its evolution was not a problem. In fact it was the simplest solution to the 'problem' of where to put the nostrils at a time when the nostrils were olfactory organs, not part of the respiratory system. The placoderms, like all fish, 'breathe' by passing water over blood-rich gills which extract oxygen and excrete carbon dioxide - which is why it also makes sense to have the inlet facing the oncoming water.

But we don't breathe that way. Our olfactory organs have also become airways through which air passes to our lungs when exchange of gasses takes place across a wet membrane just as it did in gills. This means that the air passage crosses the food passage in the crossover chamber known as the pharynx. Again, this wasn't a problem in our evolving air-breathing ancestors which walked on all fours and had a forward-pointing face in line with the vertebrae, the air passage and the food passage because the air passage could be easily closed off by the larynx when swallowing. Never-the-less, accidents happen and a few mammals can choke to death if food takes the wrong route.

In humans however, as a result of becoming bipedal, our skulls have had to rotate forward and our face has had to rotate downwards. This, together probably with the evolution of speech has caused our pharynx to lengthen faster than our swallowing reflex had been able to adapt to it, and the evolved 'design' which was perfectly adequate for most of our evolutionary history is now a fatal flaw in our design, accounting for about 30,000 sudden deaths a year in otherwise healthy children and adults.

So, not only do we now know how the face evolved but we also know that there was no intelligence involved in it. And we know this because we have the transitional fossils.

The study was a collaboration among researchers at Uppsala, the National Museum of Natural History in Paris, and the European Synchrotron Radiation Facility in Grenoble.

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