Your Body - The Fish That Evolved. Dr. Keith Harrison
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Fossils
Applying the scientific method to an investigation of the natural world is not difficult when we are examining the present, but becomes much harder when we start to explore the past (scientists can’t record the mating calls of dinosaurs). However, that doesn’t mean it’s impossible to explore the past using science. As long as a theory is testable, it can be scientific; and a test need not be a laboratory experiment, it can also be a prediction. For example, if birds evolved from reptiles then somewhere in the rocks should be fossils showing a mixture of reptile and bird characteristics. If palaeontologists look for these, they may eventually find them. ‘Birds evolved from reptiles’ is therefore a testable theory, even though no one knows where to look or how long it may take. In fact, in this example a fossil has already been discovered. Archaeopteryx, found in a German quarry in 1861, shows just such a mixture of characters. However, the finding of fossils may rely more on luck than judgement. Most animals and plants are not fossilised when they die, they are eaten or their bodies are destroyed by scavengers or decomposition. Only unusually are some remains protected from destruction and turned to stone. Even then, most fossils will later be destroyed by erosion of the land over geological time or may lie so deeply underground that no one knows anything about them. Fossils are found only when someone interested in fossils stumbles across rocks in the process of being eroded and notices the telltale signs, or when engineers excavate mines or quarries. The chances of any fossil being discovered in the brief period when it’s visible are therefore very remote and we shall never have a complete and accurate record of all the animals and plants that lived at a particular time or in a particular place. Studying palaeontology is like trying to analyse a game of football when all you can see are the shadows, and clouds keep drifting across the sun.
Evolution, Darwin and Natural Selection
E volution is an old idea; in Europe, it can be traced back to the ancient Greeks more than 2,500 years ago. For centuries, Christians rejected this idea because it contradicted the opening words of the Bible which say God created the Earth and all the species on it, including ourselves, in six days. Here we see the true history of the alleged conflict between science and religion. Science may not be contrary to the idea that there is a god but it is capable of showing the universe was not created in six days.
By the end of the 18th century, with the expansion of European science and with growing numbers of naturalists studying the world around them, the possibility that species might change their appearance over time was increasingly under discussion. In addition to religious resistance, there were two other problems preventing the acceptance of this idea: the length of time such evolution would take and the fact that no one could see a mechanism by which it might occur. At that time the Earth was thought to be only several thousand years old, not long enough for evolution to have had an effect. The realisation around the year 1800 that the complex geological structure of the world had in fact been produced by the incredibly slow action of volcanoes, sedimentation and weathering, just as we see happening today, awoke scientists to the fact that the Earth must be significantly older than they thought. With that, evolution could be taken more seriously.
The timescale involved in evolution is truly beyond the grasp of the human mind. Even today, when we talk glibly about hundreds of millions of years – as I did in the opening lines of this book – our brains are simply not capable of understanding what that means. If non-Christian readers will allow me a biased example, most of us would recognise that the period between ourselves today and Christ 2,000 years ago feels like a very long time. Christ lived in ancient history but if we close our eyes we can probably picture that timescale. If I ask you to imagine 10,000 years ago, you have to imagine five times the period between ourselves and Christ. Now we are truly deep into the past. No event in any history book you have ever read had yet occurred. Our ancestors were still chipping flakes off the sides of pieces of flint and would continue to do so for millennia. This timescale is getting harder to imagine but we still think we can manage it. Now let me ask you to imagine an elapse of time 2,000 times the difference between ourselves and Christ. Such a vast age is almost incomprehensible. It fades far into the distance, well beyond our mental horizon, yet this takes us back only 4 million years. It takes us back to a time when our ancestors were about to leave the trees and walk across the plains of Africa leaving footprints virtually identical to our own. In geological terms this was only a few hours ago. The giant dinosaurs disappeared 65 million years ago, having dominated the Earth for 140 million years before that, and they were newcomers. Life has lived on this planet for more than 3,500 million years. Evolution is very, very slow, but there has never been any rush.
Darwin
In 1859, Charles Darwin published his book On the Origin of Species by Means of Natural Selection. Natural selection was the mechanism Darwin proposed to explain how evolution could work. In his book, Darwin made several critical observations: resources in nature (such as food or living space) are limited; there is competition for them; and within each species individuals are slightly different from each other.
He argued that in a competition in which the competitors show different characteristics (one leopard being able to run slightly faster than another; one mouse having slightly paler fur than another) some characteristics would give an advantage and some would not. In a struggle for life – as he put it – the characteristics giving an advantage could lead to their owner, and hence the characteristic, surviving. In this way, some characters would automatically be selected by nature, survive and be inherited by the next generation. During this process of natural selection, with some characters passing from one generation to the next and others being eliminated, the species showing the characters would change over time, there would be evolution.
Darwin is often said to be the father of evolution. In fact, he was the father of natural selection, which is how evolution works. His ideas have been tested extensively since his book appeared and his theory has long since ceased to be a theory. Evolution and natural selection are now established as facts.
Natural selection
Natural selection changes the average appearance of a species, not individual animals. To oversimplify: imagine a herd of gazelles in which each gazelle has legs slightly different in length to the others in the herd. There are therefore tall gazelles and there are short gazelles (this could equally be a room full of people of different heights, but what happens next would not bear thinking about). If all the gazelles with the shortest legs are caught and eaten by lions because they can’t run fast enough, only the gazelles with longer legs will survive. The short-legged gazelles will not have lived long enough to reach breeding age so only long-legged gazelles will produce young. All the next generation of gazelles in this herd will therefore tend to have long legs. No individual gazelle has increased the length of its legs but the average length of legs in the herd has increased. There has been evolution. Evolution therefore acts at the level of reproduction; the natural selection of one generation affects the appearance of the next generation.
Natural selection of behavior
Some characters selected by nature may be behaviours, not features governed by our genes. A group of animals – including early humans – that decides to drink at the same water-hole as predators, at the same time as the predators drink, and who then jostle for position amongst themselves rather than remaining vigilant, are unlikely to last long enough to teach this behaviour (deliberately or by example) to their children. Indeed, they are unlikely to last long enough to have children. On the other hand, a group that allows the predators to drink and disperse before they themselves journey to the water-hole, and who then post lookouts, may survive to pass this behaviour to the next generation.
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