June 25, 2007
Evolution-3: Natural selection and the age of the Earth
It is clear that many people find it hard to accept Darwin's theory of evolution by natural selection. One reason is of course because it completely undermines the need to believe in a creator, making god superfluous when it comes to explaining the nature and diversity of life, and thus people may have a negative emotional reaction that prevents them from seeing the power of the theory. As I have discussed earlier, people are quite able to develop quite sophisticated reasons to believe what they want and reject what they dislike.
Another reason that Darwin's ideas were so hard to accept is because, as Daniel Dennett says in his book Darwin's Dangerous Idea (1995), he turned the whole model of how things come to be on its head. Up until then, people had thought that to make anything always required a more complex thing. Simpler things never made more complex things. You did not find a horseshoe making a blacksmith, for example. But what Darwin was suggesting was that a very simple mechanism, natural selection, could result in simpler things becoming more complex without an external agent, but just from the ground up, as it were. What is worse was that, according to Darwin's theory, intelligence, which had been thought as a precursor to creation and often used synonymously with god, turned out to be something that occurred much later in life's evolution. In other words, intelligence itself came into being by a non-intelligent mechanism. These ideas made people who thought of human beings as possessing some special divine qualities uncomfortable, to put it mildly.
People find it hard to accept the fundamental idea of evolution that very small changes, if cumulative over very long times, can result in big changes. This should not be an entirely foreign concept, especially to those with savings accounts who are familiar with the way that interest grows when compounded. If you keep some money in a savings account at a rate even as low as 1%, it will double in 70 years, quadruple in 140 years, become eight times as much 210 years, and so on, becoming over a thousand times as large in 700 years, and over a million times as large in 7,000 years. But therein lies the difficulty. People do not fully appreciate the power of compounding because they tend not to be able to grasp time scales much longer than their own life spans.
The mathematics and statistics that are relevant to understanding how natural selection works does not come easily to people, partly because we do not have a firm intuitive grasp of geological time scales which are so large as to be almost impossible to comprehend. I once had a college first year student say that she did not think evolution could have happened. I asked her why and she said that when you saw the images drawn on 'ancient' Egyptian inscriptions, those people looked just like us today. So in her view, since there had been no visible evolutionary change over what to her was an enormous length of time, this disproved evolution!
It is not easy to grasp that even written language only goes back 5,000 years or so. When we factor in that the more appropriate unit of time for evolutionary change is the generation (which for humans is about 20 years), we see that written language emerged only 250 generations ago. It is hard for us to even imagine what life was like back then. Even the Vietnam war, which was just one generation ago, seems like ancient history to college students today, almost obscured by the murky mists of time.
So it is almost impossible to wrap our minds even around the fact that the common ancestor of humans and chimpanzees lived 300,000 generations or 6 million years ago, even though that itself is a blip compared to the origin of life itself (over 3 billion years ago) or age of the Earth (4.5 billion years ago). When we realize that the lifetime of a generation for many species is usually much less that 20 years, and is often measured in months and even days, the number of generations that have been available for evolutionary change to take place is staggeringly huge.
Although he could not quantify it at that time, Darwin knew that his theory of natural selection required very long time scales in order to be feasible. But he was born at a time when Biblical cosmology was dominant and the idea of an Earth that was less than 10,000 years old was widespread. This would not have been long enough for his ideas to work and it is unlikely that he would have hit upon his great discovery if not for having been born at a fortuitous time. In another example of how science is deeply interconnected in its theories, Darwin's theory was made possible because of the work of his contemporary and later friend, geologist Charles Lyell and his theory of uniformitarianism.
Prior to Lyell, ideas in geology were strongly influenced by the book of Genesis and it was believed that the Earth had had a series of catastrophes (floods, volcanic eruptions, earthquakes, etc.) that had produced its major geologic features. The advantage of this theory of catastrophism was that it enabled people to believe that the Earth was quite young, since it made it plausible that major geological fractures like the Grand Canyon and Niagara Falls could come into being suddenly.
Lyell in his three volumes The Principles of Geology: Being an Attempt to Explain the Former Changes of the Earth's Surface, by Reference to Causes now in Operation (published over the years 1830-33) advanced evidence that the Earth had been around for a very long time and in particular, from his study of fossils, that human beings were much older than had been thought. Darwin read the first volume of this work on his life-changing trip on the Beagle (which lasted from 1831 to 1836) and it opened his eyes to a new way of seeing the diverse life forms in the exotic faraway places he visited. Lyell's work not only gave Darwin the large window of time necessary to fit his own theory, it also was a precursor of Darwin's central idea that very small changes, accumulated over very long times, could produce dramatic effects.
Although Lyell's estimate of the age of the Earth was only about 250 million years, smaller in comparison to current estimates by a factor of almost twenty, this was still a huge increase from earlier ideas, and Darwin saw in it an opening that the Earth was possibly very old, old enough that made it possible for the evolution of life as he saw it to occur and it encouraged him in his work. But after Darwin published his landmark On the Origin of Species in 1859, the old Earth theory of Lyell received a major setback when in1864, one of the most eminent physicists of that time, William Thomson (later Lord Kelvin), said that his calculations of the rate of cooling of the Earth's magma suggested that the Earth became a solid body between 20 and 400 million years, a disturbingly low lower limit. But it got worse, with later calculations reducing even the upper limit to much less than what Lyell had proposed, coming down to about just 10 million years. This was much less than what Darwin needed for his theory to work, and Thomson in 1868 explicitly challenged the validity of natural selection on these grounds. (David Quammen, The Reluctant Mr. Darwin (2006), p. 211.)
While this was undoubtedly a setback, Darwin doggedly persevered, accumulating more biological evidence for his theory, confident that future work in physics would vindicate him that the Earth was much older. But conclusive support on this question would only come after his death in 1882. Following the discovery of radioactivity, Rutherford and others in 1907 found evidence of rocks that were 1.6 billion years old. Further studies since then have increased the age to the current estimates of 4.5 billion years, more than enough for the theory of evolution to work.
Once again, we see how the interconnectedness of science can provide powerful constraints when it comes to constructing new theories, because theories in one area (such as biology) have to be consistent with theories in seemingly disparate areas (like physics and chemistry and geology). When creationists attack the theory of evolution and try to replace it with ad hoc theories of great floods, they are also severing ties with an entire network of scientific theories, and adding on yet more ad hoc hypotheses to fill in the obvious gaps does not help. When they reject a comprehensive theory like the theory of evolution by natural selection without replacing it with another one that is consistent with the findings of other scientific theories, they are pretty much rejecting the foundations of modern science.
As the philosopher of science Pierre Duhem wrote long ago in his book The Aim and Structure of Physical Theory (1906): "The only experimental check on a physical theory which is not illogical consists in comparing the entire system of the physical theory with the whole group of experimental laws, and in judging whether the latter is represented by the former in a satisfactory manner." (emphasis in original)
POST SCRIPT: The Lion Sleeps Tonight
Hank Medress, vocalist of the group the Tokens, died last week at the age of 68. Here he is singing their big hit The Lion Sleeps Tonight.