Reality Check

Darwinism and the Age of Earth

When Darwin and Wallace proposed the processes of evolution in 1858, including common descent, speciation, and natural selection, they were making extraordinary claims. This required extraordinary evidence, and Darwin provided that in the data he had gathered on the voyage of the Beagle. Furthermore, the theory was eminently falsifiable!

While the falsification criterion is no longer considered adequate for defining good science, it certainly remains an important indicator. This is especially the case when a theory makes a highly risky prediction that strongly contradicts existing knowledge. Natural selection not only clashed with common religious beliefs based on scripture, it also implied a deep disagreement with basic physics.

At the time of Darwin and Wallace, most people believed that the age of Earth was about 6,000 years, as estimated by Bishop Ussher in the seventeenth century from his reading of the Bible. Geologists were just beginning to gather evidence for a much older Earth, and this knowledge had a great influence on Darwin, who took Charles Lyell's classic Principles of Geology with him on the voyage of the Beagle.

In the first edition of On the Origin of the Species by Means of Natural Selection, Darwin made a crude estimate of Earth's age, based on geology, of several hundred million years. This, he suspected, was sufficiently long for the processes of natural selection to take place and produce the wide range of species on Earth.

The great physicist William Thomson, later to become Lord Kelvin, disputed Darwin's estimate, arguing that Earth was much younger. Thomson had made major contributions to thermodynamics, formulating the second law of thermodynamics and establishing the absolute temperature (Kelvin) scale. At the time, the only known sources of energy that could account for solar radiation were chemical and gravitational. Thomson calculated the age of the Sun for each mechanism and found that gravity gave the largest value, of a few tens of millions of years. Earth could not be older than the Sun, and this age was a factor of ten lower than Darwin's estimate of the age of Earth. Using thermodynamics, Kelvin also calculated that the temperature of Earth would have been too high even as recently as a million years ago to allow for life.

Thus, based on the best physics knowledge of Darwin's day, evolution by natural selection was highly suspect. Darwin admitted as much in a letter to Wallace: "Thomson's views on the recent age of the world have been for some time one of my sorest troubles." If Thomson's conclusions had been correct, evolution by natural selection would have been falsified.

But Thomson's conclusions were wrong, and Darwin's theory was not falsified. Thomson cannot be faulted, for he used the best information available at the time. With the discovery of nuclear energy early in the twentieth century, a new source of energy became known that was far more efficient than either gravity or chemical reactions. This provided a mechanism for a much longer-lived sun. Furthermore, the natural nuclear radioactivity of Earth generates significant heat and upsets Thomson's calculation for the rate of cooling of Earth.

By mid-twentieth century, the nuclear processes that fuel the Sun were well established and described by theory. By the end of the century, the observation of neutrinos from the Sun (including an observation in which I participated) had directly confirmed the validity of a nuclear source of energy for the Sun and a potential lifetime on the order of ten billion years. Prior to this, radioactive dating also had verified that Earth is several billions of years old and paleontologists have found signs of life going back almost that far.

Evolution by natural selection ranks as one of the greatest scientific advances of all time. It was extraordinary and risky, and not just in regard to biology. It implied that Earth was far older than anyone expected. Although no one stated it so explicitly at the time, Darwinism can be said, in hindsight, to have "predicted" the discovery of a new force in nature to account for the Sun's energy. If no such force had been been found to exist, Darwinian evolution would have been disproved.

This story should be more widely told in textbooks and other science literature because it is a great example of good science in action. Here we have a revolutionary new idea clashing with older knowledge at several levels and requiring another dramatic discovery in a seemingly unrelated field in order to survive. When the new idea does survive, as happened with evolution, we gain additional confidence that science truly relates to an objective reality that is really out there.

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