For centuries scholars sought to determine the earth’s age, but the answer had to wait for careful geologic observation, isotopic analyses of the elements and an understanding of radioactive decay.
Aristotle thought the earth had existed eternally. Roman poet Lucretius, intellectual heir to the Greek atomists, believed its formation must have been relatively recent, given that there were no records going back beyond the Trojan War. The Talmudic rabbis, Martin Luther and others used the biblical account to extrapolate back from known history and came up with rather similar estimates for when the earth came into being. The most famous came in 1654, when Archbishop James Ussher of Ireland offered the date of 4004 B.C.
Within decades observation began overtaking such thinking. In the 1660s Nicolas Steno formulated our modern concepts of deposition of horizontal strata. He inferred that where the layers are not horizontal, they must have been tilted since their deposition and noted that different strata contain different kinds of fossil. Robert Hooke, not long after, suggested that the fossil record would form the basis for a chronology that would “far antedate ... even the very pyramids.” The 18th century saw the spread of canal building, which led to the discovery of strata correlated over great distances, and James Hutton’s recognition that unconformities between successive layers implied that deposition had been interrupted by enormously long periods of tilt and erosion. By 1788 Hutton had formulated a theory of cyclic deposition and uplift, with the earth indefinitely old, showing “no vestige of a beginning—no prospect of an end.” Hutton considered the present to be the key to the past, with geologic processes driven by the same forces as those we can see at work today. This position came to be known as uniformitarianism, but within it we must distinguish between uniformity of natural law (which nearly all of us would accept) and the increasingly questionable assumptions of uniformity of process, uniformity of rate and uniformity of outcome.
By the late 19th century the geologists included here had reached a consensus for the age of the earth of around 100 million years. Having come that far, they were initially quite reluctant to accept a further expansion of the geologic timescale by a factor of 10 or more. And we should resist the temptation to blame them for their resistance. Radioactivity was poorly understood. Different methods of measurement (such as the decay of uranium to helium versus its decay to lead) sometimes gave discordant values, and almost a decade passed between the first use of radiometric dating and the discovery of isotopes, let alone the working out of the three separate major decay chains in nature. The constancy of radioactive decay rates was regarded as an independent and questionable assumption because it was not known—and could not be known until the development of modern quantum mechanics—that these rates were fixed by the fundamental constants of physics.
It was not until 1926, when (under the influence of Arthur Holmes, whose name recurs throughout this story) the National Academy of Sciences adopted the radiometric timescale, that we can regard the controversy as finally resolved. Critical to this resolution were improved methods of dating, which incorporated advances in mass spectrometry, sampling and laser heating. The resulting knowledge has led to the current understanding that the earth is 4.55 billion years old.
That takes us to the end of this series of papers but not to the end of the story. As with so many good scientific puzzles, the question of the age of the earth resolves itself on more rigorous examination into distinct components. Do we mean the age of the solar system, or of the earth as a planet within it, or of the earth-moon system, or the time since formation of the earth’s metallic core, or the time since formation of the earliest solid crust? Such questions remain under active investigation, using as clues variations in isotopic distribution, or anomalies in mineral composition, that tell the story of the formation and decay of long-vanished short-lived isotopes. Isotopic ratios between stable isotopes both on the earth and in meteorites are coming under increasingly close scrutiny, to see what they can tell us about the ultimate sources of the very atoms that make up our planet. We can look forward to new answers—and new questions. That’s how science works.