The Debate Over Earth's Age

In the late 19th century, a heated scientific debate unfolded, pitting two giants of science against one another: William Thomson, better known as Lord Kelvin, a world-renowned physicist and mathematician and Charles Darwin, the father of the theory of evolution. The debate itself centered on the question of Earth’s age and highlighted the tension between scientific disciplines, the disputes among religious denominations in Europe and the developing nature of scientific understanding.  

In pre-19th century Europe, it was widely believed that the world was as old as recorded human history, with the Earth's age estimated to be around 6,000 years. The person who brought the question of Earth’s age back into the scientific spotlight  was Irish Archbishop James Ussher. In 1650 Ussher published a literal interpretation of the Book of Genesis, in which he calculated Earth’s age based on the genealogies described in the text. According to his calculations, the world was created in 4004 BC. Not satisfied with this level of precision, he even specified that creation took place on October 27th, at precisely six o’clock in the evening. While today such a claim would be met with a mild chuckle, Ussher’s conclusion was far from arbitrary - it was based on quite extensive historical research, though based on flawed assumptions.

It was believed that the Earth began as a scalding hot mass, with its outer layer gradually cooling. The question was how to calculate the rate of cooling. Earth’s inner structure | Illustration: Maxx-Studio, Shutterstock

Throughout history, various scientists have attempted to estimate Earth's age using a range of creative (and incorrect) methods, from assumptions regarding the link between sea level changes and Earth's age, to measuring mineral concentrations in oceans, comparing fossils, and more. In the early 19th century, a group of researchers proposed, based on geological evidence, that the Earth was much older than previously believed. The most prominent among them was geologist Charles Lyell, who in 1830, published his book Principles of Geology, in which he hypothesized that Earth was millions of years old. Lyell was a key figure in Charles Darwin’s life and had a profound influence on his thinking. Lyell’s principles of uniformitarianism, which posited that Earth’s geological features were the result of slow, gradual and ongoing processes over vast time periods, played a crucial role in shaping Darwin’s worldview.   

By the late 19th century, the question of Earth's age had become a hotly debated topic within the European scientific community. At the center of the controversy was the debate between Lord Kelvin and Darwin. Today, it’s hard to find someone who doesn’t know Charles Darwin, but in this debate, he was undoubtedly on the losing side. The debate is often likened to the story of “David and Goliath” - Charles Darwin and the geologists, armed with the modest sword of geological thought and evolutionary theory, versus Lord Kelvin, who wielded the full power and prestige of advanced mathematical physics.

The young and unknown scientist who lost the debate to the renowned physicist. Charles Darwin, drawing from an early 20th-century lexicon | Source: Nicku, Shutterstock

Millions or Hundreds of Millions of Years?

In 1859 Darwin published the first edition of  “On the Origin of Species”, which sparked a long-lasting controversy in the scientific community and challenged the prevailing creationist belief that species originated from a single or several acts of divine creation. Unlike Jean-Baptiste Lamarck’s evolutionary theory, which posited that an inner force or “guiding hand” drives organisms towards increasing complexity,  Darwin’s theory asserted that there was no predetermined plan and that even we, humans, exist by mere chance. 

According to Darwin's theory of natural selection, the development of new species is very slow and requires a very long time. Thus, the theory implies that the Earth must be very ancient. But exactly how ancient? Darwin never provided an exact figure. In fact, in the first edition of his book, he included a rough estimate of the Earth's age based on erosion rates in a particular region in England but removed this section in subsequent editions due to criticism. Based on the biological diversity he observed and the understanding that species change gradually over extended periods, he estimated that the Earth must be at least several hundred million years old.

About three years later Lord Kelvin published his calculation regarding Earth’s age, which contradicted both the theory of evolution and geological theories. In the later and more refined version of his calculations, Kelvin estimated that Earth was no more than 10 million years old. Historians argue that Darwin’s theory influenced Kelvin's conclusion. A devout Christian throughout his life, Kelvin was willing to accept Lamarck’s version of evolution, as well as other other theistic evolutionary theories, but he struggled to accept Darwin’s theory and its implication that humans, too, were created by random processes rather than a deliberate act of creation.  Aware that many in the scientific community of his time were inclined to reject Darwin’s theory on the grounds that Earth had not existed long enough to allow for slow and random natural selection, Kelvin, who was already an internationally renowned scientist, lent his authority to this viewpoint.

A calculation driven by religious motivations that led to a false conclusion, but helped introduce modern scientific practice into the field of geology. Lord Kelvin, portrayed on a Scottish banknote |  Source: Prachaya Roekdeethaweesab, Shutterstock

Kelvin's Calculation

Lord Kelvin was a brilliant mathematician and physicist who began his academic education at the age of ten. By sixteen he had published his first scientific work, became a lecturer at the University of Glasgow by the age of twenty-two. Kelvin made significant contributions across many scientific fields and in recognition of his work he was granted the title of Lord in 1982. 

Lord Kelvin was influenced by the calculations of French scientist Georges-Louis Leclerc de Buffon, who hypothesized that the Earth was initially formed by the aggregation of interstellar dust driven by gravity, and that our world began as a  molten mass of iron. In 1778 de Buffon cast iron balls of various sizes, heated them to their melting point and measured the time required for them to cool down. From these measurements he inferred how long it would take for an iron sphere the size of the Earth to cool to its current temperature, reaching the conclusion that the Earth was only 75,000 years old.

An original and interesting experiment, even if the conclusion was incorrect: a chapter on the story of de Buffon from the animated series “Once Upon a Time... The Discoverers”:

Kelvin expanded on the concept behind de Buffon’s calculation by applying an established mathematical theory and field measurements. He assumed that the entire cooling process occurred through heat conduction from the Earth's core to its surface and that the Earth began as a completely molten body. Kelvin relied on the heat theory formulated by Jean-Baptiste Joseph Fourier, which, among other things, links changes in temperature over time to changes in temperature at different locations. In the case of our planet’s cooling process, this relationship links the temperature change from Earth's formation to the present day with the differences in temperature at various depths of the Earth.

This theory could also be used to predict future temperatures: by knowing the initial conditions, such as the current temperature distribution inside the Earth, it’s possible to calculate the future temperature change. It’s also possible to extrapolate backward and calculate past conditions, as Kelvin attempted to do. By knowing the current temperature distribution and assuming certain initial conditions, the duration of the cooling process, and by extension the Earth's age—can be calculated. Kelvin obtained data regarding the temperatures at various depths from measurements taken in mines and deep drills, where it was observed that temperature indeed increases with depth. Based on his understanding of how temperature changes with depth, Kelvin inferred the rate at which the Earth's interior cooled and estimated that the Earth was between 10 and 40 million years old, periodically updating his estimate.

Unlike Darwin, Kelvin enjoyed significant support within the scientific community, leading to his "victory" over Darwin in the debate about Earth's age. His theory was elegant, mathematically sound, and logically refined - but entirely incorrect.

A convincing mathematical theory. Here’s is how Kelvin calculated the age of the Earth - an explanatory video by Robert Hanlon: 

What Was Missing in Kelvin’s Calculation?

Kelvin assumed that all the heat from Earth's interior reached the crust solely through heat conduction. In reality, a non-negligible amount of heat is also transferred via heat convection. However, the most critical factor Kelvin could not have accounted for was the existence of an additional heat source deep within the Earth - radioactive decay, which wasn’t discovered until the late 19th century.

Although Kelvin’s calculation was incorrect, it made a significant contribution to the development of geology.  His application of thermodynamics and mathematical methods to estimate the Earth's age highlighted the importance of quantitative approaches in geology, and encouraged geologists to adopt more mathematical methods. 

Today, the scientific community unanimously agrees that the question of Earth's age  has been resolved. Based on radiometric dating of rocks that formed on Earth since its beginning and of meteorites, we estimate that the Earth has existed for about 4.54 billion years. This vast timespan provides ample time for the occurrence of the evolutionary processes that transformed it into a planet brimming with life. 

Radiometric dating allows scientists to calculate the age of rocks and minerals and determine the age of Earth. Billion-year-old zircon mineral | Image: Henri Koskinen, Shutterstock

Summary

The debate between Lord Kelvin and Charles Darwin demonstrates how much courage was required of Darwin to insist upon the truth while facing enormous resistance from both the scientific community and society at large. Even Darwin’s wife was concerned about the consequences and urged him not to publish the theory. This teaches us that Darwin’s greatness was not only the result of sharp intellect and hard work, but also of his ability to exercise mental flexibility and observe findings as they were, even when they contradicted the belief and value system he was brought up on.   

In contrast, while Kelvin’s scientific integrity and greatness are beyond question, the story raises questions about the objectivity of science and its independence from cultural influences. Kelvin’s motivation to calculate the age of the Earth was partly driven by his religious beliefs. This blending of personal faith and scientific inquiry underscores the idea that, while science strives for objectivity, it is often shaped by the broader cultural context in which it operates.