The Scottish geologist James Hutton made a proposal in 1788 that, at the time, was extraordinarily controversial. He described Earth as a “beautiful machine”, constantly subjected to long-term decay and regeneration, that could only be understood over many millions of years. This may not sound that contentious, but the challenge this posed to humanity’s sense of time was substantial. Popular contemporary estimates of Earth’s age, such as Bishop Ussher’s calculation that it was created in 4,004 BC, were dwarfed by the magnitude of what Hutton described.