The Earth is approximately 2 to 3 billion years old.

The age of the Earth was one of the great unsettled questions of nineteenth-century science. Geologists argued for hundreds of millions of years based on sedimentation rates; physicists, following Lord Kelvin, calculated a much younger Earth cooling from a molten state. By the early twentieth century, radioactive decay had provided a new clock, and estimates were climbing toward a billion, then two billion years. When students opened earth science textbooks in the early 1950s, the figure most commonly given was somewhere between two and three billion years, a number that felt impressively ancient even if it still underestimated the truth.

The problem was lead. Clair Patterson, a graduate student at the University of Chicago, was attempting to establish a precise age for the Earth using uranium-lead ratios in ancient rocks. The method was straightforward in principle: uranium-238 decays to lead-206 at a known rate, so the ratio of the two isotopes reveals how long a sample has been cooling. But Patterson kept getting wildly inconsistent results. The culprit, he eventually discovered, was everywhere: the laboratory itself, the glassware, his reagents, even the air. Lead contamination from industrial sources, primarily from leaded gasoline, which had been pumped into the atmosphere since the 1920s, had saturated the global environment so thoroughly that it was impossible to get clean measurements.

Patterson’s solution was to build the world’s first ultra-clean laboratory, where he could work in an environment as free from contamination as humanly possible. He also switched from Earth rocks to meteorite samples, ancient fragments from the early solar system that had formed at the same time as the planets and had not been subjected to the geological processes that reset Earth’s own rocks. In 1956, he published his result: the Earth was approximately 4.55 billion years old, a figure since refined to 4.54 billion years. The two-to-three-billion-year figure that textbooks had carried through the early 1950s was wrong by nearly two billion years.

Patterson’s work had an unexpected second act. In cataloguing the sources of lead contamination in his samples, he had documented that industrial lead had permeated every corner of the environment, ocean sediments, polar ice, human blood. He compared pre-industrial lead levels locked in ancient Greenland ice cores with contemporary measurements and found that modern humans carried lead concentrations hundreds of times higher than pre-industrial norms. Patterson spent the rest of his career arguing, against fierce opposition from the petroleum and paint industries, that leaded gasoline and lead paint were poisoning the population. He was blacklisted from government advisory panels, had research contracts cancelled, and was targeted by industry lobbying campaigns. His findings were eventually vindicated: leaded gasoline was phased out in the United States by 1996.