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 and the slow accumulation of rock layers. Physicists, following Lord Kelvin's calculations of a cooling molten sphere, insisted the planet could be no more than a few tens of millions of years old. The discovery of radioactive decay at the turn of the twentieth century revolutionized the debate. Uranium and thorium, it turned out, produced helium as they decayed, and the accumulation of helium in ancient rocks suggested ages far greater than Kelvin had imagined. By the 1920s, estimates were climbing toward a billion years. By the 1940s, careful measurements of uranium-lead ratios in minerals had pushed the figure to two billion, then three billion. 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 by nearly half.

The problem was contamination. Clair Patterson, a geochemist working under Harrison Brown at the University of Chicago, was attempting to establish a precise age for the Earth using uranium-lead isotope ratios. The method was elegant in principle: uranium-238 decays to lead-206 at a precisely known rate, so measuring both isotopes in a sample reveals how long it has been cooling since formation. But Patterson kept getting wildly inconsistent results. His measurements were noisy, unreliable, scattered across orders of magnitude. The culprit, he eventually discovered, was everywhere: the laboratory itself, the glassware, his reagents, the solder in the equipment, even the air he breathed. Lead contamination from industrial sources had saturated the modern environment so thoroughly that it was impossible to get clean measurements.

The largest single source was leaded gasoline, which had been pumped into the atmosphere since the 1920s. Tetraethyllead, added to gasoline as an anti-knock agent, was burned in engines and released as fine particulate lead into the air, where it settled on every surface. Patterson realized he would need 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 made a crucial methodological shift: instead of measuring Earth rocks, which had been subjected to billions of years of geological recycling and melting that reset their isotopic clocks, he turned to meteorites. These ancient fragments from the early solar system had formed at the same time as the planets and had remained largely unchanged since.

In 1956, Patterson published his result in the journal Geochimica et Cosmochimica Acta. The Earth, he announced, was approximately 4.55 billion years old. The figure has since been refined to 4.54 billion years, but Patterson's essential finding has stood for seventy years. The two-to-three-billion-year estimate that textbooks had carried through the early 1950s was wrong by nearly two billion years. The correction was not a matter of better instruments or more careful measurements alone; it required fundamentally rethinking the problem and finding samples that had been shielded from Earth's own turbulent history.

Patterson's work had an unexpected second act. In cataloguing the sources of lead contamination that had nearly derailed his research, he had documented that industrial lead had permeated every corner of the environment: ocean sediments, Antarctic ice cores, human blood. He compared pre-industrial lead levels locked in ancient Greenland ice with contemporary measurements and found that modern humans carried lead concentrations hundreds of times higher than natural background levels. Lead, he realized, was poisoning the population on a civilization-wide scale. He spent the rest of his career arguing, against fierce opposition from the petroleum and paint industries, that leaded gasoline and lead-based paints represented a catastrophic public health threat. He was blacklisted from government advisory panels, had research contracts cancelled, and was subjected to industry-funded smear campaigns. His findings were eventually vindicated. Leaded gasoline was phased out in the United States by 1996, and childhood blood lead levels dropped by more than eighty percent in the following decades. Patterson had set out to date the Earth and ended up documenting one of the largest preventable environmental poisonings in human history.