Lead is where the radioactive decay chains stop. Uranium-238 decays through a long series of steps to lead-206; uranium-235 ends at lead-207; thorium-232 ends at lead-208. Every one of those atoms of lead was once something heavier. Transmutation of base metal is therefore happening continuously, everywhere, in the direction opposite to the one the alchemists wanted, and it always has been.
This has a practical consequence. Because the decay rates are known and the end products accumulate, the ratio of lead isotopes to their parent uranium in a mineral gives its age. Clair Patterson used exactly this method on meteorite samples to establish in 1953 that the Earth is about 4.55 billion years old, a figure that has held.
Patterson's difficulty was that his measurements kept being contaminated, and tracking down the source led him to the conclusion that the modern environment carried far more lead than any natural background. The reason was tetraethyl lead, added to petrol from 1923 to stop engine knock on the recommendation of Thomas Midgley, who also invented chlorofluorocarbons. Leaded petrol dispersed lead into the air of every city in the world for most of the twentieth century.
Patterson spent decades documenting this against sustained opposition from the lead industry, which funded contrary research and attempted to have him removed from advisory positions and from his funding. He was right. Leaded petrol was phased out in the United States from 1975 and banned progressively worldwide, with the last country ending its use in 2021, and blood lead levels in populations fell sharply and measurably wherever it was withdrawn.
Lead has no known biological function and no established safe level of exposure. Its effects on the developing nervous system are irreversible. The whole episode is the clearest available case of a substance's usefulness being weighed against its harm over a century, badly, and at scale.