Geologists can measure the concentrations of these two isotopes of lead in rocks, and so measure their age. This enables us to estimate the length of time since the Earth's crust became solid. In the nineteenth century scientists attempted to work out the age of the Earth by estimating how long it would take molten rock to cool down to its present temperature. Lord Kelvin decided that this was no more than 100 million years. Unfortunately, Charles Darwin needed the Earth to be much older than this for human life to have evolved.
Evolution is a very slow process.
The discovery of radioactivity solved the problem - the Earth was much older than Kelvin's estimate. Radioactive minerals in the rocks were generating sufficient heat to slow down the cooling.