Mammals, which for 150 million years had been small, rodent-sized creatures, rapidly evolved to massive proportions in the wake of the Cretaceous-Tertiary extinction 65 million years ago.
Geological timekeeping continues to be a lively science, with new methods emerging all the time.
Then followed years of study and debate involving him and other investigators and politicians over control of lead in the environment.
In the end, his basic views prevailed, resulting in drastic reductions in the amount of lead entering the environment.
But Kelvin didn't, and couldn't, know that radioactive atoms such as uranium were breaking down and keeping the planet warmer than it would be otherwise.And by comparing the ratios of those atoms to atoms from meteorites, they could estimate how long ago it was that the Earth formed along with the rest of the solar system.In 1956 the American geologist Clair Patterson (left) announced that the Earth was 4.5 billion years old.While some physicists used these discoveries for applications ranging from nuclear weapons to nuclear medicine, others applied them to understanding the natural world.The sun was once thought to burn like a coal fire, but physicists showed that it actually generates energy by slamming atoms together and creating new elements.