Scientists measure the ages of rock layers on Earth using radiometric dating.
Radioactivity also gave the history of life an absolute calendar. By measuring the atoms produced
by these breakdowns inside rocks, physicists were able to estimate their ages (right). 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 (above)
announced that the Earth was 4.5 billion years old. Darwin had finally gotten the luxury of time he had craved.
The dates that radioactive clocks have put on evolutionary history are astonishing. Life is well over
3.5 billion years old, and until about 600 million years ago, the planet was dominated by microbes.
Radioactive clocks have shown that evolution can change its pacethe Cambrian Explosion of about
535 million years ago saw the relatively rapid emergence of many
major lineages of animals in just a
few million years. 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. Some of these methods have helped to pin down the evolution of
our hominid ancestors; anatomically
modern humans evolved about 100,000 years ago. While thats nearly 20 times older than the Earth
was once thought to be, its a geological eye blink.