Radioactive carbon dating origin of life
He came up with that figure by estimating how long it had taken for the planet to cool down to its current temperature from its molten infancy.
Atoms can fuse together to create new elements; they can also spontaneously break down, firing off subatomic particles and switching from one element to another in the process (see figure, right).
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 pace the 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.
Nineteenth century geologists recognized that rocks formed slowly as mountains eroded and sediments settled on the ocean floor.
But they could not say just how long such processes had taken, and thus how old their fossils were.