Discovery of radioactive dating
Life is well over 3.5 billion years old, and until about 600 million years ago, the planet was dominated by microbes.
This process is called radiometric or radioactive dating.
Darwin had argued that the Earth was immensely old which gave his gradual process of evolution plenty of time to unfold.
The great physicist Lord Kelvin had countered that the planet was actually relatively young perhaps 20 million years old.
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 that's nearly 20 times older than the Earth was once thought to be, it's a geological eye blink.