The abundance of fissionables in the Earth's crust (as well as deeper down) is currently quite a bit less than when the planet formed.
The radioisotope decay curves, as well as the daughter products of such elements, point to a much higher ratio when the Earth was young. U-235 comprised nearly half of all the uranium present at the birth of the solar system.
There is even evidence of a cluster of natural nuclear reactors operating in what is now Oklo, Gabon (Africa). Since the isotopics were much better some 1.7 billion years ago, nature saw fit to make use of a cache of uranium ore and run a reactor to about the 100 kilowatt level, for over 1 million years, as it turned out.
There are those who say that the resulting radioactivity influenced the flora and fauna, giving a healthy kick-start to evolution, perhaps with respect to primates and their hominid descendants.
The current Religion of Globular Worming can't hold a candle to that...
