Attaining Complex Life Is One Thing. Maintaining It Is Something Else Entirely.

The book Randall references - Rare Earth: Why Complex Life Is Uncommon in the Universe by paleontologist Peter Ward and astronomer Donald Brownlee - made a case that cut against the optimistic assumptions of the Search for Extraterrestrial Intelligence movement. Simple microbial life may be widespread across the universe. Complex animal life - the kind that produces civilizations and technology - is another matter entirely. The conditions required to produce and sustain it are so specific and so demanding that the authors argued it is genuinely rare on a cosmic scale.

The astrobiological revolution Randall describes carries two distinct insights. The first is that life on Earth may have been seeded from the stars - panspermia, the idea that the building blocks of life arrived here from elsewhere in the cosmos. The second is the one Randall finds equally significant - attaining the stage of complex animal life is extraordinarily difficult, but maintaining it once achieved is something else entirely. The geological record reveals an unending succession of planetary disasters that have repeatedly threatened to undo what evolution took hundreds of millions of years to build. Mass extinction events are not rare interruptions to the story of life on Earth. They are a recurring feature of it - and the fact that complex life has survived them at all, Randall suggests, may be one of the most remarkable facts in the entire history of this planet.