Half a billion years… that’s how long the Earth existed as a barren world.  At least half a billion years of hell before the planet’s molten seas of liquid rock cooled to give the world a solid surface.  Only then did life appear.  Only then did our world’s fantastic history of microbes evolving to mollusks evolving to dinosaurs evolving to us begin. 

But what, exactly, was that beginning?

When scientists tell the story of life on Earth they usually call on a theory known as abiogenisis.  It's a narrative of life emerging from non-life.  Through random chemical trial and error, (and a lot of time), the raw materials of living cells are assembled like jiggling tinker-toys.  Once nature hits on the right self-replicating combination of molecules, life takes off and the evolution of form and function begins.  It’s a story backed up by a lot of science.

But is that the only way to tell the story of life’s origin on Earth?  There is, in fact, another narrative about our planet’s ascent to a full biosphere. 

Panspermia, that’s the name for the alternative to abiogenesis.   The idea is simple.  Life did not begin on Earth but was transported here from somewhere else in the Universe.  Like seedpods floating on the wind till they fall and take root, life on Earth is the child of a distant parent.  In the panspermian view (or at least one version of it), an ancient asteroid or comet from another solar system fell on Earth some three or four billion years ago.  Hardy cells buried deep in the rock matrix of that alien asteroid were then freed in the impact seeding our world with its first microbes.

It is, without doubt, a mind-blowing idea.  In fact, there’s a lot of science fiction based on panspermian conceits.  For example if you add an intelligent species doing the seeding, you find panspermia as the core idea of Ridley Scott’s Prometheus.  You can also find echoes of it my favorite science fiction series The Expanse. 

Over the last few years, we caught 3 different alien comets passing through our solar system (the most recent is 3I/Atlas).  Because of those visits, the science behind panspermia’s science fiction-y sounding premise has gotten a wee kick upward on the plausibility scale. 

Of course, there’s a basic problem with panspermia that’s pretty simple too. Maybe life did start somewhere else and accidently ended up here.  But as an explanation for life itself, panspermia just kicks the can down the road (a road measured in light-years).  Even if Earth-life did not start on Earth, it had to start somewhere.  That means if you do accept panspermia’s super-cool story of microbes hitching a ride on alien asteroids, scientists are still left with the same basic question.  How did life start?  How did non-life make the remarkable transformation to living form?

So yes, it is really possible that life started somewhere else and ended up here on Earth. But the origin of life itself still remains the greatest question of all.

— Adam Frank 🚀