Comets Almost Never Delivered the Ingredients
A hundred thousand simulated histories. In most of them, the cargo burned.
The idea that comets seeded early Earth with the building blocks of life is old enough to feel like a fact. A Cambridge-led model, reported by the university and by Sci.News on 28 October 2026, says the delivery was at best a fluke.
Richard Anslow and colleagues used hydrogen cyanide as a proxy for the fragile organics. Across 100,000 simulated histories of Earth's cometary bombardment, 54 percent recorded no impact that delivered HCN at a concentration considered useful for prebiotic chemistry. The comets hit too fast and too steep. Anslow told IFLScience the impacts heat to tens of thousands of degrees. Successful delivery, in the model, is on the order of one in a million, and even then the chemistry would have had to start immediately, before the puddle diluted.
The same runs make Mars a slightly better candidate. Weaker gravity, thinner air, more cargo surviving. Venus did worse than Earth. Anslow's line: it would be odd if it happened here and not there.
The Rational Explanation
This is one proxy molecule in a model. Asteroids, dust and chemistry on Earth itself are not ruled out. "One in a million" is the model's threshold, not a measured rate from the Archaean.
What We Don't Know
What did deliver the ingredients, if comets did not. The paper narrows a story. It does not replace it.