Comets May Not Have Delivered Life to Earth, New Study Finds

For decades, textbooks have offered a romantic origin story for life on Earth: comets, falling from the cold outer solar system, delivered the chemical ingredients that made biology possible. A new study says the delivery probably never arrived intact.

According to reports from CNN and Gulf News, the research, published on Wednesday, 28 October in the journal Scientific Reports, simulated the early Earth's bombardment 100,000 times — and found, on average, that no comet impact succeeded in delivering life's building blocks.

Why comets were the favourite delivery service

The idea is older than most people realise. The broader notion that life's ingredients arrived from space, known as panspermia, was scientifically postulated in 1908. The specific comet-delivery theory dates back to at least 1961.

It made sense on paper. Comets are icy bodies rich in organic molecules. Early Earth, more than four billion years ago, may have been struck by more than 100,000 comets larger than 100 metres across during a period of heavy bombardment. If even a fraction of those visitors dropped their cargo gently, the young planet would have been seeded with the chemistry life needed.

The new study tests that "gently" assumption — and finds it fails. Using updated models of comet formation and evolution, the researchers simulated impacts under early-Earth surface conditions. The extreme temperatures and pressures generated by a comet striking at high speed, they found, would generally destroy the delicate organic molecules essential to the emergence of life.

The hydrogen cyanide test

The study focuses on one molecule: hydrogen cyanide, or HCN, a naturally occurring compound of hydrogen, carbon and nitrogen that has been detected in dozens of comets, including Halley's and Hale-Bopp. HCN matters because, combined with water, it can lead to amino acids — the molecules that make up proteins — and other studies have previously identified it as a potential key ingredient for the origin of life.

Lead author Richard Anslow, who conducted the research while completing his doctorate at the Institute of Astronomy at the University of Cambridge, told CNN: "Based on what we know about the surface environment of the early Earth, on average our simulations found that no impacts succeeded."

In 54% of the simulations, no comet hit Earth at all. In the rest, at least one impact occurred, but only a very small proportion produced as many as three collisions. And even a successful impact, the authors argue, would be unlikely to deliver HCN in a useful concentration: if cometary material had been deposited into Earth's oceans simultaneously, the resulting concentration would have reached only about 0.75 micromoles per litre — approximately 13,000 times below the concentration considered necessary for the chemical reactions examined in the study.

An ocean impact makes things worse, not better. The chemical would quickly dissolve and disperse, rather than pooling where prebiotic chemistry could use it. Over 500 million years, the researchers estimate, comets delivered about 2.63 quadrillion tonnes of material — roughly 0.19% of the mass of Earth's oceans. Impressive in absolute terms, but spread far too thin to seed life on its own.

What this does — and does not — prove

The study does not definitively rule out a cometary contribution to life. It tests one molecule, HCN, under one set of early-Earth conditions, and its authors are careful about that limit. Queenie Chan, an associate professor of Earth sciences at Royal Holloway, University of London, told CNN that hydrogen cyanide might have reacted to form other biologically important compounds before comets entered Earth's atmosphere — a pathway the simulations do not close off.

What it does challenge is the simplest version of the story: that comets rained down intact building blocks and life followed. If the ingredients did not survive the fall, they must have been made here — in Earth's own oceans, vents or atmosphere — or delivered by a gentler mechanism scientists have not yet identified.

That is why a simulation study matters beyond the laboratory. Every origin-of-life theory is, at heart, a story about where we came from. This one suggests the answer may be less cosmic, and more terrestrial, than we liked to believe.

Related: How Hubble challenged the supernova theory of the early universe's clearing fog.