A new study led by the University of Cambridge challenges the long-standing idea that comets helped kick-start life on our home planet, suggesting that the icy bodies were largely unable to deliver the concentrated chemical ingredients needed for prebiotic chemistry.
An artist’s impression of cometary impact. Image credit: Scarlett White.
“Historically, scientists thought that comets were important for the origin of life on Earth, because they carried carbon and nitrogen-bearing molecules, so these giant space snowballs were almost like a ‘get out of jail free card’ that brought all the molecules a planet would need to seed life,” said lead author Dr. Richard Anslow, a researcher with the Institute of Astronomy at the University of Cambridge.
“It’s kind of a romantic idea, but our results suggest it’s highly unlikely.”
Dr. Anslow’s team used computer simulations to investigate whether comets bombarding the early Earth could have supplied hydrogen cyanide (HCN), a molecule that plays a central role in several proposed pathways for producing the building blocks of life.
Laboratory experiments have shown that sufficiently concentrated hydrogen cyanide can help synthesize sugars, lipids and the components of genetic material.
Comets contain hydrogen cyanide and other organic compounds, making them potential carriers of the raw materials for life.
But delivering these molecules intact to a planet’s surface is difficult. According to the researchers, small comets can break apart in the atmosphere, while the extreme temperatures generated during high-speed impacts can destroy organic molecules that survive the descent.
To assess how often comets might overcome these obstacles, the authors modeled the evolution of short-period comets and their impacts on Earth, accounting for atmospheric entry, impact velocities and the survival of hydrogen cyanide during collisions.
The results suggest that successful delivery was exceedingly rare: across 100,000 simulated histories of Earth’s cometary bombardment, 54% recorded no impacts that delivered hydrogen cyanide at concentrations considered sufficient for prebiotic chemistry.
“It would really have to be a one-in-a-million occurrence, because not only would you need an impact at just the right speed and just the right angle, you would also need everything to happen after that point, in terms of the concentration and the speed at which the chemical origins of life got going,” Dr. Anslow said.
“We can’t rule out for certain that it happened that way. But it seems to be a really rare occurrence — at best, a one-off fluke.”
The scientists also compared comet impacts on Earth, Mars and Venus.
Their simulations found that the Red Planet was more favorable for delivering concentrated hydrogen cyanide, largely because its weaker gravity and thinner atmosphere allow more comets to reach the surface without losing their chemical cargo.
Under the model’s assumptions, Mars experienced substantially more successful deliveries than Earth or Venus.
“This is not to say it definitely happened on Mars, of course, but suggests it would be unlikely if it happened on Earth and didn’t happen on Mars,” Dr. Anslow said.
“Or the other, perhaps more likely, alternative, is that it didn’t happen on either planet.”
The study appears today in the journal Scientific Reports.
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R.J. Anslow et al. 2026. Cometary impacts an unlikely pathway towards the origins of life on Earth. Sci Rep 16, 31688; doi: 10.1038/s41598-026-74214-y
