origin of life, USA News: A landmark study has dismantled a theory about the origin of life that has persisted for more than a hundred years. Scientists long theorised that comets seeded our planet with the essential molecular ingredients for life. However, new research, published in the journal Scientific Reports, found that these icy celestial bodies strike Earth’s atmosphere at speeds and angles far too extreme for delicate organic compounds to remain intact. Rather than the long-theorised steady shower of life-giving chemicals raining down from space, any successful delivery would amount to nothing more than an extraordinary fluke.
Lead author Dr Richard Anslow from Cambridge’s Institute of Astronomy stated that while it is a romantic idea, their results suggest it is highly unlikely. The team’s computer simulations centred on hydrogen cyanide (HCN), a compound composed of hydrogen, carbon and nitrogen—three elements fundamental to all living organisms. HCN is particularly significant because the energy stored in its carbon-nitrogen triple bond makes it highly reactive and capable of driving the creation of complex molecules, including sugars, fats, and nucleobases, the building blocks of DNA.
Yet the modelling revealed that even if every scrap of cometary material had been deposited into Earth’s oceans simultaneously, the resulting concentration would reach just 0.75 micromoles per litre. This is roughly 13,000 times below the threshold required to trigger the formation of key life-giving compounds. Simulations estimated that comets delivered about 2.63 quadrillion tonnes of material to Earth over 500 million years, a figure equivalent to just 0.19 per cent of the mass of the world’s oceans. The frequency of comet strikes fell sharply during Earth’s initial 500 million years, leaving only a small number of windows for hydrogen cyanide to arrive from space. During that turbulent early era, more than 100,000 comets wider than 100 metres are thought to have hit the planet, yet the vast majority of organic cargo would have been incinerated upon impact. Even had HCN somehow reached the surface, it would have dispersed rapidly, leaving insufficient time for prebiotic chemistry to take hold.
Dr Anslow explained that it would have to be a one-in-a-million occurrence because an impact would require just the right speed and angle, alongside subsequent precise conditions for chemical origins to begin. While researchers cannot rule it out entirely, they view it as a rare occurrence or a one-off fluke. Asteroids, which bombard Earth far more often than comets, may have generated the necessary molecules through the heat of their impacts rather than carrying them directly from space. Another possibility is that the chemical precursors of life arose entirely through reactions within Earth’s own atmosphere, without any extraterrestrial contribution.
Intriguingly, the researchers identified Mars as a marginally more plausible recipient of cometary organic material. The Red Planet’s smaller size and much thinner atmosphere mean comets would have had a better chance of reaching its surface intact during the period when such impacts were most frequent. Dr Anslow noted this does not mean it definitely happened on Mars, but rather suggests it would be unlikely if it happened on Earth and not on Mars, or alternatively, that it did not happen on either planet.
The findings could also reshape the hunt for life beyond our Solar System. As successors to the James Webb Space Telescope, such as the proposed Habitable Worlds Observatory, come online, astronomers may use these results to refine their targets. The research team suggests that identifying planetary systems where worlds orbit in close proximity could prove particularly valuable, since tightly packed planets would allow comets to decelerate by bouncing between them, improving the odds of organic molecules surviving delivery. The study received funding from the Royal Society, the Leverhulme Centre for Life in the Universe, and the Science and Technology Facilities Council, part of UK Research and Innovation.
