The Mystery of Life’s Origin on Earth Just Got More Mysterious

3 min readSince time immemorial, humanity’s greatest thinkers have wrestled with what is arguably the most burning question of our inquisitive species: Why are we here? Over the millennia, the proposed answers have meandered from the divine to the demonstrable. Then, in 1953, the famous Miller-Urey experiment made it seem increasingly plausible that life emerged from the complex chemistry of early Earth.However, exactly where life’s building blocks came from remains an open question. Some scientists favor “panspermia,” the idea that life didn't begin on Earth at all. In this view, hardy microbes rode here aboard comets, asteroids, or rocky debris blasted off other worlds, survived the trip through space, and seeded our planet. A softer version of the idea says space rocks delivered life’s raw ingredients rather than life itself, in the form of organic molecules made in the protoplanetary disk around our young sun. Still others argue that Earth needed no outside help and produced everything life required on its own.But just as important as pinpointing this much sought-after answer is ruling out a few choices. To that end, a new study led by scientists at the University of Cambridge and published in the journal Scientific Reports models the possibility that comets ferried chemicals, specifically hydrogen cyanide (HCN), to the planet’s surface during its infancy more than four billion years ago. A 2022 study found that cyanide on early Earth likely served as a critical reductant (an electron donor in a chemical reaction), and the cyanide also served as a vital intermediate in Stanley Miller and Harold Urey’s famous experiments by producing amino acids when exposed to electricity (possibly from volcanic lightning).Cometary bombardment is one of the leading theories of how HCN arrived on Earth (though other studies explore whether Earth's own chemistry could have produced the molecule). So the Cambridge team, led by Richard Anslow, modeled whether comets could have delivered it. They found the opposite of what the theory predicts: The chances of comets delivering HCN intact to the surface are vanishingly small, bordering on impossible.“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,” Anslow said in a press statement. “It’s kind of a romantic idea, but our results suggest it’s highly unlikely.”The theory that comets provided the chemical substrate of life runs head first into two major problems. Anslow’s previous work showed that “bouncing comets”—comets whose speeds are impacted by the gravitational influence of multiple planets—could arrive gently enough to deliver their fragile cargo intact. But these new models suggest that such a billiard-ball effect likely didn’t occur in Earth’s neighborhood. Using advances in comet-impact simulations and a better understanding of comet-atmosphere interactions, the researchers concluded that comets usually traveled too fast and at too sharp an angle. On the off chance that HCN did survive the journey, the concentrations would often be too low to be useful, and prebiotic chemistry would need to occur quickly upon arrival. Together, these two requirements push the comet-delivery theory from plausible to a “one-in-a-million” occurrence, Anslow says.“We can’t rule out for certain that it happened that way,” Anslow said in a press statement. “But it seems to be a really rare occurrence—at best, a one-off fluke.”Strangely, Earth’s planetary neighbor Mars would’ve had an easier go of it because it’s smaller and its atmosphere is less thick. This would’ve increased the likelihood of comets actually reaching its surface in higher numbers, but the dissolution of the Red Planet’s magnetic field roughly four billion years ago doomed the world to a lifeless existence (as far as complex life goes, that is).Neither Anslow’s previous work nor this new study rules out the possibility that comets helped deliver life’s ingredients elsewhere in the Milky Way or beyond. But when it comes to the age-old question of why we’re here, these icy wanderers are likely not the answer.Darren lives in Portland, has a cat, and writes/edits about sci-fi and how our world works. You can find his previous stuff at Gizmodo and Paste if you look hard enough.