And it turns out that Miller under-reported the richness of his prebiotic soup. When Bada’s lab inherited the leftover residues after Miller’s death in 2007, Bada examined them using modern instruments. He detected more than eight times the number of amino acids found by Miller.
The surprising success of the Miller-Urey experiment has made it a classic in many textbooks, but there was also a problem at its core — namely, Urey’s hypothesis that the early atmosphere was flush with methane and ammonia. “In fact, it was carbon dioxide with nitrogen, or carbon dioxide with a little methane,” Bada says. Miller knew about the problem, and when he replicated his experiments in the 1980s using a more plausible mix of gases, most turned up no amino acids.
Seeding an RNA World
About a decade after Miller’s unsuccessful second attempt, the European Space Agency added a young German chemist named Uwe Meierhenrich to its Rosetta mission to Comet 67P/Churyumov–Gerasimenko. Meierhenrich helped develop instruments that allowed the Philae lander to probe the comet’s surface for organics. But to test Philae’s abilities, he’d need a comet on Earth.
Meierhenrich found what he needed at Leiden University in the Netherlands, where astrochemists were making artificial cometary ice to test how these dirty snowballs form. Their technique was akin to Miller’s simulated primordial soup. The Leiden scientists replicated the frigid vacuum of interstellar space, then introduced the chemicals found in cometary ice and hit them with ultraviolet light like that emitted by stars. This process produced mere micrograms of comet, but it was enough to test Rosetta’s instruments. The equipment worked impeccably — and detected 16 amino acids.
Although researchers suspected they’d see amino acids on 67P — having already detected them in certain primitive meteorites — nobody expected that the organics would form instantaneously in a simple ice analog on Earth. As doubt grew over the Miller-Urey experiment, Meierhenrich’s research lent strong support to an alternative hypothesis: that the organic ingredients of life came from outer space.
And support for a cometary origin has only grown since Meierhenrich published his initial results in 2002. Twelve years later, the Philae lander descended onto 67P and detected organics that matched the ice from Leiden.