Stanley Miller performed some of the most famous origin of life experiments, showing that the chemicals thought to be present in the early Earth's atmosphere might react to form amino acids, the building blocks of proteins. But these experiments haven't aged well, through no fault of their own; other scientists have since revised their estimates of what was present in the early atmosphere, raising some doubts as to whether the Miller experiments are especially relevant. A paper released by Nature Chemistry neatly dodges this issue by showing that it might not matter what the Earth looked like—the shockwave of a comet impact can make biological materials regardless of the composition of the atmosphere it crashes into.

In the years since Miller's experiments, we've been better able to image the composition of comets, and have even returned samples of some of the material shed by the comet Wild 2 as it approached the Sun. These have revealed a mixture of simple organic compounds like ammonia and ethanol, but nothing as complex as an amino acid, chemicals that form the building blocks of proteins.

Still, the authors of the paper note two important facts about comets. The first is that a lot of them probably hit the Earth during its early years. Estimates of the heavy bombardment period indicate that over 1013 kilograms of organic material were delivered to Earth by comets every year, and the heavy bombardment periods went on for hundreds of millions of years. The second item is that these comets didn't arrive peacefully. The extreme conditions of a planetary impact have the potential to foster some unusual chemistry that would never occur under equilibrium conditions.

Lacking access to a comet, the authors looked closer to home: they were based at Lawrence Livermore Lab, which happens to host some serious computing power. So, they ran molecular dynamics simulations of what might happen to a typical cometary mixture as a blazing hot shockwave passed through, and was followed by a rapid decompression. These were pretty elaborate calculations, with femtosecond time resolution, and molecular interactions that considered quantum effects. Simply modeling the decompression that followed a shockwave for 50 picoseconds involved about 80,000 CPU hours. They also reran the model to simulate different speeds and angles of impact, which produce different pressure/temperature combinations within the shockwave that passes through the comet.

One of the problems facing origin-of-life research is that building complex organic chemicals requires a reducing environment, but the early Earth's atmosphere is now thought to have been weakly oxidizing. None of this matters as the comet hits. A typical shockwave quickly reaches conditions where the simple compounds break down, liberating hydrogen ions. These create local reducing environments no matter what the atmosphere looks like.

That environment promotes the formation of carbon-nitrogen bonds, essential parts of both the amino acids that form proteins, and the nucleotides that are part of DNA. As the pressure peaks, complex chains of these bonds form, some as many as ten atoms long. Once the shockwave passes, however, the sudden drop in pressure allows further reactions that break many of these chains apart. One of the common products that remains is glycine, the simplest amino acid.

The glycine is accompanied by hydrogen cyanide and formaldehyde, both of which can engage in further reactions that the authors suggest might ultimately build more complex amino acids.

Right now, most scientists think that life originated in an RNA world, where proteins didn't exist, and amino acids simply acted as co-factors for some key chemical reactions. So this doesn't necessarily help us understand how life first got started. It may, however, provide some insight into how life started using amino acids in the first place, starting it on the road towards the production of proteins. If basic amino acids were plentiful, then evolution might have simply worked with what was already around.

Nature Chemistry, 2010. DOI: 10.1038/NCHEM.827 (About DOIs).

Listing image by NSF