Ever since its formation, the Earth has been bombarded by comets, and scientists think they now have evidence that these cosmic missiles could have carried the building blocks for life along with them when they impacted.
In 2009, NASA's Stardust mission rendezvoused with the comet Wild-2 and returned to Earth with a sample of an amino acid the probe had picked up from comet's outgassing. Tests showed it had too much carbon-13 to have come from Earth, but skepticism remains that simple amino acids could have kick-started the development of life on the planet.
To test that hypothesis, a team of scientists at the University of California, Berkeley, and the University of Hawaii, Manoa, built a pseudo-comet in the lab – a snowball of carbon dioxide, ammonia, and various hydrocarbons such as methane, ethane, and propane – then chilled it in an ultra-high-vacuum chamber down to 10 degrees above absolute zero.
The comet was then zapped with high-energy electrons to simulate the cosmic rays it would encounter had it travelled through space. The team then analyzed the result to see what had formed using the Mars Organic Analyzer, an instrument designed for ultrasensitive detection and identification of small organic molecules in the Solar System.
On analysis, the laboratory comet was found to have not only nine different amino acids but also at least two dipeptides – linked pairs of amino acids that are capable of forming proteins (polypeptides), enzymes, and more-complex molecules such as sugars.
"It is fascinating to consider that the most basic biochemical building blocks that led to life on Earth may well have had an extraterrestrial origin," said UC Berkeley chemist Richard Mathies, coauthor of the paper to be published in The Astrophysical Journal, in a statement.
Previous scientific theories have largely revolved around the famous 1953 Miller-Urey experiment, in which simulated lightning was fired into a primordial soup of water, methane, ammonia, and hydrogen. This produced over 20 amino acids, and further research in the area is continuing.
Of course there are still plenty of people – particularly here in the US – who will tell you that humans were created by a supernatural deity (who curiously burdened humans with such useless organs as the appendix and men's nipples) and therefor such scientific research is pointless.
But for scientists, the news is rather exciting. ®