We've often found trace evidence of DNA on meteorites, but it wasn't clear whether these DNA pieces originated in space, or were accidental byproducts of contamination by scientists. Now we have our best evidence yet that they actually come from space.
While it often gets bogged down in fanciful and pseudoscientific interpretations, the basic principle of panspermia — namely, that the building blocks of life are found throughout the universe and can be carried by meteors, asteroids, and other small bodies — is far from ridiculous. The main issue that we've never been able to find any truly convincing evidence for this idea, and it's definitely the sort of extraordinary claim that requires some pretty extraordinary evidence.
We might quite have reached that level of proof, but the latest find by NASA scientists offers some intriguing support for the notion that meteorites carry the basic components of DNA. Dr. Michael Callahan of NASA's Goddard Space Flight Center explains:
"People have been discovering components of DNA in meteorites since the 1960's, but researchers were unsure whether they were really created in space or if instead they came from contamination by terrestrial life. For the first time, we have three lines of evidence that together give us confidence these DNA building blocks actually were created in space."
So just what did the researchers find? First, they examined twelve carbon-rich meteorites. By pulverizing the samples and then extracting samples with the help of some formic acid, the scientists were able to separate and analyze the various compounds that make up the meteorites' internal structures.
The big discovery was the presence of adenine and guanine, which are two of the four nucleobases that make up DNA. They also found hypoxanthine and xanthine, which do not play a direct role in DNA but show up in many related processes. What's even more exciting is the discovery of three molecules related to the nucleobases: purine, 2,6-diaminopurine, and 6,8-diaminopurine.
Those last two, beyond being downright unpronounceable, are crucial pieces of evidence because they almost never show up in terrestrial biology. Their presence seems to argue against the DNA simply being the result of earthly contamination. Indeed, as Dr. Callahan explains, it makes much more sense for these unusual molecules to originate from the meteorite than on Earth:
"You would not expect to see these nucleobase analogs if contamination from terrestrial life was the source, because they're not used in biology, aside from one report of 2,6-diaminopurine occurring in a virus (cyanophage S-2L). However, if asteroids are behaving like chemical 'factories' cranking out prebiotic material, you would expect them to produce many variants of nucleobases, not just the biological ones, due to the wide variety of ingredients and conditions in each asteroid."
That's one line of evidence. Another came from analysis of an ice sample from the Antarctic region in which nine of the twelve meteorites were found. While the DNA samples were present in the meteorites at an average concentration of several parts per billion, their presence in the ice as several orders of magnitude smaller, only about one part per trillion, and those unusual molecules were entirely absent from the ice. That makes the odds of contamination by the surrounding terrestrial environment very unlikely.
Finally, one last piece of evidence for the meteorite origins of the DNA is that the nucleobases could be produced in non-biological reactions. That makes it a lot easier to accept a space-based origin, as Dr. Callahan explains:
"In the lab, an identical suite of nucleobases and nucleobase analogs were generated in non-biological chemical reactions containing hydrogen cyanide, ammonia, and water. This provides a plausible mechanism for their synthesis in the asteroid parent bodies, and supports the notion that they are extraterrestrial. In fact, there seems to be a 'goldilocks' class of meteorite, the so-called CM2 meteorites, where conditions are just right to make more of these molecules."
Of course, this doesn't prove that we have meteorites to thank for the existence of life on Earth. But the researchers do suggest that the huge variety and utility of the DNA building blocks found in the meteorite might well have played at least a supporting role in the origins of life on Earth billions of years ago.