Scientists Wonder: Is the Origin of Life Extraterrestrial?

The origins of life are unknown. Did it spontaneously arise from hydrothermal vents, or when lightning struck a pool of sludge? How did all the necessary ingredients happen to be in the right place at the right time? Is the emergence of life on Earth inevitable? Or did meteorites bring in some of the necessary ingredients?

About 10 years ago, scientists at NASA and elsewhere used cutting-edge techniques to look at meteorites and find definitive evidence of several molecules considered prebiotic, including the purine nucleobases guanine and adenine that are part of DNA. While the scientists didn’t find everything needed to encode genetic information, the discovery established that there was active organic chemistry happening on asteroids.

Now, Yasuhiro Oba (Hokkaido University, Japan) and colleagues have analyzed three meteorites and found something new: In addition to the previously detected purine nucleobases as well as uracil, they discovered the “missing” pyrimidine nucleobases (cytosine and thymine) that make up the rest of the DNA/RNA alphabet.

To do this, the team developed a new, milder extraction method. “It has long been considered that hot formic acid is best for extracting nucleobases from natural samples,” Oba explains. “But this sometimes decomposes, or hydrolyzes, organic molecules. Instead, we used ultrasonication in cold water.”

Ultrasonication forces intense ultrasound waves through liquids, creating shock waves that “sort” the molecules within the medium. When Oba’s group used this technique on finely powdered meteorite samples in water, it revealed a greater diversity of organic molecules.

But how can scientists tell that organic molecules, nucleobases or otherwise, are actually extraterrestrial? Oba’s group could test this directly with one of the meteorites they sampled:  the “Murchison” meteorite, discovered 1969 in Murchison, Australia. The researchers compared soil at the Murchison impact crater to the meteorite sample to ensure the nucleobases arrived with the impactor and weren’t terrestrial in origin.

“Although some nucleobases were identified in the soil sample,” Oba says, “the concentration and molecular distribution are clearly different from those detected in the Murchison meteorite.”

The Murchison meteorite is 7 billion years old, so it formed while the Sun was still a protostar. The presence of prebiotic molecules original to the rock (and not its crash-landing) could support the theory that life on Earth has an extraterrestrial origin.

Michael Callahan (Boise State University), who performed the aforementioned analyses of nucleobases in meteorites, says the new study has “improved detection limits and enabled the identification of pyrimidines.” But he cautions that the pyrimidines are found in such low concentrations that the result inhibits more speculative conclusions. “If these results are representative of typical pyrimidine concentrations in meteorites,” he explains, “then geochemical synthesis on early Earth would likely have been responsible for the emergence of genetic material rather than inputs from extraterrestrial delivery.”

Then again, it isn’t clear whether these meteorites are representative of the general population of space rocks that make it to Earth, now or in the distant past. Sample-return missions from the asteroids Ryugu and Bennu will help us understand the evolution of extraterrestrial organic molecules. The methods pioneered by Oba’s group could prove invaluable in determining the true composition of these pristine asteroids, as well as the origin of complex organic molecules in interstellar space.

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