Sid Perkins (Science Dec 12 2014) — A high-powered laser in the Czech Republic has now provided provocative evidence that the hellish conditions produced when an asteroid or comet slams into Earth could have created some key building blocks of life on Earth.
In a lab experiment intended to duplicate the high temperatures and pressures of such an impact, researchers transformed a solution containing a simple pre cursor into adenine, guanine, cytosine, and uracil—the information-bearing nucleobases in RNA, which many believe to have been the first genetic molecule to encode.
“This is, I believe, the first time that all four nucleobases have been made in one set of reaction conditions,” says Steven Benner, an astrobiologist at the Foundation for Applied Molecular Evolution in Gainesville, Florida.
Researchers have long sought to identify ways that the nucleobases that make up RNA or DNA could be created from simpler substances. That, could help scientists ascertain where and how life might have originated, says Svatopluk Civiš, a physical chemist at the J. Heyrovský Institute of Physical Chemistry in Prague.
In recent years, Civiš adds, researchers proposed that a substance called formamide was a possible source of such genetic building blocks. This minimalist chemical, which forms when hydrogen cyanide reacts with water, would have been abundant on early Earth and has the major elements needed for prebiotic chemicals—namely, hydrogen, nitrogen, carbon, and oxygen. Indeed, some teams have already produced individual nucleobases in lab experiments that relied on catalysts to drive chemical reactions between formamide and other ingredients. Other teams have made nucleobases from different grab bags of simple chemicals.
In the new research, Civiš and his colleagues fired their institute’s 1-kilojoule laser into a formamide-bearing solution that also included clay. In that mixture, intended to represent a chemical-rich pool on ancient Earth’s surface, the one-third-of-a-nanosecond-long pulses generated intense pressure, temperature spikes exceeding 4200°C, and a cascade of radiation including ultraviolet and x-ray wavelengths—just the sort of conditions expected when an object such as a comet or asteroid strikes the ground. These extreme conditions sparked reactions that, besides producing substances such as hydrogen cyanide, carbon monoxide, ammonia, and methanol, also created the four RNA nucleobases, the researchers report this week in the Proceedings of the National Academy of Sciences.
In addition to being a nice bit of lab work, Benner says, “this paper has real prebiotic relevance since it uses as a starting material a compound that was undoubtedly present in sizable amounts on early Earth.”
Previous studies showed that some classes of meteorites already contain nucleobases such as adenine and guanine. But the new results suggest that besides merely delivering nucleobases, celestial bodies could have also created them when they struck the planet, says Raffaele Saladino, an organic chemist at Tuscia University in Viterbo, Italy. He and his colleagues have studied formamide under more benign lab conditions, but he says the Prague experiments “are the first time that formamide has been studied in the context of a meteorite impact.”
Such impacts were common in Earth’s early history. During a period aptly dubbed the Late Heavy Bombardment, which began about 4 billion years ago and lasted some 150 million years, large objects pummeled our planet and moon as well as Mercury, Venus, and Mars. Although many researchers have suggested that such impacts and their resulting effects on climate would have effectively sterilized Earth’s surface, erasing any life that may have already started, the new study hints that this period seeded our planet with the raw ingredients necessary for life to develop. “This paper nicely correlates the Late Heavy Bombardment and the energy it delivered to Earth about 4 billion years ago with the formation of RNA and DNA nucleobases from formamide,” Benner says.
Science 12 December 2014:
Vol. 346 no. 6215 p. 1279