Asteroid impacts may have formed life’s building blocks

A high-powered laser in the Czech Republic has now produced thought provoking evidence that the hellish circumstances produced when an asteroid or comet slams into Earth could have shaped some key building blocks of life on Earth. In a lab experiment proposed to duplicate the high temperatures and pressures of such an impact, scientists used the laser to make adenine, guanine, cytosine, and uracil, the four organic compounds in RNA at the same time, which many consider to have been the first molecule to encode genetic info.

“This is, I trust, the first time that all four nucleobases have been produced in one set of reaction situation,” says Steven Benner, an astrobiologist at the Foundation for Applied Molecular Evolution in Gainesville, Florida.

Scientists have long sought to recognise ways that the nucleobases that make up RNA or DNA (which uses thymine not uracil) could be produced from simpler constituents. That, in turn, could help researchers ascertain where, how, and under what circumstances life might have initiated, says Svatopluk Civiš, a physical chemist at the J. Heyrovský Institute of Physical Chemistry in Prague.

In recent years, Civiš says, scientists suggested that a simple matter called formamide was a conceivable source of such genetic building blocks. This simple chemical, which forms when hydrogen cyanide reacts with water, would have been lavishly available on early Earth and has the major elements necessary for prebiotic chemicals—namely, hydrogen, nitrogen, carbon, and oxygen. In fact, some teams have previously made separate nucleobases in lab experiments using various catalysts to drive chemical reactions between formamide and other constituents. And other groups have done so beginning with different grab bags of simple chemicals.

In the new study, Civiš and his associates fired their institute’s laser into a formamide-bearing solution that also contained clay. In that mixture, planned to represent a chemical-rich pool on ancient Earth’s surface, the one-third-of-a-nanosecond-long pulses produced intense pressure, temperature spikes beyond 4200°C, and a cascade of radiation comprising ultraviolet and x-ray wavelengths—just the sort of circumstances predictable when an object such as a comet or asteroid strikes the ground. Reactions sparked in the experiment’s harsh surroundings, besides making substances such as hydrogen cyanide, carbon monoxide, ammonia, and methanol, also formed the four RNA nucleobases, the scientists report online today in the Proceedings of the National Academy of Sciences.

In addition to being a nice bit of lab work, Benner states, “this paper has real prebiotic significance since it uses as a starting material a compound that was certainly present in substantial amounts on early Earth.”

Preceding studies show that some classes of meteorites at present comprise nucleobases such as adenine and guanine, says Aaron Burton, an astrobiologist at NASA’s Johnson Space Center in Houston, Texas. But the new outcomes propose the celestial bodies could have also shaped them when they struck the planet, says Raffaele Saladino, an organic chemist at Tuscia University in Viterbo, Italy. While he and his associates have studied formamide under more benign lab circumstances, the Prague experiments “are the first time that formamide has been studied in the background of a meteorite impression,” he notes.

Such impacts were a noteworthy factor in Earth’s early history. During a period aptly dubbed the Late Heavy Bombardment, which initiated about 4 billion years ago and lasted some 150 million years, large objects pummelled our planet and moon as well as Mercury, Venus, and Mars. Though many scientists have proposed that such impacts and their subsequent effects on climate effectively sterilized Earth’s surface of any life that may have previously started, the new study hints that this period seeded our planet with the raw constituents essential for life to develop. “This paper nicely associates the Late Heavy Bombardment and the energy it conveyed to Earth about 4 billion years ago with the formation of RNA and DNA nucleobases from formamide,” Benner says.

Even if extra-terrestrial bodies carried nucleobases to Earth in early times, Burton says, impacts “offer an additional route to making these vital molecules during periods of high shower.”