Simple, single-celled life is believed to have existed on Earth as early as 3.9 billion years ago – almost immediately, when the Earth was cool enough for liquid water to surface. How could life arise so quickly? Researchers may have taken a big step forward in answering this question: a team from Japan and the USA has detected so-called nucleobases in three meteorites – important building blocks for the genetic material DNA. The scientists explain that these complex molecules probably formed in space before the formation of the solar system in the journal Nature Communications.
“Our investigations show that there is a large variety of nucleobases in meteorites,” report the researchers led by Yasuhiro Oba from the University of Hokkaido in Japan. “These nucleobases may have served as building blocks for the formation of DNA and RNA on early Earth.” In order for living beings to be able to reproduce and adapt to their environment through evolution, their blueprint must be able to be stored and passed on. The carrier of this plan is usually the DNA, while the RNA helps to copy this information. The actual information is stored in the sequence of nucleobases.
The central building block of all life on earth is carbon: Up to four other atoms can bond to a carbon atom, complex compounds, long molecular chains and molecular rings can arise. In chemistry, such carbon-based compounds are called “organic” because they form the basis of life. It has long been known that many organic substances can form in space. Even amino acids and sugar molecules have already been detected in gas clouds and in meteorites that have fallen to Earth.
The molecules are arguably stable enough to survive the turbulent early stages of planets
As a result, the hypothesis gained weight that the rapid emergence of life on earth was pushed by an influx of life building blocks from space. But how far did this cosmic support go? Very far, as the study by Oba and his colleagues now shows: With the help of new analysis methods, the researchers were able to detect a large variety of nucleobases in three intensively studied meteorites. These include adenine, thymine, guanine and cytosine – these four substances are the key information carriers in DNA.
The key to the Japanese team’s success was the sensitivity of its measuring instruments. “Our analysis technique is optimized to detect nucleobases in the lowest concentration down to one molecule in a trillion molecules,” say the scientists. In fact, they found the nucleobases in concentrations of up to one in a billion. The abundances found coincide with predictions from models of the chemical evolution in dense gas clouds from which stars and planets form.
Oba and his colleagues conclude that the building blocks of DNA formed before the sun and earth formed. They are apparently stable enough to survive the turbulent formation of the planets, accumulate in dust and rock fragments, and then make their way to Earth through meteorites.
In contrast, the formation of such molecules on the young earth is difficult. “We therefore suspect,” say the researchers, “that nucleobases delivered from space contributed to the emergence of the genetic characteristics of the first life on earth.”