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Meteorites could have been responsible for delivering the basis of life's genetic code.
Analyses of three meteorites suggest that nucleobases, the crucial components of DNA, could have formed in space and then fallen to Earth to provide the raw material for the origin of life itself.
Could meteorites have brought the building blocks of life to Earth?
A new study, published in Nature Communications, suggests this could be the case. Analysing a group of meteorites known to be rich in organic materials, the study is one of the first to demonstrate that both types of nucleobase, the compounds that encode genetic information in DNA, could have originally had an extraterrestrial origin.
After the formation of nucleobases in outer space, the meteorites would have been able to deliver the molecules to Earth, later becoming the basis of all genetic coding for life.
Dr Helena Bates, who researches asteroids at the Museum and was not involved in the study, says, 'During the formation of Earth, many common elements and compounds with low boiling points, known as volatiles, would not have been present. We believe that many of these, such as water, were delivered afterwards by meteorites, which may have also delivered organic molecules.
'This paper finds that nucleobases were among those organic compounds potentially delivered by meteorites, and suggests that these molecules may have contributed to the emergence of genetic material on Earth.
'It's very strange to think that the molecules which make up DNA can be found in rocks from space. It suggests that these molecules are all over the place, which has a lot of implications when it comes to considering the potential distribution of life within our Solar System.'
There are many different theories for what conditions were needed for life to form on Earth. One of the most famous is the primordial soup hypothesis, which suggests that intense ultraviolet radiation and lightning on the early Earth provided the energy for chemical reactions between compounds such as water, ammonia and methane to begin.
The theory suggests these reactions then led to the creation of molecules such as DNA and RNA, which subsequently became part of a self-replicating system which would ultimately become life. Experiments have confirmed that this theory might work, with subsequent analysis showing that more than 20 amino acids crucial for life were synthesised.
However, there are some gaps between the conditions simulated in these experiments and what we now understand about the early evolution of Earth's atmosphere. The challenges of the primordial soup hypothesis have led some scientists to look elsewhere for how the earliest molecules formed.
The bottom of the ocean presents some promising candidates, with alkaline vents on the ocean floor being rich in the compounds needed for the formation of nucleic acids.
Other theories include reactions involving ice and glaciers, radioactive beaches, and meteorites bringing these materials to Earth. The latter are already known to have played a crucial part in the formation of life as we know it, with a type known as the carbonaceous chondrites believed to have delivered most of the planet's water.
Meteorites also contain a variety of other organic molecules. Charlotte Bays is a PhD student at the Museum who was not involved with the study but researches these organic compounds. She says, 'The intricacies of how these molecules form in the interstellar medium is still widely debated, as there are a number of different ways in which they could be synthesised.
'What's interesting about this paper is that it finds nucleobases, which make up the structure of RNA and DNA, in these meteorites. While guanine and adenine have previously been identified, the discovery of cytosine, uracil and thymine complete the set needed for forming base pairs.
'The occurrence of these kind of complex molecules that we're seeing in these meteorites get around some of the issues of them forming on Earth, as the delivery of these initial building blocks would have paved the way for critical biorelevant molecules such as DNA and RNA to form later.'
The study, a collaboration between Japanese and American scientists, studied three carbonaceous meteorites which fell in Murchison, Australia; Lake Murray, USA; and Tagish Lake, Canada.
Samples were taken from these meteorites and ground into a fine powder, allowing the organic components of each meteorite to be extracted. All five of the purines and pyrimidines which make up DNA and RNA were detected in the meteorites at concentrations similar to experiments replicating the conditions of the early universe.
The researchers ruled out that these compounds were the result of contamination after the meteorites landed on Earth, as the nucleobase concentrations were different from the soil found in the areas they landed. The typical precursors for their biological formation were also mostly absent, suggesting the molecules formed in a way unlike those on Earth.
The next step is to analyse samples of asteroids which may have been similar to the parent bodies of these meteorites, potentially allowing the researchers to confirm their findings.
Previous missions have already detected organic molecules on Itokawa, a siliceous asteroid, while missions to the asteroids Ryugu and Bennu are expected to be analysed in the coming years. This paper adds to this work, demonstrating the presence of nucleobases on other asteroid types.
'Two of the meteorites are thought to come from B- and C-type asteroids, like Bennu and Ryugu, which are common in the Solar System,' Charlotte says. 'However, the Tagish Lake meteorite has been suggested to come from a D-type asteroid, which are rarer.
'As nucleobases were found in this meteorite as well, it could be that these molecules are common across the Solar System, as these meteorites and their parent asteroids come from very different places.'
NASA's Lucy mission hopes to visit eight asteroids in the coming decade, including C and D-type asteroids, allowing scientists to discover even more about how these bodies contributed to the formation of the Solar System.