Could meteorites give Earth the hope of the first life?
According to a new study recently published in Nature Communications, this possibility cannot be ruled out. With the help of the new analytical method, the scientists found the last two of the five information cells of DNA and RNA that had previously been undiscovered in samples from meteorites.
While it is unlikely that such a relatively large molecule as DNA would form in meteorites, the discovery suggests that it was possible that these genetic components could have been sent to Earth, which may have contributed to the development of instruction molecules on early Earth.
Theory of the origin of life
There are various theories about what conditions life would need to first appear on Earth. One of the most representative is known as the primitive soup hypothesis. It believes that intense ultraviolet radiation and intense lightning on early Earth provided energy for the beginning of chemical reactions between compounds such as water, ammonia and methane.
The theory is that these reactions led to the creation of molecules such as DNA and RNA, which subsequently became part of a self-replicating system and eventually evolved life.
Experiments have confirmed that this theory may be feasible, and subsequent analyses have shown that more than 20 amino acids that are vital to life can also be synthesized. But there are some gaps between the conditions simulated in these experiments and our current understanding of the early evolution of Earth's atmosphere.
Some scientists have also begun to look elsewhere for how the original molecules formed. Other promising candidates include seafloor hydrothermal vents, which are rich in compounds needed to form nucleic acids. There are also theories involving reactions to ice and glaciers, radioactive beaches, and meteorites from outer space.
Asteroids like Bennu could be the parent of carbonaceous chondrite meteorites. Carbonaceous chondrite meteorites have been found to provide large amounts of water to earth. (NASA's Marshall Space Flight Center, Fllickr, CC BY)
In particular, meteorites have been thought to have played a key role in the formation of life as we know it, for example, carbonaceous chondrite meteorites may have provided most of the Earth's water.
The basic building block of heredity
The crux of the matter is where the key molecules or precursors that make up the needs of life come from. DNA and RNA contain instructions for building every living thing on Earth, and they have a total of 5 basic information components, also known as bases. Such organic molecules can be divided into purines and pyrimidines.
Common bases in DNA and RNA. (Photo: BruceBlaus, Wikimedia Commons, CC BY)
Previously, when searching for extraterrestrial samples, scientists found only 3 of the 5 bases necessary for life. However, this recent analysis identified the last two bases that have not been found.
The newly discovered bases, namely cytosine and thymine, have been elusive in previous analyses, possibly because their structures are relatively more subtle and may have degraded by the time the sample was extracted.
In early experiments, scientists would often create some sort of "meteorite tea" in which they would put the meteorite particles into a hot bath, extract the molecules from the sample into solution, and then analyze the molecular makeup of these "extraterrestrial soups." Because the two bases, cytosine and thymine, are so delicate that scientists have not been able to find evidence of their existence. Even in this new experiment, the team was initially skeptical about seeing them in the sample.
However, in the end, two factors may have contributed to this new discovery. First, the team extracted these compounds using cold water, rather than high-temperature formic acid, in other words, it was more like a "cold drink" than a "hot tea." In addition, formic acid is highly reactive and has the potential to destroy fragile molecules in the sample. Second, the researchers employed a more sensitive analytical approach that allowed for a chance of being discovered even if the number of these molecules was very small.
More and more fragments
Although, this discovery cannot yet be considered to provide conclusive evidence to answer the question of whether the emergence of life on Earth was aided by the help of space or from the soup of primordial life on the nascent Earth. However, it complements the full set of bases that make up life today.
At present, in addition to the five bases related to life, scientists have also found many organic molecules in meteorite samples, including many precursors of life molecules. They're like more pieces in a puzzle, constantly bringing evidence and providing scientists trying to understand the origin of life with more compounds to experiment in the lab.
In addition to this, the discovery of bases also raises some interesting questions. Purines and pyrimidines can be synthesized in extraterrestrial environments, and it was expected that more diverse bases would be found in meteorites, but for now, they do not appear to exhibit structural diversity in carbon-grained meteorites like other organics such as amino acids and hydrocarbons.
The debate over the origin of life may be hard to put to rest, but more discoveries keep us seeing more possibilities.
#创作团队:
Written by: M Ka
Typography: Wenwen
#参考来源:
https://www.nasa.gov/feature/goddard/2022/life-blueprint-in-asteroids
https://www.sciencenews.org/article/all-of-the-bases-in-dna-and-rna-have-now-been-found-in-meteorites
https://www.nhm.ac.uk/discover/news/2022/april/meteorites-could-have-brought-dna-precursors-earth.html
#图片来源:
Cover image: NASA Goddard/CI Lab/Dan Gallagher
First image: NASA