So where exactly did the building blocks for life (DNA and its chemical precursors) come from? This is a question that has both puzzled and fascinated scientists for decades, and a couple of possible answers have been offered.
One theory asserts that life emerged from a primordial soup of chemical elements that existed on Earth billions of years ago. A second alternative looks to the heavens, claiming that those building blocks were deposited here by comets, asteroids, and meteorites that smashed into the planet at some time in the far distant past.
For quite a while the primordial soup theory, in its various versions, has been the most commonly accepted hypothesis. But some strong evidence in favor of the extraterrestrial seeding theory has just recently been discovered, by a team of space scientists and biochemists from Japan and NASA who re-examined pieces of three meteorites that crashed on earth in 1950, 1969, and 2000.
During this new study , which was just published in the journal Nature Communications , the scientists discovered traces of chemical compounds known as cytosine and thymine, which are essential elements in the formation of RNA and DNA. These chemicals were not detected in the meteorite remnants in past studies, nor have they ever before been found in any other meteorite or comet samples recovered from the earth’s surface. All the other chemical precursors of DNA had been found, which means cytosine and thymine represent the last two pieces of the life-generating puzzle.
This research does not prove that life on earth evolved from chemicals carried to the planet’s surface by meteorites or other space objects. But the presence of DNA precursors in meteorite samples is consistent with that possibility, and that’s why this new discovery represents a meaningful leap forward in the search for the truth about how life got started on Earth.
The latest study revealed that all the building blocks of life are to be found in meteorites. Specifically, all the necessary chemical precursors of DNA and RNA have been found in meteorites. Shown here is the structure of the DNA double helix. The atoms in the structure are color-coded by element and the detailed structures of two base pairs are shown in the bottom right. (Zephyris / CC BY-SA 3.0 )
Finding the Last Building Blocks of Life In Meteorites
The genetic coding found in DNA , and put into application by its RNA messengers, is essential for the development of life as we know it.
For DNA and RNA to form in the first place, however, two fundamental chemical building blocks (nucleobases) must be available. These are the purines (guanine and adenine) and the pyrimidines (cytosine, uracil, and thymine). Without those chemicals, DNA and RNA would have no pathway for coming into existence.
Previous studies of meteorite remnants have revealed the presence of purines, and uracil. These discoveries are what prompted scientists to speculate that meteors or other space bodies may have been the original DNA and RNA chemical factories, rather than any type of primordial soup. If enough meteorites hit the earth billions of years ago, when such collisions were more common, they might have delivered sufficient quantities of DNA precursor chemicals to make that next step possible.
The one problem was that the two remaining chemicals needed to support DNA synthesis, cytosine and thymine, had never been found inside any meteorite—which was actually quite surprising.
“The lack of pyrimidine diversity in meteorites remains a mystery, since prebiotic chemical models and laboratory experiments have predicted that these compounds can also be produced from chemical precursors found in meteorites,” the scientific team from Japan and NASA noted in their Nature Communications article.
In their search for the missing pieces, the scientists, who were led by Associate Professor Yasuhiro Oba from Japan’s Hokkaido University, performed a new chemical analysis of the rocky remains of three well-known meteorites. These were the Murray, Murchison, and Tagish Lake meteorites, which crashed in Kentucky in 1950, Australia in 1969, and British Columbia in 2000 respectively.
Earlier testing showed these rocky visitors from outer space were carrying the compounds guanine, adenine, and uracil, just like so many other meteorites. Using a new technique that is designed to detect miniscule chemical traces more easily, the scientists were able to confirm that they also carried cytosine and thymine, at concentration levels of a few parts per billion.
“These compounds are present at concentrations similar to those predicted by experiments replicating conditions which existed prior to the formation of the solar system,” the researchers wrote in their Nature Communications article.
Even though the meteorites studied only hit the earth recently, their chemical composition would have been the same as similar objects that crashed into the planet millions or billions of years ago.
“We now have evidence that the complete set of nucleobases used in life today could have been available on Earth when life emerged,” Danny Glavin, a study co-author who works at the Goddard Space Flight Center, declared in a NASA press release .
The research team believes these nucleobases would have been generated by photochemical reactions of a type that could only occur in an interstellar environment. The chemicals would have been free-floating at first, before being absorbed into the bodies of asteroids (the sources of almost all meteorites) as the solar system was in formation.
The 60-tonne, 2.7 meter-long (8.9-foot-long) Hoba meteorite in Namibia is the largest known intact meteorite and it likely also contains all the building blocks of life. (Calips assumed / CC BY-SA 3.0 )
Did Life on Earth Really Begin in Space? Perhaps
Previous NASA research has suggested that the emergence of life on earth billions of years ago was problematic, since water and carbon-based molecules were seemingly quite rare on the planet at that time.
Conversely, ancient asteroids, meteorites, and comets would have contained large quantities of both water and carbon-based molecules. If enough of them hit the earth, they could have seeded the planet with everything it needed to support the eventually synthesis of DNA and RNA, and ultimately the development of life itself.
But this does not prove that this is what actually happened. There is a difference between a viable theory and a proven theory, and as of now the extraterrestrial seeding hypothesis is very much in the former category.
Top image: In a recent breakthrough study, scientists have proven that meteorites carry all the building blocks of life, which is to say every chemical necessary for the formation of DNA and RNA. This photo is of the world famous Perseid Meteor Shower that occurs annually in Utah, US. Source: UtahHome
By Nathan Falde