Did organic molecules, originating in extraterrestrial sources, contribute to the origin of life on Earth? Was life on our planet seeded from outer space? While the theory of panspermia – that the origin life on Earth came from outer space – is rejected, scientists are exploring how life’s building blocks can be found in meteorites, objects from the early solar system.
No, life did not exclusively originate in space, but meteorite impacts did contribute to the chemical compounds that led to the formation of nucleobases, the molecules responsible for DNA and RNA.
Why am I writing about this topic?
In 2020, I wrote an article regarding the Japanese Space Agency’s (JAXA) mission to collect samples from the asteroid Ryugu. The latter is an asteroid relatively close to Earth – given the distance involved in astronomical missions. Asteroid particles were collected from Ryugu by the Hayabusa2 spacecraft. Scientists have been examining these samples for evidence of an organic compounds.
Ryugu is a remnant from the early stages of the solar system’s formation. Roughly 900 metres across, it is older than the Earth, and provides a snapshot of the materials which were predominant in the early phases of the solar system. It was found to contain all five nucleobases necessary for the making of DNA and RNA molecules, the basic building blocks of life.
Such compounds were abundant in the early solar system. They form a crucial step in the transition from the nonliving world to life.
We all learned in high school biology about the building blocks of DNA – adenine, guanine, cytosine, thymine. These molecules, combined with phosphates and sugars, form the nucleotides that combine into now famous double-helix structure of DNA.
These nitrogen-containing organic molecules form the now famous letters of genetic information in DNA.
Uracil is necessary for the development of RNA. Without these nucleobases; adenine and guanine – known as purines – and cytosine, thymine and uracil – known as pyrimidines – life as we know it would be impossible.
This is the not first time that scientists have examined meteoritic and asteroid material for signs of organic compounds. The 1864 Orgueil meteorite, which slammed into southwestern France, contained various nucleobases of life. Meteorite explorations were still in their infancy at the time, but curiosity regarding the origins of life was taking on a scientific turn with the rise of geology.
The Murchison meteorite in Western Australia, which impacted in 1969, was also found to contain nucleobases. Bennu, a near-Earth asteroid has been studied by telescope for decades, but in 2018, a NASA spacecraft collected particles from it. Scientists have found the presence of nucleobases as well.
There were differences found in the relative abundances of the nucleobases when comparing the different meteoritic samples.
Ryugu contained roughly equal amounts of the purine nucleobases (adenine and guanine) and pyrimidine nucleobases (cytosine, thymine, and uracil). The Murchison meteorite mostly contained purines, while the Bennu and Orgueil samples mostly contained pyrimidines.
Bennu and Ryugu asteroids are a type of carbonaceous asteroids, the most common type in the solar system.
The question of abiogenesis, the origin of life, is separate from the subject area of evolutionary biology. Exploring the prebiotic chemistry of the early Earth and solar system is a complex subject, and scientists are working on the chemical pathways which led from inorganic matter to organic life.
The findings from the Ryugu asteroid do not conclusively prove that life was seeded from outer space, as science fiction fans would have us believe. However, the tantalising evidence that nucleobases formed in the early solar system opens up an exciting avenue of enquiry.
The distinction between the biological and geological realms is becoming blurry. Early in the 20th century, Soviet geologist and biochemist Vladimir Vernadsky (1863 – 1945) first suggested that life can be considered a geological force that shapes the Earth’s ecosystems.
He helped to popularise the concept of a biosphere, which consists of considering all living matter on Earth as an interconnected system – what we would today call the ecosystem. He looked for the origins of the animate world in inanimate matter.
Alexandr Oparin (1894 – 1980), a Soviet biologist and biochemist developed the theory of the chemical evolution of organic molecules from prebiotic sources. (I wrote about him in this article). He pioneered experiments which reproduced the prebiotic conditions of Earth, providing a chemical pathway for the emergence of organic compounds.
The Ryugu findings do not decisively prove that Vernadsky and Oparin were correct. It does bolster their findings that organic molecules can arise from non-animate worlds, such as that which prevailed in the early solar system. By piecing together the various lines of evidence, the mystery of life’s origins is beginning to be answered.

