RNA Formation on Early Earth Investigated
New research suggests that ribonucleic acid (RNA), a molecule critical for protein synthesis, may have readily formed on early Earth approximately 4.3 billion years ago. This finding implies that RNA could be common in the universe.
Understanding RNA
RNA, or ribonucleic acid, is a molecular cousin to DNA, which carries genetic information. RNA exists in three primary forms:
- Messenger RNA (mRNA): Carries genetic instructions from DNA for protein synthesis.
- Ribosomal RNA (rRNA): Forms ribosomes, essential for protein production.
- Transfer RNA (tRNA): Facilitates the actual synthesis of proteins from mRNA.
Due to its relative simplicity compared to DNA and its dual capacity to carry genetic information and catalyze reactions, RNA is hypothesized to have been central to the origin of life on Earth, a concept known as the "RNA world" hypothesis. In this scenario, early single-celled life forms would have utilized RNA for self-replication.
Challenges in RNA Formation Research
Understanding the conditions under which RNA's precursor molecules assembled and reacted has been a long-standing challenge. Chemists investigate pathways that could lead to RNA formation, such as the six-step Discontinuous Synthesis Model (DSM).
Previous assumptions held that borates, which are common oxyanions found in seawater and contain boron and oxygen, might hinder specific reactions within the chemical pathway to RNA.
New Experimental Findings
A team of biochemists led by Yuta Hirakawa from Tohoku University in Japan and the Foundation for Applied Molecular Evolution in Florida has presented findings that revise the understanding of borates' role. Their experiments involved combining RNA ingredients—the five-carbon sugar ribose, phosphates, and the four RNA nucleobases (adenine, guanine, cytosine, and uracil)—with borates and basalt.
The mixture was heated and subsequently dried, simulating conditions believed to have been present around underground aquifers on early Earth. The experiments resulted in the formation of RNA. Contrary to prior beliefs, borates were found to support steps in the DSM model by stabilizing ribose molecules, which are often unstable, and by facilitating phosphate production.
Corroborating Evidence and Hypotheses
These findings align with recent discoveries from NASA's OSIRIS-REx mission, which returned samples from asteroid Bennu. The detection of ribose in the Bennu sample means that all the necessary ingredients for RNA have now been identified in extraterrestrial material.
Hirakawa's team hypothesizes that the impact of a large protoplanet, approximately 500 kilometers in diameter and containing RNA's building blocks, could have delivered these materials to Earth. They estimate this event and the subsequent RNA production occurred 4.3 billion years ago.
While RNA has been produced in laboratories through directed chemical reactions, the researchers suggest their method represents the first instance of RNA production in a laboratory setting without direct human chemical intervention. Critics, however, argue that assembling the constituent building blocks still constitutes a form of human intervention.
Implications for Astrobiology
Early Mars experienced similar asteroid impacts, and borates have been detected on the planet. These conditions suggest that RNA could also have been produced on Mars. If RNA formed rapidly on Earth, geologically speaking, it may have provided an accelerated pathway for the emergence of early simple organisms on our planet.