Before humans, before dinosaurs, before even so much as a trilobite appeared, Earth is thought to have been covered in primordial ooze from which life somehow emerged.
What would eventually form the genes of every life-form on Earth was probably floating around on our pre-pre-prehistoric planet. Kind of like Alien Origin, the nucleosides and phosphates that would eventually become life could have crashed here via asteroids and comets that got in the way of the young and volatile planet as it went through growing pains, which were more like a volcanic rage. Nucleosides — adenine, cytosine, guanine, thymine, and uracil* — bond with phosphates to form nucleotides, and those nucleotides form the nucleic acids DNA and RNA.
Meteorites were recently found containing all these nucleotides. Where exactly they came from is unknown, but whatever their origin was, how could these nonliving organic molecules possibly have created life? This is the question chemists Luis Escobar and Felix Müller from Ludwig Maximilian University of Munich (LMU) sought to answer by recreating RNA-peptide particles (which can encode genetic information) in a lab to see how they would have behaved under the conditions of primitive Earth. They coauthored a study recently published in Nature.
“Evolution is based on the adaption of a system to the surrounding conditions,” Escobar and Müller told SYFY WIRE. “On early Earth, RNA and peptides could have existed under high PHs and temperatures. When conditions became milder, it enabled more evolutionary pathways.”
The LMU researchers are now changing how we think life emerged on this planet. If RNA and peptides could replicate, that might have pushed them to morph into the first microbial forms of life that would then continue evolving into more complex organisms until they finally became trilobites, and dinosaurs, and eventually primates like us. When Earth calmed down from its mood swings that ended up in volcanoes erupting (and other phenomena most living things wouldn’t want to deal with), precursors to life made of RNA and peptides evolved further.
Not all nucleosides have to be able to encode information to create viable RNA. Some can be found in messenger or transfer RNA. Messenger RNA (mRNA) is a single strand of RNA which is involved in synthesizing proteins, and transfer RNA (tRNA) is also necessary for protein synthesis. It carries amino acids to the ribosome, which translates the genetic code in those amino acids, which is how it becomes a link between mRNA molecules and amino acids. Modified non-coding nucleosides in mRNA and tRNA mostly keep the RNA in a cell stable and prevent it from degrading. They necessarily take away a nucleoside’s ability to code.
“The nucleoside modifications that we chose for our RNA-peptide world concept are present in, or at least adjacent to, the anticodon loop where the decoding process takes place,” Müller and Escobar said. “Our amino acid-nucleoside conjugates remained stable under harsh prebiotic conditions.”
Artificially producing RNA-peptide particles, which could code for genes just like they do in a living organism, meant studying the structure of both mRNA and tRNA in order to replicate it. The trio of nucleotides on mRNA are known as codons. The tRNA sequence that mirrors them is the anticodon, which determines which amino acid attaches itself to which hydroxyl group.
What surprised Müller, Escobar, and their team was that amino acids weren’t only where they were expected to be, on the ribose, a sugar that assists with DNA and RNA synthesis. Modified versions of them were unexpectedly found near the anticodon. The most challenging aspect of this for the researchers was chemically synthesizing modified molecules and then getting them into strands of RNA. Seeing that the mashups of amino acids and nucleosides survived gave the researchers what they needed to suggest that early RNA molecules were able to attach peptides to themselves, which ended up in both the RNA and peptides evolving together.
Escobar and Müller believe that this connection was what made it possible for life to emerge on Earth. While the early molecules did fade out of existence as Earth grew out of its adolescence, are still found in RNA helping it keep its structure intact.
“We thought that a very early stage of peptide synthesis could have occurred connected to the nucleobase,” they said. “These modifications are not only present in biological systems, but the are considered to be molecular fossils of an early Earth.”
*Thymine is the fourth nucleoside in DNA, while uracil takes its place in RNA.