Where did humans come from? Well Terra, of course. But where did Terra come from?
tRNA molecules for two amino acids, Phenylalanine and Aspartate. tRNA in modern cells adds amino acids to growing polypeptide (protein) chains.
One of the defining characteristics of Earth's biosphere that makes her unique is her genetic code, which is based on DNA. It's much of what makes Terra - well, Terra.
But what if Terra wasn't the first biosphere to exist on Earth? What if there was life with a different genetic code - say, RNA - that lived first, and whose child might have been Terra?
It's a speculative theory that scientists have been tossing around for quite a while now, along with others. The theory proposes that in the beginning, Earth life was not DNA-based but RNA-based, and DNA came from RNA via evolution. DNA eventually took over because it was so much better than RNA for storing information. RNA is like DNA, but with much more flexibility in its functional capabilities.
Here's the problem: Both RNA and DNA are complicated molecules to make, to say the least. This is because of the myriad interdependent interactions the two must have to help each other exist. And their replication abilities don't make things any easier. In the never-ending process to create life, scientists have been trying for decades now to create both out of the so-called "primordial soup" that would have been present on early Earth and contained biochemicals. Amino acids have been made, but RNA and DNA - which are both nucleic acids - have been quite hard to make as an interacting system. Until now.
Scientists at the University of Tokyo took a sample of one species of RNA molecule and in a rather lengthy experiment observed what they would do. To their surprise, the molecules diversified and cooperated to form a complex interacting system - albeit one much simpler than true organisms. The end result was five species working together to help each other replicate.
How the experiment was set up and run. Species concentrations were also mapped. From the article: a The RNA replication system. The original host RNA replicates via translation of the self-encoded replicase, by which mutant host RNAs and parasitic RNAs could be generated. b Schematic representation of long-term replication experiments in water-in-oil droplets. (1) RNA replication was performed at 37 °C for 5 h. (2) Droplets were 5-fold diluted with new droplets containing the translation system. (3) Droplets were vigorously mixed to induce their random fusion and division. c Concentration changes of host and parasitic RNAs of different lengths. Host RNA concentrations were measured by RT-qPCR, and parasitic RNA concentrations were measured from corresponding band intensities after gel electrophoresis. Parasitic RNA concentrations were not plotted in rounds where they were undetectable.
This new research is exciting because it provides evidence for how organisms were born from mere molecules via Darwinian evolution. The experiment also shows that Earth's first life-forms being RNA-based is a viable possibility.
In fact, the fact that the starting system was able to undergo Darwinian evolution is what makes this study unique, according to Ryo Mizuuchi, Project Assistant Professor of the University of Tokyo. "Honestly, we initially doubted that such diverse RNAs could evolve and coexist. In evolutionary biology, the 'competitive exclusion principle' states that more than one species cannot coexist if they are competing for the same resources. This means that the molecules must establish a way to use different resources one after another for sustained diversification. They are just molecules, so we wondered if it were possible for nonliving chemical species to spontaneously develop such innovation."
What's also interesting to note about the results of the experiment is that, despite Darwinian evolution, the molecules "learned" to cooperate. As Mizuuchi describes, cooperation isn't how evolution is supposed to work; competition is. In Darwinian evolution, if there's one resource and five of you, you do your best to wipe out your buddies so you can get to the resource before they do. While the study authors do note that competition may have been a factor in the evolution of the end replication system, the fact that there was cooperation at all despite a common resource warrants further research into the viability of cooperation as the main driving factor in evolution rather than competition.
There is evidence of "cooperative evolution" in nature as well, not just in the lab among molecules. For example, most species of trees and plants in many kinds of ecosystems form underground networks, partnering with fungi to exchange nutrients, resources, and even information to help each other survive. Plants will warn each other of an oncoming insect invasion, and trees will literally donate resources to young or struggling plants to help them grow and flourish. And on a more familiar level, cells will differentiate and work together to form highly complex systems - a.k.a. organisms, like you - to ensure everyone gets their fair share of resources. Cells will even kill themselves if it means everyone else gets to survive.
Of course, we see competition in nature as well - e.g. parasites, predation, and more. But all of these observations and evidence beg the question: Are competition and death really how life is supposed to live and develop, or are we missing something? Perhaps Terra's more like us as organisms than we thought she was.
Original article: Mizuuchi, R., Furubayashi, T. & Ichihashi, N. Evolutionary transition from a single RNA replicator to a multiple replicator network. Nat Commun13, 1460 (2022). https://doi.org/10.1038/s41467-022-29113-x.