Well, perhaps your great-to-the-hundred-millionth-grandmother was.
Understanding the origins of life and the mechanics of the earliest beginnings of life is as important for the quest to unravel the Earth’s biological history as it is for the quest to seek out other life in the universe. We’re pretty confident that single-celled organisms – bacteria and archaea – were the first ‘creatures’ to slither around on this planet, but what happened before that is a matter of intense and often controversial debate.
One possibility for a precursor to these organisms was a world without DNA, but with the bare bone molecular pieces that would eventually result in the evolutionary move to DNA and its associated machinery. This idea was put forward by an influential paper in the journal Nature in 1986 by Walter Gilbert (winner of a Nobel in Chemistry), who fleshed out an idea by Carl Woese – who had earlier identified the Archaea as a distinct branch of life. This ancient biomolecular system was called the RNA-world, since it consists of ribonucleic acid sequences (RNA) but lacks the permanent storage mechanisms of deoxyribonucleic acids (DNA).
A key part of the RNA-world hypothesis is that in addition to carrying reproducible information in their sequences, RNA molecules can also perform the duties of enzymes in catalyzing reactions – sustaining a busy, self-replicating, evolving ecosystem. In this picture RNA evolves away until eventually items like proteins come onto the scene, at which point things can really gear up towards more complex and familiar life. It’s an appealing picture for the stepping-stones to life as we know it.
In modern organisms a very complex molecular structure called the ribosome is the critical machine that reads the information in a piece of messenger-RNA (that has spawned off the original DNA) and then assembles proteins according to this blueprint by snatching amino acids out of a cell’s environment and putting them together. Ribosomes are amazing, they’re also composed of a mix of large numbers of RNA molecules and protein molecules.
But there’s a possible catch to all this, and it relates to the idea of a protein-free RNA-world some 4 billion years ago. In a new, and provocative study by Harish and Caetano-Anollés in the open-access PLos One, an argument is made for the co-evolution of RNA and proteins as a more plausible way for something as intricate as the ribosome to arise. In the regular RNA-world hypothesis this is not how it happens, RNA is effectively exclusive until eventually proteins come along. However, the authors of this new work applied the tools of phylogenetics – the measurement of evolutionary distance or age between key molecular sequences and structures – to ‘date’ different parts of modern ribosomes. Intriguingly both the RNA and protein pieces of the ribosome seem to follow a similar span of evolution, suggesting that this fabulously complicated molecule was gradually aggregated in a world made of both proteins and RNA and that the proteins were already being made via some other mechanism involving RNA. Once ribosomes developed, protein-manufacture could really take off in new ways, but this may not have been their origin.
It’s certainly a fascinating idea, a ‘ribonucleoprotein-world’ hypothesis, although time will tell if it becomes accepted by evolutionary molecular biologists. It does also hint at the notion of a ribosome being one of the first results of what we might recognize as a symbiotic relationship. So, your grandmother may well have been a molecule, a ribosome in fact.