Scientists have been working for generations to untangle the mysteries of how life began on Earth, and one previously fringe theory just gained a lot of ground.
The ‘RNA World’ theory says that the so-called primordial soup of the early Earth was teeming with DNA’s single-stranded sister RNA, which carries the instructions for sustaining life.
Now, a team of researchers at The Salk Institute have unlocked a crucial piece of that puzzle and even built it in the lab: an obscure but essential class of molecules called RNA polymerase ribozymes.
RNA polymerase ribozymes are not well understood, but scientists now suspect that these substances made it possible for RNA to not just replicate but actually evolve in the gel and muck of the early planet.
These scatterplots show how, across multiple rounds of evolution, new RNA polymerase ribozymes emerged. A lower-quality version (top) drifted away from their original sequence and tended toward randomness, while a higher-quality version (bottom) retained their original function and evolved new sequences
Even though little is known about RNA polymerase ribozymes, scientists know that they can copy strands of RNA.
In laboratory experiments, scientists showed that’s not all they can do.
Apparently they can evolve, as well.
Their experiments showed that RNA polymerase ribozymes can not only copy RNA but they can get better at doing it.
READ MORE: What is the ‘RNA World’ theory?
Life on Earth may have started thanks to a modified version of modern-day DNA’s sister molecule that was integral to evolution
RNA is structurally similar to DNA, except one of the four fundamental pieces – thymine – is substituted for uracil. This changes the shape and structure of the molecule and scientists have long believed this chemical to be the cause of Earth’s first lifeforms.
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Over time, they observed, the RNA polymerase ribozymes constructed copies of themselves could also copy RNA.
And the copies were not only functional, but in fact they got better at copying RNA – the same kind of evolution described by Charles Darwin.
As Darwin described evolution, the ‘fitter’ an organism is, the more likely it is to reproduce and pass on its genetic material.
He did not know about DNA, but DNA is how genes are passed on.
The new experiment suggests that a type of evolution may have been happening in the RNA World scenario.
As certain RNA polymerase ribozymes got better at copying RNA, they became more likely to stick around and keep performing their role.
Lower-quality ones, on the other hand, copied the RNA less reliably and their sequences drifted away, becoming more random over multiple courses of evolution.
There may not have even been DNA to pass on, but evolution was happening, the authors of the new study said.
‘We’re chasing the dawn of evolution,’ said senior study author and Salk President Gerald Joyce in a statement.
‘By revealing these novel capabilities of RNA, we’re uncovering the potential origins of life itself, and how simple molecules could have paved the way for the complexity and diversity of life we see today.’
The study was published in the journal Proceedings of the National Academy of Sciences.
Eventually, RNA came together to form the more familiar double-stranded structure of DNA, giving rise to single-cell organisms and then more complex forms of life.
These are all just theories, since we can never go back to the beginning of the planet and confirm how it all began.
But building RNA polymerase ribozymes in the lab and watching them help RNA evolve adds weight to the RNA World theory because it shows that this was theoretically possible.
Early Earth may have contained the ingredients for life long before anything lived on our planet. Scientists now have evidence that these materials could have even evolved on their own
RNA is like the single-stranded version of DNA, and it contains the bits of genetic material that cells use to communicate.
Proteins, the basic materials of living things, are constructed using the instructions contained in RNA.
It’s an unsung hero of life. It’s always working, and we would die without it, but DNA tends to get the credit.
‘We’ve long wondered how simple life was at its beginning and when it gained the ability to start improving itself,’ said first study author Nikolaos Papastavrou, a research associate in Joyce’s lab.
‘This study suggests the dawn of evolution could have been very early and very simple. Something at the level of individual molecules could sustain Darwinian evolution, and that might have been the spark that allowed life to become more complex, going from molecules to cells to multicellular organisms.’
Some scientists claim life was delivered on asteroids from the stars, while others say the building blocks for life came from space and assembled into lifeforms on Earth. An older idea argues that the basic molecules of life assembled themselves here on Earth from non-organic compounds.
Without RNA, most functions of the body would not be possible. RNA carries instructions on how to build proteins, which underlie almost everything cells do.
Long before animals and single-cell organisms existed, the early Earth may have been home to strands of genetic material – and more importantly, the chemicals that can make them replicate.
Joyce and Papastavrou’s team’s work will need to be replicated by other researchers, but for now it represents a mark in favor of one theory for how life came to be.
The team said they hope they can even discover how new RNA functions are invented through the same process of evolution.
