Complex life evolved out of the chance coupling of small molecules

Complex life, as we know it, started completely by chance, with small strands of molecules linking up, which eventually would have given them the ability to replicate themselves.

A simple RNA molecule such as this may have been responsible for the evolution 
of complex life as we know it [Credit: Wits University]

In this world, billions of years ago, nothing existed that we would recognise today as living. The world contained only lifeless molecules that formed spontaneously through the natural chemical and physical processes on Earth.

However, the moment that small molecules connected and formed larger molecules with the ability to replicate themselves, life started to evolve.

“Life was a chance event, there is no doubt about that,” says Dr Pierre Durand from the Evolution of Complexity Laboratory in the Evolutionary Studies Institute at Wits University, who led a project to find out how exactly these molecules linked up with each other.

Very simple ribonucleic acid (RNA) molecules (compounds similar to Deoxyribonucleic acid (DNA)) can join other RNA molecules to themselves though a chemical reaction called ligation. The random joining together of different pieces or RNA could give rise to a group of molecules able to produce copies of themselves and so kick start the process of life.

While the process that eventually led to the evolution of life took place over a long period of time, and involved a number of steps, Wits PhD student Nisha Dhar and Durand have uncovered how one of these crucial steps may have occurred.

They have demonstrated how small non-living molecules may have given rise to larger molecules that were capable of reproducing themselves. This path to self-replicating molecules was a key event for life to take hold.

“Something needed to happen for these small molecules to interact and form longer, more complex molecules and that happened completely by chance,” says Durand.

These smaller RNA molecules possessed enzyme activity that allowed ligation, which, in turn allowed them to link up with other small molecules thereby forming larger molecules.

“The small molecules are very promiscuous and can join other pieces to themselves. What was interesting was that these smaller molecules were smaller than we had originally thought,” says Durand.

The smallest molecule that exhibited self-ligation activity was a 40-nucleotide RNA. It also demonstrated the greatest functional flexibility as it was more general in the kinds of substrates it ligated to itself although its catalytic efficiency was the lowest.

“Something needed to happen for molecules to reproduce, and thereby starting life as we know it. That something turned out to be the simple ligation of a set of small molecules, billions of years ago,” says Durand.

The study is published in the journal Royal Society Open Science.

Source: Wits University [September 20, 2017]

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Could interstellar ice provide the answer to birth of DNA?

Researchers at the University of York have shown that molecules brought to earth in meteorite strikes could potentially be converted into the building blocks of DNA.

The building blocks of DNA could have come from space [Credit: iStock]

They found that organic compounds, called amino nitriles, the molecular precursors to amino acids, were able to use molecules present in interstellar ice to trigger the formation of the backbone molecule, 2-deoxy-D-ribose, of DNA.

It has long been assumed that amino acids were present on earth before DNA, and may have been responsible for the formation of one of the building blocks of DNA, but this new research throws fresh doubt on this theory.

Meteor shower

Dr Paul Clarke, from the University of York's Department of Chemistry, said: "The origin of important biological molecules is one of the key fundamental questions in science. The molecules that form the building blocks of DNA had to come from somewhere; either they were present on Earth when it formed or they came from space, hitting earth in a meteor shower.

"Scientists had already shown that there were particular molecules present in space that came to Earth in an ice comet; this made our team at York think about investigating whether they could be used to make one of the building blocks of DNA. If this was possible, then it could mean that a building block of DNA was present before amino acids."

Before life began

In order for cellular life to emerge and then evolve on earth, the fundamental building blocks of life needed to be synthesised from appropriate starting materials -- a process sometimes described as 'chemical evolution'.

The research team showed that amino nitriles could have been the catalyst for bringing together the interstellar molecules, formaldehyde, acetaldehyde, glycolaldehyde, before life on Earth began. Combined, these molecules produce carbohydrates, including 2-deoxy-D-ribose, the building blocks of DNA.

DNA is one of the most important molecules in living systems, yet the origin 2-deoxy-D-ribose, before life on earth began, has remained a mystery.

'One-pot'

Dr Clarke said: "We have demonstrated that the interstellar building blocks formaldehyde, acetaldehyde and glycolaldehyde can be converted in 'one-pot' to biologically relevant carbohydrates -- the ingredients for life.

"This research therefore outlines a plausible mechanism by which molecules present in interstellar space, brought to earth by meteorite strikes, could potentially be converted into 2-deoxy-D-ribose, a molecule vital for all living systems."

The research is published in the journal Chemical Communications.

Source: University of York [September 14, 2017]

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