Exactly how life on Earth began more than 3.5 billion years ago remains the most intriguing, poorly understood and hotly contested areas of biology. In 1871, Charles Darwin speculated that it may have begun in a “warm little pond”. Others have claimed the ultimate birthplace lay in mineral-rich waters spewing from boiling hot springs.
Now a team has suggested a surprising alternative: that life had icy origins, part of a wider trend of radical new thinking about the possibilities for life and how it emerged from the non-living world (an event known as abiogenesis).
This frigid scenario appears puzzling because the chain-like molecules that carry hereditary information (DNA and RNA) from generation to generation prefer, like Goldilocks, environments that are not too hot or not too cold. These molecules, known as nucleic acids, are larger than individual atoms but still small enough to be knocked about by collisions with the surrounding sea of molecules.
The degree of pounding depends on temperature: too low and there’s not enough motion to shake molecules apart and bash them together to make beautiful chemistry; too high and all chemical structures are smashed to smithereens.
Now an idea that combines two other likely ingredients of genesis has been put forward by Philip Holliger of the MRC Laboratory of Molecular Biology in Cambridge.
One ingredient came from the realisation that though all life relies on two types of nucleic acid – deoxyribonucleic acid, DNA, and ribonucleic acid, RNA. DNA carried the genetic code down through the generations, while RNA acts as a “messenger” molecule governing, among other things, the construction of proteins in the cell.
And RNA, albeit delicate, is a better candidate for the first living things, because it is more versatile than DNA, being both an information carrier and able to catalyse chemical reactions.
This flexibility has led many to believe that there was an “RNA world” before the current DNA world (though the RNA world lives on, since recent research shows that RNA plays a far more important role than a mere message-carrier).
The second ingredient of genesis comes from the idea that a key factor in the origin of evolution was the emergence of the cell membrane, one that could create an environment to nurture the precious RNA machinery of life.
These ingredients have been linked by Dr Holliger to overcome a problem with the RNA world: no known RNA enzyme can copy a stretch of RNA as long as itself. After screening RNA sequences for the ability to copy other RNA, his team has found an RNA enzyme capable of making a similar-sized strand, and that this worked better at sub-zero temperatures.
The reason is that refrigeration provides the second key ingredient of life: as a salty solution of delicate RNA freezes, ice crystals form to leave briny pockets of concentrated RNA. Now if it stayed frozen, that would be that. But if it thaws and freezes again, so that replicating RNA can pass in and out of these icy cradles, they can compete for ingredients and adapt to a chilly environment. That gets evolution up and running.
The Cambridge team is still not quite there yet: the RNA strands are not in themselves enzymes, because the long chains of nucleic acid do not fold up the correct way – but it is only a matter of time.
More extraordinary, this work is part of a broader move to extend the possibilities for life. Holliger and his team has been using what he calls “artificial cells” to breed not just RNA and DNA through synthetic evolution, but other so-called xenonucleic acids or XNAs, not seen in nature. He concludes that living information can be passed on by chemicals other than DNA and RNA.
His team has found at least six unnatural nucleic acids capable of sharing information. One of these XNAs, a molecule referred to as hexitol nucleic acid or HNA was capable of evolution and folding into biologically useful forms. Another, called threose nucleic acid or TNA, can bind with RNA to copy information from the pre-RNA world to the RNA world. Thus XNAs could have acted as chemical stepping-stones to life.
Meanwhile the American genomics wizard Craig Venter describes in his new book, Life at the Speed of Light, how teams based at his institutes in La Jolla, California, and Rockville, Maryland, are attempting to use a cocktail of enzymes, ribosomes, lipids and other molecules, including a synthetic DNA genome to create new cells and life forms without the need for pre-existing cells, creating life from the bottom up.
Taken together, these new flavours of genetic information and life without pre-existing cells may lead to a dramatic surge in the possibilities for living things, not just in the imaginations of terrestrial scientists, but across the furthest reaches of the cosmos.
Roger Highfield is the director of external affairs, Science Museum.