Life may have had a very cold start

Exactly how life on Earth began more than 3.5 billion years ago remains one of 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 mineralrich 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. This scenario appears puzzling because the chain-like molecules that carry hereditary information (DNA and RNA) from generation to generation prefer environments that are not too hot or 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.

Now an idea that combines two other likely ingredients of genesis has been put forward by Dr. Philip Holliger of the MRC Laboratory of Molecular Biology in Cambridge.

One ingredient came from the realization that though all life relies on two types of nucleic acid — deoxyribonucleic acid, DNA, and ribonucleic acid, RNA — the latter, though delicate, is a better candidate for the first living things because it is more versatile than DNA, being an information carrier and able to catalyze chemical reactions. This flexibility has led many to believe that there was an “RNA world” before the current DNA world.

The second ingredient comes from the idea that a key factor in 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 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 enzyme capable of making a similar-sized strand, and that this worked better at sub-zero temperatures.

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.

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. The 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.

Even more excitingly, this is part of a broader move to extend the possibilities for life. Holliger’s team has been using “artificial cells” to breed not just RNA and DNA through synthetic evolution, but other xenonucleic acids or XNAs, not seen in nature. He concludes that living information can be passed on by chemicals other than DNA and RNA.

Meanwhile the genomics wizard Craig Venter describes in his new book, Life at the Speed of Light, how his U.S. institutes are attempting to use a cocktail of enzymes, ribosomes, lipids and other molecules, including a synthetic DNA genome to create new life forms without the need for pre-existing cells, creating life from the bottom up.

Taken together, these new flavours of genetic information may lead to a surge in the possibilities for living things, not just in the imaginations of scientists, but across the cosmos.