Origin-of-life puzzle cracked

1. Robert F. Service

The origin of life is a set of paradoxes. To get it started, there must have been a genetic molecule—something like DNA or RNA—capable of passing along blueprints for making proteins, the workhorse molecules of life. But modern cells cannot copy DNA and RNA without the help of proteins themselves. Worse, none of these molecules can do their jobs without fatty lipids, which provide the cell membranes needed to contain them. In yet another chicken-and-egg complication, protein-based enzymes (encoded by genetic molecules) are needed to synthesize lipids.

Now, researchers say they see a way out. A pair of simple compounds, which would have been abundant on early Earth, can give rise to a network of simple reactions able to produce all three classes of biomolecules—nucleic acids, amino acids, and lipids. The new work, published online this week in Nature Chemistry, does not prove that this is how life started, but it may help explain a key mystery. “This is a very important paper,” says Jack Szostak, a molecular biologist and origin-of-life researcher at Massachusetts General Hospital in Boston. “It proposes [how] almost all of the essential building blocks for life could be assembled in one geological setting.”

Scientists have long touted their own favorite scenarios for which set of biomolecules formed first. “RNA World” proponents suggest RNA may have been the pioneer; not only does it carry genetic information, but it can also serve as a proteinlike chemical catalyst. Metabolism-first proponents argue that simple metal catalysts, found in minerals, may have created a soup of organic building blocks that could have given rise to the other biomolecules.

The RNA World hypothesis got a big boost in 2009. Chemists led by John Sutherland at the University of Cambridge in the United Kingdom reported that two relatively simple precursor compounds, acetylene and formaldehyde, could undergo a sequence of reactions to produce two of RNA's four nucleotide building blocks, showing a plausible route by which RNA could have formed on its own in the primordial soup. Critics, though, pointed out that acetylene and formaldehyde are still somewhat complex molecules themselves. That raised the question of where they came from.

So Sutherland and his colleagues set out to see if they could find a route to RNA from even simpler starting materials. They succeeded. Sutherland's team now reports that it created nucleic acid precursors starting with just hydrogen cyanide (HCN), hydrogen sulfide (H2S), and ultraviolet (UV) light. What is more, Sutherland says, the same conditions also create the starting materials for amino acids and lipids.

His team argues that early Earth was a favorable setting for those reactions. HCN is abundant in comets, which pelted the newborn planet. Impacts would also have produced enough energy to synthesize HCN from hydrogen, carbon, and nitrogen. Likewise, Sutherland says, H2S was thought to have been common on early Earth, as was the UV radiation that could drive the reactions and metal-containing minerals that could have catalyzed them.

Sutherland cautions that the reactions for the three sets of building blocks are different enough from one another—requiring different metal catalysts, for example—that they likely would not have all occurred in the same location. Instead, slight variations in chemistry and energy could have favored the creation of one set of building blocks or another in different places on land. “Rainwater would then wash these compounds into a common pool,” says Dave Deamer, an origin-of-life researcher at the University of California, Santa Cruz.

Could life have kindled in that common pool? That detail is almost certainly lost to history. But the idea and the “plausible chemistry” behind it are worth careful thought, Deamer says. Szostak agrees. “I am sure that it will be debated for some time to come.”

Quelle: http://www.sciencemag.org/content/347/6228/1298.full