Mystery that is life

Some 3.9 billion years ago, a shift in the orbit of the sun’s outer planets sent a surge of large comets and asteroids careening into the inner solar system. Their impacts gouged out the large craters visible on the moon’s face, heated Earth’s surface into molten rock and boiled off its oceans into a mist.

Yet rocks that formed on Earth 3.8 billion years ago, almost as soon as the bombardment had stopped, contain possible evidence of biological processes. If life can arise from inorganic matter so quickly and easily, why is it not abundant in the solar system and beyond? If biology is an inherent property of matter, why have chemists so far been unable to reconstruct life, or anything close to it, in the laboratory?

Which came first, the proteins of living cells or the genetic information that makes them?

How could the metabolism of living things get started without an enclosing membrane to keep the necessary chemicals together? But if life started inside a cell membrane, how did the nutrients get in?

Scientists like Francis Crick, the chief theorist of molecular biology, have quietly suggested that life may have formed elsewhere before seeding the planet, so hard does it seem to find a plausible explanation for its emergence on Earth. In the last few years, however, four advances have renewed confidence that a terrestrial explanation for life’s origins will emerge.

Cell-like structures from chemicals

One is a series of discoveries about the cell-like structures that could have formed naturally from fatty chemicals likely to have been present on primitive Earth. Researchers Jack W Szostak, David P Bartel and P Luigi Luisi, published a manifesto in Nature in 2001, declaring that the way to make a synthetic cell was to get a protocell and a genetic molecule to grow and divide in parallel, with the molecules being encapsulated in the cell. If the molecules gave the cell a survival advantage over other cells, the outcome would be “a sustainable, autonomously replicating system, capable of Darwinian evolution,” they wrote.

One of the authors, Szostak, of the Massachusetts General Hospital, has since managed to achieve a surprising amount of this programme.

Simple fatty acids, of the sort likely to have been around on the primitive Earth, will spontaneously form double-layered spheres, much like the double-layered membrane of today’s living cells. These protocells will incorporate new fatty acids fed into the water, and eventually divide.

Living cells are impermeable and have elaborate mechanisms for admitting only the nutrients they need. But Szostak and his colleagues have shown that small molecules can easily enter the protocells. If they combine into larger molecules, however, they cannot get out: just the arrangement a primitive cell would need. If a protocell is made to encapsulate a short piece of DNA and is fed with nucleotides, the building blocks of DNA, the nucleotides will spontaneously enter the cell and link into another DNA molecule.

Szostak’s experiments have come close to creating a spontaneously dividing cell from chemicals assumed to have existed on the primitive Earth. But some of his ingredients, like the nucleotide building blocks of nucleic acids, are quite complex. Nucleotides consist of a sugar molecule, like ribose or deoxyribose, joined to a base at one end and a phosphate group at the other. Prebiotic chemists discovered with delight that bases like adenine will easily form from simple chemicals like hydrogen cyanide. Last month, John Sutherland, a chemist at the University of Manchester,  reported in Nature his discovery of an unexpected route for synthesising nucleotides from prebiotic chemicals. Instead of making the base and sugar separately from chemicals likely to have existed on the primitive Earth, Sutherland showed how under the right conditions the base and sugar could be built up as a single unit, and so did not need to be linked.

RNA molecules that replicate

Gerald F Joyce, an expert on the origins of life at the Scripps Research Institute in La Jolla, Calif, has developed RNA molecules that can replicate. Besides carrying information, RNA can also act as an enzyme to promote chemical reactions. Joyce reported in Science earlier this year that he had developed two RNA molecules that can promote each other’s synthesis from the four kinds of RNA nucleotides.

Another advance has come from studies of the handedness of molecules. Some chemicals, like the amino acids of which proteins are made, exist in two mirror-image forms, much like the left and right hand. In naturally occurring conditions they are found in roughly equal mixtures of the two forms. But in a living cell, all amino acids are left-handed, and all sugars and nucleotides are right-handed.

Left-handed nucleotides are a poison because they prevent right-handed nucleotides linking up in a chain to form nucleic acids like RNA or DNA. Researchers like Donna Blackmond of Imperial College London have discovered that a mixture of left-handed and right-handed molecules can be converted to just one form by cycles of freezing and melting.

With these four recent advances, those who study the origin of life have much to be pleased about. However there is little agreement on the kind of environment in which life originated.