WERE little balls of fat the precursors for life on Earth? These chubby balls can reproduce and evolve all by themselves in computer simulations, and can even help form the long chains of RNA that are vital for life as we know it.
The most popular theory for how life originated is the 鈥淩NA world鈥�, which says self-replicating RNA molecules started the process off. But how did complex chains of RNA arise from simple organic molecules floating in the primordial soup?
One possibility is that nucleotides 鈥� building blocks of RNA 鈥� could stick to the surface of clay particles. This would concentrate the molecules, helping them join together. Others believe lipid membranes enclosed groups of simple molecules into tight compartments, allowing them to connect.
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But Doron Lancet, a biologist at the Weizmann Institute of Science in Israel, is proposing a radical alternative 鈥� the 鈥渓ipid world鈥�. He argues that balls of detergent-like molecules provide a simpler way to kick-start replication and evolution, as well as explaining how the first RNA molecules formed.
These balls, known as micelles, are well known to chemists. They consist of lipid molecules, which have a water-loving head attached to a water-hating tail. In solution, the hydrophilic heads form the surface of a ball, tucking their hydrophobic tails inside. This is all that was needed for the first steps towards life, says Lancet. 鈥淲e think a ball like this can grow and split and give rise to an almost identical copy.鈥�
While RNA molecules encode information in their sequence of bases, micelles carry it in the ratios of different types of heads in the ball, Lancet explains. What鈥檚 more, some of these heads behave like catalysts, converting other heads from one type into another. When Lancet simulated these balls in a computer model, complex webs of reactions took place inside them. Sometimes a micelle formed that was able to split into two new micelles with the same composition of molecules as the original 鈥� forming a self-replicating, slowly evolving ball.
At some point in the lipid world, RNA must have arrived. Lancet envisages micelles made of a mixture of molecules 鈥� some with catalytic heads and some with heads made of nucleotides. As the hydrophobic tails form a ball, the heads are forced into close proximity. The catalytic heads can then join the nucleotides into more complex molecules. Add in molecules with heads made of amino acids and the same process could make proteins, suggests Lancet. 鈥淪uch amino-acid-bearing lipids exist even today, so the idea is not so far-fetched.鈥�
Proponents of the RNA world are dubious. 鈥淎ll life is based on DNA,鈥� points out Enzo Gallori, an RNA expert at the University of Florence. 鈥淕enetic polymers are fundamental and I think they mark the beginning of life. It鈥檚 difficult to imagine another possibility.鈥�
The idea can鈥檛 be tested with today鈥檚 molecular tools, so for now the lipid world exists only on Lancet鈥檚 computer. But he hopes that in the future, scientists will recreate and analyse it inside a test tube.
