LIFE on Earth has two basic talents. It can replicate, or copy itself, and it can metabolise nutrients for energy. Scientists have long been divided over which came first. The latest contribution to the debate argues strongly that the honour should go to replication.
The rival metabolism-first camp argues that life could not have existed without the simple molecules that produce energy, so they must have come first. For instance, Günter Wächtershäuser, a chemist turned patent lawyer in Munich, Germany, has hypothesised that one of the first key molecules to emerge from the primordial soup was acetic acid. This molecule lies at the heart of the series of reactions known as the citric acid cycle, which forms the basis of metabolism in the living cells of modern organisms (Science, vol 276, p 245). A primitive metabolic system based on acetic acid could have been harnessed by other processes, eventually leading to replicating molecules with a genetic code.
Now Addy Pross of Ben-Gurion University of the Negev in Beer Sheva, Israel, is saying that the metabolism-first approach could never work. He argues that if primitive metabolic systems ever emerged spontaneously, the principles of thermodynamics would ensure that they would not last long. The second law of thermodynamics decrees that ordered things tend towards disorder unless energy is put in to keep them together. So a metabolising molecule, which is inherently fragile, would break up before it could evolve towards more complexity, or acquire the ability to copy itself (Origins of Life and Evolution of the Biosphere, vol 34, p 307).
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On the other hand, replicating molecules like DNA have been shown to be good at harnessing external energy sources, such as those present in hydrothermal vents, to overcome the thermodynamic trend towards disorder. Pross says that natural selection would favour any new, faster replicators that emerged because of mutations, and the replicator that resulted would usually be more complex than the original. Any replicators that acquired rudimentary metabolism in this way would be even more capable of overcoming the thermodynamic forces that favour disorder, he says.
Some computer models have shown that stable metabolic systems can exist without any genetic material for replication. But Pross says that there is no experimental evidence that such systems can form spontaneously from a soup of relevant molecules, let alone be stable for any length of time, whereas such evidence exists for replicating molecules like RNA.
This is a significant contribution to an ongoing debate, says Dieter Braun, an origin-of-life researcher at the Ludwig Maximilian University in Munich, Germany. “He opens the playground of the discussion quite nicely.”
Pross’s idea has profound philosophical implications. It tells us that the essence of life is replication, while everything else is subordinate. Metabolism is simply another adaptation to aid in replication, like a butterfly’s wings or a cheetah’s speed.