LIFE could not have arisen from an 鈥渁ncestral commune鈥� in which rudimentary life forms, not yet sophisticated enough to survive independently, pooled their metabolic talents. That is the conclusion reached by a Canadian mathematician who has analysed the interactions among such a loose-knit group of protocells.
If he is right, it undermines a leading modern view on the origin of life. In recent years, geneticists have traced the lineage of more than 30 genes all the way back to the base of the tree of life, and have found that no single origin fits all genes. This puzzling result led Carl Woese, an evolutionary biologist at the University of Illinois at Urbana-Champaign, to hypothesise that modern life arose not from a single first cell, but from several, each carrying some 鈥� but not all 鈥� of the genes needed to grow and replicate (快猫短视频, 22 June 2002, p 10).
鈥淭here never was a universal common ancestor,鈥� says Woese. The members of this ancestral commune would all have benefited from genes and gene products leaking out of their neighbours until eventually they evolved into self-sufficient cells.
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But Peter Antonelli, a mathematician at the University of Alberta in Edmonton, Canada, has concluded that this loose-knit cooperative could not have held together. Using an arcane mathematical technique previously used by Japanese engineers to study electric motors, Antonelli wrote equations describing both the ecological interactions, such as competition for nutrients, and biochemical sharing that would have had to exist between protocells. When his colleague Solange Rutz analysed the equations, she found they were mathematically unstable, that is, the associations could not persist over time (Nonlinear Analysis: Real World Applications, vol 4, p 743).
鈥淩emember the communes back in the sixties?鈥� says Antonelli. 鈥淭hey didn鈥檛 work, and it鈥檚 the same reason. It鈥檚 a loose conglomeration of individuals trying to make a stable community.鈥� For the cooperative to work, he explains, all its members must contribute their share of the collective metabolism at the proper rate. And the more members there are, the harder it becomes to get such precise timing right. 鈥淵ou can鈥檛 just integrate a whole lot of things all at once. It only works two at a time, and even then it鈥檚 hard as hell,鈥� he says.
If there was any sharing of genes and enzymes, he says it must have been between pairs of protocells, much as the ancestor of eukaryotic or nucleated cells assimilated a bacterium that evolved into the modern mitochondrion, though that happened much later in the evolution of life. This kind of close, pairwise association can indeed be stable, Antonelli and his colleagues found.
Reaction to Antonelli鈥檚 claim has been muted. Few biologists pack the mathematical tools necessary to follow the details of his argument. Stuart Kauffman, a theoretical biologist and expert on the origin of life at the University of New Mexico in Albuquerque, says it is intuitively plausible that the bigger the 鈥渃ommune鈥�, the harder it is to coordinate production rates. But he believes that such coordination could happen if the members were truly interdependent, since the whole commune would then grow at the rate of the slowest producer.
Woese points out that many modern bacterial communities contain associations of several interdependent species. This suggests there may be ways around the instability Antonelli finds in his equations.