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By Root 720 0

it allows organisms to change to take advantage of new opportunities and learn from failures—evolution is self-correction in action. Just as the buildup of oxygen in the air allowed animal life to appear, so the accumulation of any waste is an opportunity for new species to evolve to take advantage of it. Evolution is very different from mere adaptability, because it allows new life-forms to appear rather than old ones to adapt, leading to much greater transformations. A species may, for example, be able to adapt to a shift in its food supply by moving, but over many millennia an entirely new species may thereby come into being, able to exploit a whole new niche in the ecosystem. Think of polar bears, likely descended from an isolated population of brown bears in an ice age, but which evolved white fur and an ice-based lifestyle to become the preeminent hunter of the far north.

All this sounds comforting. The Earth, and life, will always prevail. But the self-regulating system contains a flaw, one that can seriously damage or even destroy it. This flaw is the gap in time between a perturbation and the ensuing correction: Instabilities can happen very fast, while the correcting process of self-regulation typically takes much longer. The gap between the advent of an oxygen-rich atmosphere and the appearance of animal life was a long one: a good hundred million years if not more. Major volcanic eruptions may release trillions of tonnes of carbon dioxide over just a few thousand years, outstripping the capacity of the Earth system to mop up the additional CO2 via rock weathering and other processes of sequestration, and leading to extreme global warming events. Mass extinctions happen because changing circumstances outstrip the adaptability of existing species before evolution can work its magic. Over millions of years new species can appear, but only from the diminished gene pool of the survivors—and a return to true pre-extinction levels of biodiversity may take much longer, if it ever takes place at all.

This time-lag effect was cleverly demonstrated in a modeling simulation undertaken by two British researchers, Hywell Williams and Tim Lenton (both at the University of East Anglia; Lenton is a member of the planetary boundaries expert group).2 In a computer-generated world—entirely populated by evolving microorganisms living in a closed flask—Williams and Lenton found that the closing of nutrient loops emerged as a robust property of the system nearly every time the model was run. As in the real world, the emergence of self-regulation came about because evolution allowed new species to appear that could use the waste of one species as food for themselves, recycling nutrients and leading to a stable state. Moreover, the more species that evolved, the greater the amount of recycling and the greater the overall biomass the system could support. “Flask world” had discovered the value of biodiversity.

But this world also had a dark side, for several simulations illustrated that the flaw in self-regulation—the time gap between a disturbance and the evolved correction—might occasionally be fatal. In just a few model runs, an organism appeared that was so spectacularly successful in mopping up nutrients that its numbers exploded and its wastes built up to toxic levels before other organisms were able to evolve a response. Williams and Lenton dubbed these occasional rogue species “rebel organisms.” They were unusual, but their impact was invariably catastrophic: the explosive initial success of the rebels changed the simulated global environment so suddenly and dramatically that their compatriots were killed, and—with no other life-forms around to recycle their wastes—they were themselves condemned to die too. As the last lonely rebels perished, their whole biosphere went extinct, evolution ceased, self-regulation failed, and life wiped itself out.

Like Lovelock’s Gaia, Flask world—and its rebel organisms—might just be a clever idea, more of a metaphor than a true representation of reality. But the parallels with our species