Why complex life probably evolved only once

Michael Le Page - The universe may be teeming with simple cells like bacteria, but more complex life – including intelligent life – is probably very rare. That is the conclusion of a radical rethink of what it took for complex life to evolve here on Earth.

It suggests that complex alien life-forms could only evolve if an event that happened just once in Earth's history was repeated somewhere else.

All animals, plants and fungi evolved from one ancestor, the first ever complex, or "eukaryotic", cell. This common ancestor had itself evolved from simple bacteria, but it has long been a mystery why this seems to have happened only once: bacteria, after all, have been around for billions of years.

The answer, say Nick Lane of University College London and Bill Martin of the University of Dusseldorf in Germany, is that whenever simple cells start to become more complex, they run into problems generating enough energy.

"It required a kind of industrial revolution in terms of energy production," says Lane. "[Our hypothesis] overturns the traditional view that the jump to complex eukaryotic cells simply required the right kinds of mutations."  ...

Producing energy by "burning" food is playing with fire. If the energy-producing machinery straddling the membrane is not constantly fine-tuned, it produces highly reactive molecules that can destroy cells. Yet fine-tuning a larger membrane is problematic because detecting and fixing problems takes longer.

These obstacles were overcome when a cell engulfed some bacteria and started using them as power generators – the first mitochondria. ...

So if Lane and Martin are right, the textbook idea that complex cells evolved first and only later gained mitochondria is completely wrong: cells could not become complex until they acquired mitochondria.

Simple cells hardly ever engulf other cells, however – and therein lies the catch. Acquiring mitochondria, it seems, was a one-off event. This leads Lane and Martin to their most striking conclusion: simple cells on other planets might thrive for aeons without complex life ever arising. Or, as Lane puts it: "The underlying principles are universal. Even aliens need mitochondria."

via Why complex life probably evolved only once - life - 21 October 2010 - New Scientist.