Solving early life’s energy crisis (Image: Donald Fawcett/Getty)

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.

The basic principles are universal. Even aliens need mitochondria

“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.”

“It is very, very convincing, in my opinion,” says biologist John Allen of Queen Mary, University of London, on whose work Lane and Martin have drawn.

Growing costs

To become more complex, cells need more genes and more proteins – and so they need to get bigger. As the volume of any object increases, however, its relative surface area falls: an elephant has less surface area per unit of volume than a mouse, for instance. This is a major problem because simple cells generate the energy they need using the membrane that encloses them.

Lane and Martin calculate that if a bacterium grew to the size of a complex cell, it would run out of juice. It might have space for lots of genes, but it would have barely enough energy to make proteins from them.

Folds don’t help

In theory, there is an easy answer to the energy problem: create lots of folds in the cell membrane to increase its surface area, which in turn will increase the amount of energy the membrane can produce. Indeed, many bacteria have such folds. But this leads to another problem as they get larger.

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.

By increasing the number of mitochondria, cells could increase their membrane area without creating maintenance problems: each mitochondrion is a self-contained system with built-in control and repair mechanisms.

Birth of complexity

Once freed from energy restraints, genomes could expand dramatically and cells capable of complex functions – such as communicating with each other and having specialised jobs – could evolve. Complex life was born.

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.”

Journal reference: Nature, vol 467, p 929