Energy revolution key to complex life

Oct 20, 2010

The evolution of complex life is strictly dependent on mitochondria, the tiny power stations found in all complex cells, according to a new study by Dr Nick Lane, from UCL (University College London), and Dr William Martin, from the University of Dusseldorf.

"The underlying principles are universal. Energy is vital, even in the realm of evolutionary inventions," said Dr Lane, UCL Department of Genetics, Evolution and Environment. "Even aliens will need mitochondria."

For 70 years scientists have reasoned that evolution of was the key to complex life. Now, in work published today in Nature, Lane and Martin reveal that in fact mitochondria were fundamental to the development of complex innovations like the nucleus because of their function as power stations in the cell.

"This overturns the traditional view that the jump to complex 'eukaryotic' cells simply required the right kinds of mutations. It actually required a kind of industrial revolution in terms of energy production," explained Dr Lane.

At the level of our cells, humans have far more in common with mushrooms, magnolias and marigolds than we do with bacteria. The reason is that complex cells like those of plants, animals and have specialized compartments including an information centre, the nucleus, and power stations – mitochondria. These compartmentalised cells are called 'eukaryotic', and they all share a common ancestor that arose just once in four billion years of evolution.

Scientists now know that this common ancestor, 'the first eukaryote', was a lot more sophisticated than any known bacterium. It had thousands more and proteins than any bacterium, despite sharing other features, like the genetic code. But what enabled eukaryotes to accumulate all these extra genes and proteins? And why don't bacteria bother?

By focusing on the energy available per gene, Lane and Martin showed that an average eukaryotic cell can support an astonishing 200,000 times more genes than bacteria.

"This gives eukaryotes the genetic raw material that enables them to accumulate new genes, big gene families and regulatory systems on a scale that is totally unaffordable to bacteria," said Dr Lane. "It's the basis of complexity, even if it's not always used."

"Bacteria are at the bottom of a deep chasm in the energy landscape, and they never found a way out," explained Dr Martin. "Mitochondria give eukaryotes four or five orders of magnitude more energy per gene, and that enabled them to tunnel straight through the walls of the chasm."

The authors went on to address a second question: why can't bacteria just compartmentalise themselves to gain all the advantages of having mitochondria? They often made a start but never got very far.

The answer lies in the tiny mitochondrial genome. These genes are needed for cell respiration, and without them die. If cells get bigger and more energetic, they need more copies of these mitochondrial genes to stay alive.

Bacteria face exactly the same problem. They can deal with it by making thousands of copies of their entire genome – as many as 600,000 copies in the case of giant bacterial cells like Epulopiscium, an extreme case that lives only in the unusual guts of surgeonfish. But all this DNA has a big energetic cost that cripples even giant bacteria – stopping them from turning into more complex eukaryotes. "The only way out", said Dr Lane, "is if one cell somehow gets inside another one – an endosymbiosis."

Cells compete among themselves. When living inside other cells they tend to cut corners, relying on their host cell wherever possible. Over evolutionary time, they lose unnecessary genes and become streamlined, ultimately leaving them with a tiny fraction of the genes they started out with: only the ones they really need.

The key to complexity is that these few remaining genes weigh almost nothing. Calculate the energy needed to support a normal bacterial genome in thousands of copies and the cost is prohibitive. Do it for the tiny mitochondrial genome and the cost is easily affordable, as shown in the Nature paper. The difference is the amount of DNA that could be supported in the nucleus, not as repetitive copies of the same old genes, but as the raw material for new evolution.

"If evolution works like a tinkerer, evolution with mitochondria works like a corps of engineers," said Dr Martin.

The trouble is that, while cells within cells are common in eukaryotes, which often engulf other cells, they're vanishingly rare in more rigid bacteria. And that, Lane and Martin conclude, may well explain why complex life – eukaryotes – only evolved once in all of Earth's history.

Explore further: DNA may have had humble beginnings as nutrient carrier

More information: 'The energetics of genome complexity' is published in the 21 October issue of Nature. www.nature.com/nature/journal/… ull/nature09486.html

Related Stories

An answer to another of life's big questions

Feb 05, 2010

(PhysOrg.com) -- Monash University biochemists have found a critical piece in the evolutionary puzzle that explains how life on Earth evolved millions of centuries ago.

Evolution still scientifically stable

Sep 14, 2009

An international team of researchers, including Monash University biochemists, has discovered evidence at the molecular level in support of one of the key tenets of Darwin's theory of evolution.

Research breakthrough on the question of life expectancy

Aug 05, 2010

Why do we grow old and what can we do to stop it? This is the question asked by many, but it appears that we are now closer to an answer thanks to new research published by Monash University researcher Dr ...

Membrane-coat proteins: Bacteria have them too

Jan 20, 2010

Although they are present almost everywhere, on land and sea, a group of related bacteria in the superphylum Planctomycetes-Verrucomicrobia-Chlamydiae, or PVC, have remained in relative obscurity ever since ...

Analysis knocks down theory on origin of cell structure

Apr 22, 2009

(PhysOrg.com) -- Understanding how living cells originated and evolved into their present forms remains a fundamental research area in biology, one boosted in recent years by the introduction of new tools ...

Recommended for you

DNA may have had humble beginnings as nutrient carrier

18 hours ago

New research intriguingly suggests that DNA, the genetic information carrier for humans and other complex life, might have had a rather humbler origin. In some microbes, a study shows, DNA pulls double duty ...

Central biobank for drug research

18 hours ago

For the development of new drugs it is crucial to work with stem cells, as these allow scientists to study the effects of new active pharmaceutical ingredients. But it has always been difficult to derive ...

User comments : 0