Later On

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Have scientists found where life on earth originated?

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The theory of evolution solves many problems in biology, but it leaves unanswered (so far) how life originated. We may be getting very close to that. Nick Lane in New Scientist:

Peter Mitchell was an eccentric figure. For much of his career he worked in his own lab in a restored manor house in Cornwall in the UK, his research funded in part by a herd of dairy cows. His ideas about the most basic process of life – how it gets energy – seemed ridiculous to his fellow biologists.

"I remember thinking to myself that I would bet anything that [it] didn’t work that way," biochemist Leslie Orgel wrote of his meeting with Mitchell half a century ago. "Not since Darwin and Wallace has biology come up with an idea as counter-intuitive as those of, say, Einstein, Heisenberg and Schrödinger."

Over the following decades, however, it became clear that Mitchell was right. His vindication was complete when he won a Nobel prize in 1978. Even today, though, most biologists have yet to grasp the full implications of his revolutionary ideas – especially for the origin of life.

"Mitchell’s ideas were about how cells are organised in space, and cellular energy generation is a feature of that," says geochemist Mike Russell of NASA’s Jet Propulsion Laboratory in Pasadena, California. "The problem is that most ideas on the origin of life lack both spatial organisation and a supply of energy to drive replication or growth."

A few researchers, including Russell, have been rethinking the origin of life in the light of Mitchell’s ideas. They think the most counter-intuitive trait of life is one of the best clues to its origin. As a result, they have come up with a radically different picture of what the earliest life was like and where it evolved. It’s a picture for which there is growing evidence.

Before Mitchell, everyone assumed that cells got their energy using straightforward chemistry. The universal energy currency of life is a molecule called ATP. Split it and energy is released. ATP powers most of the energy-demanding processes in cells, from building proteins to making muscles move. ATP, in turn, was thought to be generated from food by a series of standard chemical reactions. Mitchell thought otherwise. Life, he argued, is powered not by the kind of chemistry that goes on in a test tube but by a kind of electricity.

The energy from food, he said, is used to pump positively charged hydrogen ions, or protons, through a membrane. As protons accumulate on one side, an electrochemical gradient builds up across the membrane. Given the chance, the protons will flow back across, releasing energy that can be harnessed to assemble ATP molecules. In energy terms, the process is analogous to filling a raised tank with buckets of water, then using the water to drive a waterwheel.

Mitchell dubbed his theory chemiosmosis, and it is not surprising that biologists found it hard to accept. Why would life generate energy in such a complicated and roundabout way, when simple chemical reactions would suffice? It just didn’t make sense.

It might be counter-intuitive, but chemiosmosis has turned out to be ubiquitous in the living world. Proton power drives not only cell respiration, but photosynthesis too: energy from the sun is converted into a proton gradient in essentially the same way as the energy of food.

And proton gradients are often harnessed directly, rather than being used to make ATP. They drive the rotation of the bacterial flagellum, as well as the active transport of numerous substances in and out of cells. So proton power is central to energy generation, movement and maintaining the internal environment – some of the most basic features of life.

This suggests that proton power is no late innovation but evolved early in the history of life, an idea supported by the tree of life. The first branch in the tree is between the two great groups of simple cells, bacteria and archaea. Both of these groups have proton pumps and both generate ATP from proton currents, using a similar protein. The obvious explanation is that both inherited this machinery from a common ancestor – the progenitor of all life on Earth.

Think about the properties of that common ancestor, however, says Bill Martin of the University of Düsseldorf in Germany, and you come up with a very strange beast indeed…

Continue reading.

Written by LeisureGuy

19 October 2009 at 3:45 pm

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