Tuesday, August 25, 2015
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Matt Ridley is the author of provocative books on evolution, genetics and society. His books have sold over a million copies, been translated into thirty languages, and have won several awards.

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The origin of life

Electrochemical echoes of life's membranes at alkaline vents

What better subject for the origin of a new year than the origin of life itself? A new paper claims to have nailed down at last the conditions, location and path by which life started, slicing through two Gordian knots.

Knot No. 1 is the chick-and-egg problem of energy. Living things burn energy at a furious rate to stay alive. Every time a bacterium divides, it uses up 50 times its own mass of energy-currency molecules (called ATP)-and that's with efficient and specialized modern protein machinery to do the job. When starting out, life would have been a far more wasteful process, needing more energy, yet would have had none of its modern machinery to harness or store energy.

Knot No. 2 is entropy. Life uses energy to make order out of chaos. So the putative location preferred by previous evolutionists-Alexander Oparin's primordial soup in Charles Darwin's "warm little pond" with a little lightning-is just too unconstrained: Life would just keep dissolving away before it got started.

Before the cell existed, life needed a controlled supply of concentrated energy in a confined space. Comparing gene sequences leads to the conclusion that at the very root of life's family tree lie the "chemi-osmotic" bacteria and archaea (single-celled creatures like bacteria). These are microbes that effectively charge their electrochemical batteries by converting carbon dioxide into methane or the organic compound acetate.

Where did they originate? In 2000 explorers found vents in the mid-Atlantic, the Lost City hydrothermal field, that are quite unlike the better known hot, acidic "black smoker" deep-sea vents: They last for much longer, are highly alkaline and modestly warm.

Now Nick Lane of University College London and William Martin of Heinrich Heine University in Düsseldorf, Germany, have concluded that this environment carries uncanny echoes of life's method of storing energy.

Cells store energy by pumping charged "ions" usually of sodium or hydrogen across membranes, effectively creating an electrical voltage. This is a peculiar but universal feature of life, whose ubiquity has never been explained. The scientists think it was originally borrowed from vents like those at Lost City.

Four billion years ago, at such a vent, hot alkaline fluid rich in hydrogen met acidic oceans saturated in carbon dioxide. There would have been natural proton gradients across the thin iron-nickel-sulfur walls of the vents, with a voltage very like the membrane potential of modern chemi-osmotic microbes. The authors write: "In our view-and given the near universality of proton gradients across life-this is no coincidence."

Today's microbes that live at these hydrothermal vents use proton gradients to add electrons to a protein called ferredoxin, which in turn converts carbon dioxide into organic molecules needed for growth. The vents' rocks are riddled with interconnected micro-pores, where these organic chemicals might once have accumulated, some of them further accelerating the reaction.

Drs. Lane and Martin have an ingenious suggestion for how life then became free-living. A geochemical proton gradient could freely drive a biochemical sodium-ion pump, in effect allowing a membrane to exclude sodium, forming another electrical energy storage mechanism-one that would have been much more reliable as the first leaky membranes began to seal off into cells. This would explain why many primitive energy proteins use both sodium and hydrogen (protons) indiscriminately, and why all living cells have less sodium in them than seawater.

In effect, the energy reactions that happen chemically at alkaline hydrothermal vents were borrowed and refined by living cells, which is why there are so many chemical similarities today.