The 'primordial soup' theory--which posits that early life began in a soup of organic molecules before evolving out of the oceans millions of years later--is fatally flawed, according to researchers writing in BioEssays. Instead the authors claim it was the Earth's chemical energy, from hydrothermal vents on the ocean floor, which kick-started early life.

In rejecting the soup theory the team turned to the Earth's chemistry to identify the energy source which could power the first primitive predecessors of living organisms: geochemical gradients across a honeycomb of microscopic natural caverns at hydrothermal vents. These
catalytic cells generated lipids, proteins and nucleotides giving rise to the first true cells.

The team focused on ideas pioneered by geochemist Michael J. Russell, on alkaline deep sea vents, which produce chemical gradients very  similar to those used by almost all living organisms today - a gradient of protons over a membrane. Early organisms likely exploited these
gradients through a process called chemiosmosis, in which the proton gradient is used to drive synthesis of the universal energy currency, ATP, or simpler equivalents. Later on cells evolved to generate their own proton gradient by way of electron transfer from a donor to an acceptor. The team argue that the first donor was hydrogen and the first acceptor was CO2.

"Modern living cells have inherited the same size of proton gradient, and, crucially, the same orientation – positive outside and negative inside – as the inorganic vesicles from which they arose" said co-author John Allen, a biochemist at Queen Mary, University of London.

"Thermodynamic constraints mean that chemiosmosis is strictly necessary for carbon and energy metabolism in all organisms that grow from simple chemical ingredients [autotrophy] today, and presumably the first free-living cells," said lead author Nick Lane. "Here we consider how the earliest cells might have harnessed a geochemically created force and then learned to make their own."

This was a vital transition, as chemiosmosis is the only mechanism by which organisms could escape from the vents. "The reason that all organisms are chemiosmotic today is simply that they inherited it from the very time and place that the first cells evolved – and they could not have evolved without it," said Martin.

"Far from being too complex to have powered early life, it is nearly impossible to see how life could have begun without chemiosmosis", concluded Lane. "It is time to cast off the shackles of fermentation in some primordial soup as 'life without oxygen' – an idea that dates back to a time before anybody in biology had any understanding of how ATP is made"

The soup theory was proposed in 1929 when J.B.S Haldane published his influential essay on the origin of life in which he argued that UV radiation provided the energy to convert methane, ammonia and water into the first organic compounds in the oceans of the early earth. However critics of the soup theory point out that there is no sustained driving force to make anything react; and without an energy source, life as we know it can't exist.




Citation: Nick Lane, John F. Allen, William Martin, 'How did LUCA make a living? Chemiosmosis in the origin of life', BioEssays, January 2010; doi: 1002/bies.200900131