Nutritious but not vital

The textbook view of life on Earth arising from a fermenting primordial soup of organic molecules may have to be rewritten. In a paper recently published in the journal BioEssays, Nick Lane, John Allen and William Martin argue that it was chemical energy from hydrothermal vents on the ocean floor which gave birth to early biology.

“We provide a new perspective on why that old and familiar view won’t work at all,” says Lane, a biochemist at University College London. “We present the alternative that life arose from gases [such as hydrogen, carbon dioxide, nitrogen and hydrogen sulphide], and that the energy for first life came from harnessing geochemical gradients created by mother Earth at a special kind of deep-sea hydrothermal vent – one that is riddled with tiny interconnected compartments or pores.”

It was the eminent evolutionary biologist JBS Haldane who in 1929 proposed the organic soup concept, and this quickly caught on in the public imagination. In Haldane’s theory, ultraviolet light from the Sun provided the energy required to convert methane, ammonia and water into primitive organic compounds in the oceans. The problem, however, is the lack of a mechanism in this environment to sustain the reaction between these chemical elements.

Lane and his colleagues turned to geochemistry to identify the missing energy source, and in particular they looked at geochemical gradients across a honeycomb of microscopic natural caverns in deep-sea hydrothermal vents. These catalytic cells generate lipids, proteins and nucleotides, and, say the scientists, it was these that gave rise to the first biological cells.

The researchers focused on the work of Michael Russell on alkaline deep sea vents, which produce chemical gradients similar to those used by virtually all living organisms today.

Based on the new thinking, early organisms are thought to have exploited chemical gradients through a process known as chemiosmosis, in which a difference in proton numbers across a membrane is used to drive synthesis of the universal biological energy carrier Adenosine Triphosphate (ATP), or its precursor molecules. Primitive cells then evolved to generate their own chemical gradients by way of electron transfer from a donor to an acceptor molecule. Lane and his colleagues argue that the first donor was hydrogen, and the first acceptor carbon dioxide.

Lane says that thermodynamic constraints necessarily lead to chemiosmosis being the basis for energy metabolism in organisms that arise from simple chemical ingredients (a process known as autotrophy), and by implication this includes the first free-living cells. These earliest cells may have harnessed an inorganic geochemical force and used this to make their own.

“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,” says Martin.

Fermentation may have provided a convincing explanation of the origin of life owing to its primitive simplicity, but Lane says it is now impossible to see how life could have begun without chemiosmosis.

“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.”

Soup may be nutritious, but it is not necessarily vital.

Further reading

Lane et al. “How did LUCA make a living? Chemiosmosis in the origin of life,” BioEssays (2010)