New theory for origin of life Mineral cells might have incubated first living things. 4 December 2002 JOHN WHITFIELD
Iron sulphide honeycomb: a fertile environment.
Life on Earth may have begun in rocks on the ocean floor. More than 4 billion years ago, tiny cavities in minerals may have served as the first cells, two biologists are proposing1. Other researchers argue that the idea leaves many questions unanswered.
The key to the new theory is iron sulphide. Hot springs deposit a honeycomb of this mineral on the ocean floor, with pockets a few hundredths of a millimetre across. This would have been the ideal place for life to get going, say William Martin, of Heinrich-Heine University in Dusseldorf, Germany, and Michael Russell of the Scottish Universities Environmental Research Centre in Glasgow, UK.
"There are lots of theories [of the origin of life] but ours is the first to start with the cell," says Martin. Most scientists assume that self-replicating molecules or proteins came first.
Iron sulphide speeds up the reactions that join inorganic molecules together into organic ones - some bacteria still use it. The hot water flowing out of springs and into the honeycomb's holes is rich in the raw ingredients for these reactions, such as ammonia and carbon monoxide. Bacteria can use such compounds as fuel.
Confined to iron sulphide cells, simple compounds could become concentrated, sparking the creation of complex molecules such as proteins and genetic material. Outside the cells "they just diffuse off into the ocean", says Martin. He hopes to recreate such conditions in the lab.
"I think it's a beautiful thing - it's important to have all-embracing theories," says evolutionary biologist Ford Doolittle of Dalhousie University, Halifax, Canada. We'll never have much definite information on the origin of life, he says. "But then, just because we'll never know why the Roman Empire fell doesn't mean it isn't worth talking about."
Other scientists agree that iron sulphide honeycombs are fertile environments, but point to the gap between simple organic reactions and the chemistry of life. "The main problem is to explain how, where and why molecules such as enzymes emerge," explains biochemist Pier Luigi Luisi of the Federal Institute of Technology in Zurich, Switzerland. "If one can't explain that it's just blackboard work."
Life escaped the rocks when it evolved a cell wall, say Martin and Russell. Controversially, they argue that the two main kingdoms of primitive life, the bacteria and the archaebacteria, have such different cell walls that they must have arisen twice.
Others disagree. "It's quite impossible that it could be right," says evolutionary biologist Thomas Cavalier-Smith of the University of Oxford, UK. Bacteria and archaebacteria have got hundreds of genes in common, he says. They share other features, such as the way that they insert proteins into their membranes.
It's quite impossible that it could be right: Thomas Cavalier-Smith University of Oxford
Martin counters that the tendency of unrelated bacteria to swap DNA makes genetics a bad guide to history. Each kingdom left its rocky nursery about 3.8 billion years ago, he suggests. The oldest non-controversial bacterial fossils are about 2.5 billion years old, although some believe there is evidence for life 3.5 billion years ago.
It may be that no theory is going to fit all the evidence. The trick is to pick which bits to ignore, says John Raven of the University of Dundee, UK. "To create a coherent hypothesis we have to say 'this bit of data doesn't fit, but we're going ahead anyway'."
References Martin, W. & Russell, M. On the origins of cells: a hypothesis for the evolutionary transitions from abiotic geochemistry to chemoautotrophic prokaryotes,and from prokaryotes to nucleated cells. Philosophical Transactions of the Royal Society B, published online, doi:10.1098/rstb.2002.1183 (2002).