Now we have the best picture yet of what that ancestor was like and where it lived, thanks to a study that identified 355 genes that it probably possessed.

“It was flabbergasting to us that we found as many as we did,” says William Martin of the University of Dusseldorf in Germany, who led the study. The findings support the idea that the last universal common ancestor (LUCA) lurked in hydrothermal vents where hot water rich in hydrogen, carbon dioxide and minerals emerged from the sea floor.

“It’s spot on with regard to the hydrothermal vent theory,” Martin says. He describes LUCA as half-living, because it may have depended on abiotic reactions in the vents to produce many of the chemicals it needed.

LUCA emerged around 3.8 billion years ago and gave rise to two kinds of simple cells: bacteria and archaea (see diagram, below). By looking for genes common to almost all cells living today, previous studies have identified around 100 genes almost certainly present in LUCA.

This tells us what LUCA had in common with modern cells, but what we really want to know is how it was different, Martin says. So his group analysed the genomes of 1800 bacteria and 130 archaea to find the genes that were the most ancient but not necessarily shared.

The 355 they found include some universal genes, such as a few involved in reading the genetic code. But others point to a very distinctive lifestyle.

One characteristic of almost all living cells is that they pump ions across a membrane to generate an electrochemical gradient, then use that gradient to make the energy-rich molecule ATP. Martin’s results suggest LUCA could not generate such a gradient, but could harness an existing one to make ATP.

That fits in beautifully with the idea that the first life got its energy from the natural gradient between vent water and seawater, and so was bound to these vents. Only later did the ability to generate gradients evolve, allowing life to break away from the vents on at least two occasions – one giving rise to the first archaea, the other to bacteria.

Revolving door

LUCA also appears to have had a gene for a “revolving door” protein that could swap sodium and hydrogen ions across this gradient. Earlier studies by Martin and Nick Lane of University College London suggest that such a protein would have been absolutely crucial for exploiting the natural gradient at vents.

One thing Martin didn’t find is genes involved in making amino acids, the building blocks of proteins. LUCA may have depended on amino acids produced spontaneously at vents, he says.

Peter Gogarten of the University of Connecticut in Storrs, who studies the evolution of early life, thinks Martin’s approach is sound. “Most of the identified genes are good candidates for having been present in LUCA,” he says.

But it’s hard to tell apart genes that are truly ancient and those that merely appear ancient because bacteria and archaea have swapped them. Martin’s team disregarded these swapped genes, and could in the process have omitted some genes that LUCA did possess, perhaps including those for amino-acid synthesis.

There are many competing ideas for how life first arose, but the hydrothermal vent theory was the leading contender even before the new findings, because it provides a detailed scenario that explains many of life’s key features.

But however plausible it appears, it will never be possible to prove that it is right, Martin says.

Source: New Scientist