At the root of the tree of life is a hypothetical organism from which all life on Earth today is descended. This 'last universal common ancestor', or LUCA, as it is known, arose at an unknown point in time, probably in the oceans more than 3.5 billion years ago. Fossils from its distant era are few and far between, so Manolo Gouy, a molecular phylogeneticist at the University of Lyon in France, and his colleagues set about finding a different means of tracing LUCA.

They analysed sequences of ribosomal RNA and proteins from a variety of modern species using mathematical models of molecular evolution. Their work shows that LUCA was a mesophile — that is, grew best at temperatures approaching 50 °C — and its descendants then adapted to higher temperatures as Earth's environment changed. Not only do the results clear up a dispute about LUCA's temperature preference, but the group's approach could also be used to learn more about this intriguing species.

Because of the scarcity of fossils from LUCA's time, researchers have instead relied on indirect evidence, delving into the genomes of modern species, to uncover LUCA's features. To this end, about 10 years ago, Gouy's group studied ribosomal RNA, a part of the cell's protein-making machinery thought to have changed little over time. Thermophilic, or heat-loving, organisms carry ribosomal RNA rich in pairs of the nucleic acids guanine and cytosine. These have stronger bonds than do adenosine and uracil pairs, so are more stable at higher temperatures.

On the basis of their reconstruction of ancestral ribosomal-RNA sequences, Gouy and his colleagues posited that LUCA lived in cooler waters. “It was a surprise because there was a hypothesis that the origin of life would have occurred in a hot environment,” says Gouy. In fact, another group of reseachers, who had reconstructed ancestral protein sequences, found just that — LUCA dwelled in a hot or thermophilic environment.

By analysing both protein and RNA sequences with new mathematical tools, Gouy and his team now show that there were two phases of environmental adaptation: first, a mesophilic LUCA, and second, its descendants, which adapted to higher temperatures (see page 942).

“We think we have demonstrated why there was a disagreement,” says Gouy. “There was a difference in the method used to reconstruct the ancestral sequences. Some assume that the evolutionary process is constant, but that's an over-simplification.” He attributes the leap forwards to computing power that didn't exist a decade ago, knowledge of many more genomic sequences and collaboration. “We can run models on much more data and our models can be much more complicated than they used to be,” says Gouy. “And I was lucky enough to get a brilliant graduate student, Bastien Boussau, who was able to work out these new models.”

As a LUCA-hunter, Gouy is after an elusive quarry. Many basic questions about LUCA remain, perhaps most importantly the date when it arose. “Any date would be unreliable, and it's a big problem,” Gouy says. But he believes that he can apply his new method to learn more about LUCA's atmospheric environment, match it to the geological record for atmospheric oxygen concentration, and pinpoint a more reliable date. “If one really wants to understand the history of the evolution of life, one would really like to anchor it in time,” he says.