Gretchen Vogel
Science
1999 January 8: 155-157.Debate on the origins of life has lately centered on a simple question: Was the cradle of life hot or cold? Many researchers argue that the first cells arose in the scalding waters of hot springs or geothermal vents, while a small but prominent band of holdouts insists on cool pools or even cold oceans. With no fossils to go by, the argument has circled a variety of indirect clues, with recent evidence favoring hotter environs. But now comes good news for the cold camp: Evidence from the genes of living organisms suggests that the cell that gave rise to all of today's life-forms was ill-suited for extremely hot conditions.
To probe the temperature preferences of early cells, Nicolas Galtier, now of Edinburgh University in Scotland, Nicolas Tourasse of the University of Texas, Houston, and Manolo Gouy of the University C. Bernard in Lyon, France, analyzed 40 living organisms for two genes that act as a sort of thermometer for an organism's ideal growing temperature. Their work suggests that in the ancestral cell, these genes could not have withstood temperatures above about 70ºC--a more moderate temperature than many have proposed. Although the evidence is indirect, other biologists say the work is a clever approach that will reinvigorate the debate about the conditions in which life began.
The notion that the last common ancestor of all life lived in very hot conditions has recently gained followers (Science, 2 May 1997, p. 700), in part because some of the organisms that populate the lowest, earliest branches of the tree of life live in extreme environments today--the so-called hyperthermophiles thrive between 80º and 90ºC. And most geologists believe the early Earth was racked by volcanoes and asteroid impacts, which create hot environments.
Galtier decided to test this theory by tracking the evolution of two temperature-sensitive RNA molecules in the cell's protein-making factory, the ribosome. The ribosome is in part made of RNA--which is itself composed of nucleotide bases--and so depends on the bonds between the bases to work properly. But those bonds are temperature sensitive: Some withstand high temperatures better than others. For example, the bases guanine (G) and cytosine (C) form a strong bond, while adenine (A) and uracil (U) form a weaker bond. Other studies have shown that the ribosomal RNA of heat-loving organisms has more G and C than A and U, presumably because the G-C bond holds up better in the heat.
Using the two ribosomal RNA molecules, Galtier's team constructed a phylogenetic tree for 40 living organisms ranging from bacteria to mammals. They then used a computer model to find the most likely proportion of G and C in the RNA molecules of the ancestor of all 40 organisms. To their surprise, the model concluded that the ancestral RNA for both molecules had only a moderate G+C content, well below that of all known hyperthermophiles and consistent with organisms that live at moderate temperatures.
To check their work, the team ran the model again with a different phylogenetic tree; the result was unchanged. To show that the model was not simply finding the average G+C content of all the organisms, they ran it again using only organisms with high G+C contents--and still found only a moderate G+C content.
Even so, it's difficult to extrapolate back billions of years, warns evolutionary biologist Norman Pace of the University of California, Berkeley, who has favored a hot origin for life. "Things get awfully murky back there," he says, calling the moderate G+C content "mud in already murky waters." And the last common ancestor of all living things must have lived some time after the very first stirrings of life.
But others welcome the result. "Statistical methods can be much more powerful than many people realize," says evolutionary biologist Ziheng Yang of University College London, who finds the analysis convincing, although he "would not take it as the last word" on the topic. Even Galtier agrees with that. But if he has his way, the evidence for a cooler ancestor will once again heat up the origins-of-life debate.