The debate over what creature is Earth's most ancestral life form blows hot and cold. For the past 15 years or so, origin-of-life researchers have favored a group of bacteria--the hyperthermophiles--that flourish around geothermal vents and hot springs at temperatures between 80° to 90°C as our very earliest ancestor. Now a team of researchers has reanalyzed the bacterial family tree and found that the honor should really go to a bacterium that likes much milder conditions.
Over the past decade, there has been ever more evidence that heat-loving bacteria weren't first on the scene (Science, 8 January 1999, p. 220). And the new findings, reported in the 16 May issue of Nature, show that the organisms at the very base of the tree of life are Planctomycetales, a cold-loving division of bacteria whose members share several peculiar characteristics, in particular a single or double membrane surrounding the chromosome.
Céline Brochier and Hervé Philippe of the Université Pierre et Marie Curie in Paris took ribosomal RNA (rRNA) sequence data from a modern day Planctomycetales and ran it through a program designed to calculate evolutionary relationships. Because every cellular organism has rRNA, it is considered a powerful tool for investigating the universal tree of life. The novel aspect of their analysis is that they focused on the parts of the rRNA molecule that mutate very slowly and so are likely to reveal ancient relationships.
"Our analysis is part of the accumulating data that life did not start at high temperatures," says Philippe. The results suggest that the hyperthermophiles evolved later and may have acquired their love of heat from the Archaea--a group that includes extremists that thrive at high pressures and temperatures of up to 115°C.
The intellectual history of the Planctomycetales is intriguing, says microbiologist Martin Embley of the Natural History Museum in London, because they were put forward as the earliest bacteria nearly 20 years ago, but were later bumped in favor of the thermophiles. The new study swings the pendulum back the other way, Embley says: "This is another piece of evidence that there are problems with the classic thermophile RNA tree."