The family tree of life may have just grown another branch. Red algae, mostly tropical seaweeds, look different from green algae and plants, but biologists believed that the appearance was misleading. Similarities in all three groups' chloroplasts--the tiny cellular factories where photosynthesis takes place--suggested a common chloroplast-containing ancestor. Now two evolutionary biologists have shaken up this family tree. They say their analysis of a fundamental gene in the nucleus places red algae on a separate branch from the one that gave rise to green algae, plants, and the nonphotosynthetic animals and fungi.
The study, reported in the current issue of the Proceedings of the National Academy of Sciences, has implications beyond the genealogy of red algae. Biology textbooks now teach that chloroplasts descended from a formerly free-living bacterium that was nearly eaten but instead took up permanent residence as part of the cell. Because most scientists believe that such a complicated event, called endosymbiosis, could have happened very few times, two groups of organisms with similar chloroplasts should have a common ancestry. But new evidence, taken from the gene for RNA polymerase by John Stiller and Benjamin Hall of the University of Washington, Seattle, suggests that's not the case.
The scientists propose three explanations for the apparent conflict between the RNA and chloroplast data. An ancestor of red algae and an ancestor of green algae might have both engulfed similar bacteria in separate events. Alternatively, an ancestor common to all higher organisms could have had a chloroplast, but many cells--including the precursor of animals--later lost it. Or perhaps the ancestor of plants and green algae engulfed an already photosynthetic red algae to gain their chloroplast--a process called secondary endosymbiosis. No matter which scenario is correct, says Hall, the history of endosymbiosis does not look as simple as many researchers had thought. "It seems like there were just organisms jumping on each other all the time," he says.
Reactions to the findings are mixed. Mitch Sogin, an evolutionary biologist at the Marine Biological Laboratory in Woods Hole, Massachusetts, says the new analysis is "very important" evidence that strengthens the case for repeated endosymbiosis. But molecular evolutionist Jeff Palmer of Indiana University is skeptical: "With more [genetic] sequences, sometimes trees settle down and change shape." He says he expects to see the red algae return to a branch common to photosynthetic organisms.