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An Earlier Debut for a Famous Alga

17 February 2009 (All day)
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Matthew Herron

Youthful appearance? New research shows Volvox split from unicellular ancestors about 150 million years earlier than was previously thought.

Life takes cooperation. That's true in politics, on the playground--and for evolution, because switching from a “me” to “we” mindset helped cells evolve to more complex organisms. New research shows that cells in a green alga called Volvox may have learned to cooperate much earlier than thought, shifting the evolutionary time frame for this model organism back hundreds of millions of years--a somewhat controversial finding.

Volvox is a perennial favorite among biologists, who study the alga to learn how multicellular organisms evolved. In Volvox, some cells coalesced to form a flagella and propel the organism, whereas others are devoted to reproduction. A study in 1989 suggested the cells specialized some 50 million to 75 million years ago. This was more than a billion years after other multicellular organisms arose, so Volvox has given researchers a relatively recent example to study.

The 1989 study analyzed just one gene and one set of fossils to give Volvox a birth date. Matthew Herron, an evolutionary biologist at the University of Arizona, Tucson, says he always suspected that the time frame was wrong. Herron and his colleagues scanned the genomes of about 45 species of green algae to see how the position of certain genes might have shifted as the organisms grew more complex. The basic idea behind this “molecular clock” technique is that the distance between the same genes on two different organisms represents the time it took for the second organism to evolve from the first. This enhanced genetic analysis, and a review of fossils, showed that green algae evolved from a unicellular organism at least 200 million years ago, the researchers report this week in the Proceedings of the National Academy of Sciences.

But some evolutionary biologists have criticized the method for giving estimates that are too rough. Herron’s retort: His team included multiple species of algae and several fossils to correct for error. The 200-million-year date is conservative, he says.

Nicholas Butterfield, a paleobiologist at the University of Cambridge in the United Kingdom, acknowledges that the researchers did attempt to account for error. “But being earnest doesn't mean you have the right answer,” he says.

Still, theoretical biologist David Penny of Massey University in New Zealand agrees that Herron's team used state-of-the-art estimation methods. And because evolutionary changes can move “sideways and backwards and occasionally forward,” he says it's important to understand when the specific genetic adaptations occurred that allowed unicellular organisms to combine, specialize, and give rise to true multicellularity.

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