Study tells evolutionary history at smaller level

Tuesday, March 11, 2008

A group of scientists led by a Brown professor has finished a ground-breaking study on evolutionary relationships in the animal kingdom.

The study, led by Assistant Professor of Biology Casey Dunn and funded by a National Science Foundation initiative, appears in this month’s edition of the journal Nature

Conducted over a period of about four-and-a-half years, the study found many significant discoveries that may change the way scientists view evolutionary history. One of the biggest surprises came when the research team discovered that comb jellyfish, also called ctenophores, may have diverged from sponges earlier than commonly thought. This could mean ctenophores might be the earliest “emerging ancestor” existing today – suggesting they are older than any other existing animal species.

It also means that sponges, which were originally thought to be older than comb jellyfish, may “have actually become simplified in the course of evolution,” Dunn said.

Dunn said this discovery requires confirmation from other studies and further examination, but that it could change the perspective on how and where evolution occurred.

Many of the hypotheses about relationships between animals in evolutionary biology are traditionally based on animal structure, Dunn said. But for this study, he and his team collected molecular data to resolve relationships between various animal phyla, which are divisions of the animal kingdom.

The scientists used a new method called EST, which stands for expressed sequence tags, to gather their data. In EST, random genes from across the genome are sampled from each animal, and the genes are analyzed to see which ones will yield the best comparison between species.

Dunn developed an “extremely clever pipeline” for this task, Gonzolo Giribet, professor of organismic and evolutionary biology at Harvard and a contributor to the study, wrote in an e-mail to The Herald.

In addition to taking this novel approach, the team also strengthened its research by surveying a broader swath of the animal kingdom. The study involves 29 animal species from 21 different phyla. Eleven of the phyla had never been studied before using the EST method.

The gene computations were the most time-consuming and difficult part of the study, the scientists said. Because of the falling price of sequencing DNA, it is much quicker to retrieve the data. A”bottleneck” occurs in the processing portion of experiments, said Mark Martindale, professor of organismal biology at the University of Hawaii and director of Kewalo Marine Laboratory, where much of the research took place.

“It took over three or four months of continuous crunching by the best computers we could commercially buy to analyze the data,” Martindale said. The team used about 80 computers in clusters across the world, he said, and the computers would sometimes shut down and data would be lost because the computers produced so much heat that the air conditioner could not keep up, Martindale said.

“You’re trying to drink from a fire hose, and you only have time to swallow so much data,” he added.

The study was funded by the NSF’s Assembling the Tree of Life initiative, which aims to reconstruct the evolutionary history of all organisms.

This is the first paper published by the group, comprised of scientists from across the world. Dunn said they plan to continue their research by adding more animal groups and revisiting those that were “recalcitrant” to being placed on a tree of life.

“It’s very important to know how animals are related to each other,” Dunn said. “A lot of the information we have about how humans work comes from studying animals such as these.”

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