The Tree of Life project, a Web site that links users through the branches of Earth's evolutionary tree, began as a graduate student's inkling.

"That little glimmer didn't really crystallize into something serious until the Web came along," said UA entomology professor David Maddison, who began composing software for the tree in 1994 along with his twin brother, Wayne Maddison, a UA ecology and evolutionary biology professor.

Since then, the ToL project has grown to more than 2,000 linked Web pages, authored by biologists around the world and containing text, photographs and references about its content and the evolution of living things on earth.

Last year, David Maddison said, the Web site had almost 2 million visitors from 130 countries.

The authors of the tree pages, well-known experts in their fields, bring together different aspects of the tree, like molecular structure or life histories of organisms.

Authors who know of other researchers can offer their recommendations, allowing the author list to grow hierarchically, Maddison said.

"The appropriate authors for the pages are the combination of people who are interested, who are top-notch in their field and who are open minded," he said. "Dogmatism doesn't work in this context."

But occasionally, there are disputes among experts about the exact relationship between organisms and the resulting shape of the branches.

The ToL team encourages dissenting contributors to each write a discussion of phylogenetic relationships that can be presented on the page.

"Everyone puts their arguments forward," said Katja Schulz, managing editor of ToL and entomology research associate. And if that's not possible, "we make sure that the authors that do contribute mention the other people's views."

As means of providing evolutionary information, "there are two very different aspects to what we do," Maddison said. "One is the shape of the phylogeny and the branching pattern of the tree. Then there's all the other encyclopedic information that gets wrapped around it."

"The shape of this tree is really important if you want to know something about biodiversity," Maddison said. "Like a river flowing along, those lines represent the flow of genes. They show the history that generated that diversity."

For the past few years, the ToL team has been working on technical improvements to the project that made data entry, editing and navigation easier.

"We did this because we see the potential to accommodate a lot more research and a lot more flexibility, and the technical tools to do this on the Web are just becoming available now," Schulz said.

"When we started this out, the audience was biology researchers," Maddison said. "But when it went up, we got so much contact from non-researches, including middle school kids. It needed to be flexible enough to serve lots of different audiences."

Expanding the content of the tree is going to be the team's main concern once all the technical changes are made, Maddison said. Right now, "there are parts of the whole structure that aren't growing, while there are other parts that are continually growing and being fleshed out," he said.

"At some point, it is going to be too big for us to take care of, and it will have to move to a bigger organization," possibly on the national level, Schulz said. "It could potentially go on forever."

Wendy Moore, an entomology graduate student, authored pages on a subfamily of beetles for the project and worked on the infrastructure of the tree. New research is continually updating the old, she said. "Since the tree is Web-based, it can represent the latest and most updated information. In a sense, it's always a work in progress. It needs continual updating."

"We're hoping to get some money, some people and some, tools to help with the content of it. But in terms of content, its never going to be done. It can't be done," Maddison said.

The Tree of Life can be accessed at www.tolweb.org.