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Trees of life: ‘plants exist as four kingdoms’. Credit: CORBIS/JIM ZUCKERMAN

A ‘tree of life’ giving the most complete picture yet of the evolutionary relationships among all of the Earth's green plants was unveiled last week at the International Botanical Congress in St Louis, Missouri.

The scenario was drawn up by the Green Plant Phylogeny Research Coordination Group, a five-year project involving around 200 scientists from a dozen nations. It is also known as ‘Deep Green’.

The team found that plants are divided into four separate kingdoms: green plants — the largest group with about 500,000 species, including all land plants and some aquatic plants such as green algae — brown and red seawater plants, and fungi. Interestingly, the fungus kingdom is the closest relative of the animal kingdom, life's fifth kingdom.

Previously, scientists divided life on Earth into animal and plant kingdoms. But the results of the Deep Green project show that, for plants, there are “four lineages of complex, nucleated organisms”.

Deep Green researchers also found that all green plants appear to come from a single lineage, not from multiple lineages as previously believed. “This indicates that there's an Eve — a common ancestor — in the primordial soup of green plants,” says Brent D. Mishler, a professor of integrative biology at the University of California, Berkeley, and a co-principal investigator of the team.

“A better understanding of this tree of life will allow scientists to better predict the biological properties of plants,” says Mishler. “Their economic importance as sources of medicines, structural materials, food and chemicals is immense.”

The Deep Green project was described as a classic example of the benefits of interdisciplinary cooperation between scientists, from taxonomists to molecular biologists. “Instead of people jealously guarding their data, they shared data and produced many important discoveries,” says Mishler.

Systematists produced data on the evolutionary origin of individual plants, for instance, then molecular biologists refined the species' position on a cladistic tree. This effort “makes a science out of the field of taxonomy”, adds Mishler. “Since Darwin, people have been speculating on phylogenetics. This effort gives us methods and explicit data to make comparisons for green plants.”

The results of Deep Green's work were presented at eight symposia during the week-long Botanical Congress, which is held every six years. More than 4,000 scientists attended.

One unexpected discovery was that plant life on land derived from fresh, not salt, water. “This overturns the traditional thinking among scientists and what is taught in every textbook in America,” says Mishler.

As universities and schools begin to teach Deep Green scientific findings, controversy may arise in the United States, since the discoveries are based on the principles of natural evolution. Indeed, some of those at the meeting warned that creationists may try to use the textbook revisions to advance their cause.

Deep Green was funded by the US National Science Foundation, the Department of Energy and the Department of Agriculture. The findings are available on: http://ucjeps.berkeley.edu/bryolab/greenplantpage.html . Congress reports are available at: www.ibc99.org .