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Summer/Fall 1999 |
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Project Explains Flowers, Identifies a Green "Eve" Scientists yesterday released the most complete analysis yet of how the world's 1 million species of plants are related to one another, overturning long-standing theories about how the first single-celled algae advanced in size and complexity to become the showy trees and flowers that stand today at the pinnacle of plant evolution. Perhaps most surprising, the five-year effort to map the entire family tree for all plants—involving more than 200 scientists in 12 countries—has determined that a rare and previously unheralded tropical flower is the closest living relative of the Earth’s first flowering plant. The unexpected discovery uproots both of the leading theories about what the first flower looked like, and apparently solves what Charles Darwin called the "abominable mystery" of how plants made the leap from primitive green monotony to full floral ebullience. That global makeover fueled an explosion in diversity among insects and other animals as well as plants. The new analysis, presented at the 16th International Botanical Congress in St. Louis, also comes to the jarring conclusion that there are at least three separate plant kingdoms rather than one, as most high school students are taught today. It finds that plants invaded land not directly from the sea, as many scientists had thought, but from fresh water, where they spent millions of years preparing for the rigors of terrestrial existence. And it concludes that the many families of green plants living on land today descended not from separate evolutionary upstarts but from a single green "Eve," a near relative of which still lives today in pristine lakes as it did more than a billion years ago. The project also confirms a counterintuitive finding, first proposed six years ago, that fungi—including yeast and mushrooms—are more closely related to people than they are to plants. "This is the first comprehensive, coordinated, large-scale attempt to reconstruct one of the major branches of life," said Brent Mishler, a professor of integrative biology at the University of California at Berkley and a spokesman for the federally funded "Deep Green" project. Beyond the intellectual gratification that comes with understanding how the world’s plants are related, the new findings could have practical benefits, said Peter Raven, director of the Missouri Botanical Garden, which is hosting the week-long meeting of 4,000 botanists. For example, Raven said, it makes sense for botanists seeking new medicinal compounds to focus on plants closely related to those already known to have therapeutic properties. But that approach has been hampered by the lack of an accurate family tree. Conversely,conservationists worried about accelerating plant extinctions want to preserve seeds and other genetic resources from a broad array of plants. But in order to decide where to concentrate their efforts, they need to know which plants represent the most disparate branches of the botanical family tree. Weed control experts might be able to mount more effective attacks against newly invading species if they knew which species the new pests were related to and what kinds of weed killers work on those near relatives. "It’s the ability to compare that gives meaning to everything in biology," Raven said. The new work was made possible by recent advances in cladistics, a field in which scientists compare the most evolutionarily relevant traits among various organisms rather than the most obvious ones, as old-fashioned taxonomists did. By comparing key traits, such as water-conducting tissues or flower shape in different species, living and fossilized, scientists can determine when and where novel "branches" erupted from ever-diversifying family tree. Equally important have been advances in genomics, a field that tracks changes in gene arrangements over millennia, allowing molecular biologists to trace evolution’s footsteps. Mishler warned that biologists who specialize in evolutionary classification are notoriously argumentative and that the new picture of plant evolution presented is sure to change as fresh data arrive and other theories are proffered. But unlike previous efforts, he said, "these new family trees really indicate relationship, not just shallow similarity." The new work sheds especially dramatic light on the emergence of flowering plants (believed to have arisen about 135 million years ago) from their non-flowering predecessors (which persist today as pine trees and related plants). Until now, scientists had thought that the first flower closely resembled either today’s magnolias or water lilies, both of which lack many of the specialized parts found in more modern flowers. No one had suspected that the debate between those two camps would be settled by the appearance of an even more primitive relative, a small, cream colored flower called Amborella, a single species of which lives on the South Pacific island of New Caledonia. Four groups of scientists yesterday offered strong evidence that Amborella—probably pollinated at first by prehistoric beetles—belongs on the lowest branch of the flowering plant family tree, with other flowers appearing later in history and "higher" up in that tree. Flowering plants have an advantage over others because their seeds are protected inside fleshy fruit. Other researchers presented data showing that green plants (including all land plants), red plants and brown plants (mostly algae and seaweeds), evolved from three different one-celled plants, and so deserve to be considered individual kingdoms. The fungi, including mushrooms and yeast, also constitute an independent kingdom. But under the new system, some former fungi (such as the so-called slime molds) have been moved to the brown plant kingdom. "The fungi are being trimmed down," Mishler said. "They are leaner and meaner." Researchers at the meeting also presented data indicating that primitive, single-celled green plants moved to fresh water before storming the land. In ponds they became multicellular, gaining the advantage of having cells that can specialize in specific tasks, including learning how to retain water, a crucial survival tactic for life on land. Many assaults on the land may have been made, but only one plant line survived to diversity into every land plant known today. New data indicate that the mother of that line, the "Eve" of green plants, was something very similar to today’s so-called coleochaetes, tiny green plants about the size of a pinhead and just one cell thick, which require fresh water that is completely free of pollutants. Some practical benefits may come from a better understanding of how plants made the transition to land, Mishler said. The first plants to climb onto terra firma, the simple mosses, are exceedingly resistant to drying, even more so than are higher plants, which lost some of that ability later in evolution. Scientists are now trying to identify genes in mosses that might be bred into crops to make them more drought-resistant. |