Aug. 6, 1999 A five-year effort to reconstruct the evolutionary relationships among all of Earth's green plants has resulted in the most complete 'tree of life' of any group of living things on the planet, scientists announced today. The team has revealed that the group traditionally thought of as 'plants' is really four separate lineages or 'kingdoms,' with one group -- fungi -- being more related to animals than to plants. The team has overturned the traditional belief that the so-called 'land-plant invasion' was led by seawater plants. Instead, the research team has found that primitive freshwater plants provided the ancestral stock from which all green plants now on land are descended and that this ancestor spawned every green plant now alive on earth.
The team of 200 scientists from 12 countries are all part of the 'Green Plant Phylogeny Research Coordination Group,' funded in the United States by the National Science Foundation, the U.S. Department of Agriculture, and the U.S. Department of Energy. The team's results were released for the first time at the International Botanical Congress where more than 4,000 scientists from 100 countries are meeting to discuss the latest results of research on plants for human survival and improved quality of life. The team sponsored eight major symposia at the Congress to release its findings to the scientists attending. The research team's discoveries hold profound ethical, intellectual, ecological, and economic implications for science, medicine, industry, and society.
"Better understanding this 'tree of life' will allow scientists to better predict the biological properties of plants," said Brent D. Mishler, a co-principal investigator for the team and Professor of Integrative Biology at the University of California, Berkeley, who spoke at a media briefing. "When you find a new plant, the first thing you want to know is how it is related to other plants. This is very important in predicting its traits. Green plant species are of particular economic value because they provide most of our food, shelter and medicines."
The team has clarified the plant 'kingdoms' and their relationships to animals. There are five main trunks, or lineages, of complex, "nucleated" organisms on the Earth's genealogical tree, four of which are classified as plants. They include the green plants, the brown plants, the red plants, the fungi, and the animals. The green plant lineage is the largest of the four plant groups, comprising about 500,000 species, including all of the Earth's land plants (trees, shrubs, grasses, flowers, ferns, mosses) and some of the aquatic plants, such as green algae.
"We can't really think of life on earth in terms of only the 'animal kingdom' and the 'plant kingdom' anymore," said Mishler. "In fact, there are five kingdoms of complex organisms on Earth today. One of the kingdoms, fungi, are more related to animals than to plants. This means that the mushrooms you eat are more related to you than to the tree on which they are growing."
The Conquest of Land by Plants
The team's work has resulted in a new understanding of how the explosion of plant life on the Earth's lands occurred. All life first arose in the ocean, then some life moved to freshwater and a number of lineages went back to the sea. It was the life forms that moved to freshwater that spawned the land plants we know of today.
"The conquest of land by the plants didn't happen from the sea; it happened from freshwater streams and lakes," said Mishler. "This overturns the traditional thinking among scientists and what is taught in every text book in America."
According to Mishler, some plants, like the kingdom of red plants - mostly seaweeds - never left the ocean. In the brown plant kingdom, most remained as seaweeds and continued living in the ocean as well, but a few, such as 'diatoms,' moved into freshwater.
The team has also discovered that all land plants alive today shared a single common ancestor, at least 450 million years ago. "Until now, it was commonly believed that within the green plant group, there were several lineages of land plants," said Mishler. "For example, people believed that mosses were derived from a different aquatic ancestor than were flowers or ferns. Now we've learned that at all levels of the green plant family tree, plants appear to be related to one another, indicating that they all share a common, freshwater ancestor. This is a real surprise."
In theory, throughout history various families would have evolved and left descendents, such that today we would see distinct subgroups related only in the distant past. Instead, at each stage of evolution only one family or lineage of green plants seems to have left the oceans and survived on land. Even at the very root of the green plant family tree, only one line leads to today's living green plants, according to Mishler.
"Plants came out onto land probably many times, but only one lineage actually made it," said Mishler. "This indicates there's an Eve -- a common ancestor -- in the primordial soup of green plants."
Some of those land plants subsequently returned to the sea to join the green plants that failed to make the transition to land, explaining why some green plants (green algae) still live in the water.
To invade the land, green plants had to adapt ways to keep from drying out and to reproduce. "In the water, you can just dump your gametes -- reproductive cells, such as egg and sperm -- into the water and they find one another," said Mishler. "Everything swims. On land, that doesn't work."
The first land adaptation for plants was to live in a wet environment where they could rely on a film of water through which their gametes could swim. Mosses and ferns still reproduce this way, but only in a wet environment. This limitation, which tends to make sexual reproduction less successful, was overcome by the angiosperms or flowering plants, which essentially rely on a whole male plant, the pollen, traveling to the female plant.
Darwin's Abominable Mystery: The Most Primitive Flower
The team's research will shed new light on which flowering plant living today is the most primitive, the one that is closest to the earliest forms. Research will be presented to support two views on which plant groups are the closest living relatives of flowering plants, and the scientific discussion at the Congress will help determine which view is most likely to be accepted. Much of the scientific world believes the Gnetophytes are the closest relative. These comprise a bizarre group of plants that includes Ephedra or "the Morman Tea," which is a shrub that grows in U.S. deserts, and Welwitschia, the strange 'pile of rubber' plant that grows in the Namibian desert, one of the driest places on earth.
"The Gnetophytes don't technically flower, but have similar reproductive structures and so people have suspected them as relatives," said Mishler. "Some phylogenies have them as the sister (closest living relative) to the flowering plants. Others have them sister to the cone-bearing plants like pines and firs and then that assemblage, sister to the flowering plants. This does have big implications for where the traits of angiosperms came from. The primitive mode of moving pollen is thought to be wind, and then the standard story is that the flowering plants explosively radiated into many types once they began associating and co-evolving with insects. However, there are indications from fossils that some non-flowering plants were beginning to associate with insects, and the Gnetophytes have insects associated with their cones."
There will be further scientific discussion on this at the Congress, in addition to discussion on when the earliest plant branched off to form the rest of the flowering plants, and on how the flower fossil record fits in with the molecular data.
Using newly available DNA sequence data and more traditional morphological and anatomical data, the team has identified some fascinating relationships within the flowering plants as well. The team has discovered that flowering plants, recently discovered to be a single, but incredibly diverse lineage, are not divided neatly into monocots and dicots, as is commonly taught, but rather some of the dicots (including magnolias and water lilies) are on a branch with the monocots (including grasses and orchids).
A Microscopic Universe Distinct from 'Plants' and 'Animals'
The team's research shows that all the plants and animals together form only a small branch on the tree of life, indicating that there is a universe of mostly single-celled, and poorly known organisms that make up the most substantial parts of the tree.
Centuries ago, the scientific discovery that the sun is merely one star in a universe full of stars, planets and other astral bodies shattered mankind's preconceptions of space and led to the exploration of the cosmos. Similarly, the research team's findings could shatter some preconceptions about life here on Earth by proving that all the familiar biodiversity (including humans, along with the other animals and plants), are only one "tiny twig" on the tree of life on Earth.
The team has traced how the most basic of one-cell, bacteria-like organisms that had neither nuclei nor chromosomes "merged" with each other to become not only multi-cellular but also multi-functional. Although life on this planet has evolved to an astonishing level of richness and complexity in the form of plants and animals, most of the Earth's life forms are still found in these ancient bacteria-like organisms, according to Mishler. The majority of these organisms have not been identified. To date, scientists have identified about 1.4 million species of organisms on earth, and estimates of the numbers of undiscovered and undescribed species of organisms range from 10 million to more than 100 million.
"There are millions of microscopic organisms that live on and in plants and animals and in the soil, air, and water," said Mishler. "They remain largely unexplored. Although some progress has been made, scientists are only just beginning to bring these creatures under the microscope much less to understand exactly how they interact with plant and animal ecosystems."
Formed in 1994, the Green Plant Phylogeny Research Coordination Group has engaged in a massive, collaborative analysis, requiring painstaking study of data on thousands of plant species from all over the world. The team was led by scientists from the United States, working with scientists from Japan, Canada, Mexico, the United Kingdom, Germany, France, New Zealand, Switzerland, Sweden, Finland, and The Netherlands. The scientists' areas of expertise include molecular biology, morphology, anatomy, chemistry, and paleontology. The web site for further information on the team's research and reference to scientific papers is: http://ucjeps.herb.berkeley.edu/bryolab/greenplantpage.html
Held only once every six years, the International Botanical Congress last met in the United States in 1969, when it was convened in Seattle, Washington. The XVI International Botanical Congress is hosted by the Missouri Botanical Garden in St. Louis.
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