Stanford Report, April 4, 2001
|Geochemists find evidence that flowers may have evolved
250 million years ago
BY MARK SHWARTZ
Daffodils, tulips, roses and other flowers are so much a part of our daily lives that we take them for granted.
But to evolutionary scientists, the question of how and when flowering plants appeared on Earth has gone unanswered for more than a century.
Mosses were the first plants to emerge on land some 425 million years ago, followed by firs, ginkgoes, conifers and several other varieties.
According to the fossil record, flowering plants abruptly appeared out of nowhere about 130 million years ago.
Where did they come from, and how could they have evolved so suddenly without any transitional fossils linking them to other ancient plant species?
"An abominable mystery" is how 19th-century naturalist Charles Darwin referred to the origin of flowering plants, and the puzzle remains as controversial today as ever.
Now a team of Stanford geochemists has entered the debate with evidence that flowers may have evolved 250 million years ago -- long before the first pollen grain appeared in the fossil record.
"Our research indicates that the descendants of flowering plants may have originated during the Permian period, between 290 and 245 million years ago," says J. Michael Moldowan, research professor of geological and environmental sciences.
"We based our findings on an organic compound called oleanane, which we found in the fossil record," he adds.
Moldowan and his collaborators, research associate Jeremy Dahl and graduate student David A. Zinniker, will present their findings at the annual meeting of the American Chemical Society (ACS) in San Diego on April 2, during a symposium titled "Biogeochemistry of Terrestrial Organic Matter."
Oleanane is produced by many common flowering plants as a defense against insects, fungi and various microbial invaders. But the chemical is absent in other seed plants, such as pines and gingkoes.
Using gas chromatography and mass spectroscopy, Moldowan and his colleagues have been able to extract molecules of oleanane trapped in oily rock deposits that are hundreds of millions of years old.
"Our work has shown that oleanane is lacking from a wide range of fossil plants," he notes, "but the chemical is found in Permian sediments containing extinct seed plants called gigantopterids."
That makes gigantopterids the oldest oleanane-producing seed plants on record -- an indication that they were among the earliest relatives of flowering plants, concludes biologist David Winship Taylor of Indiana University Southeast, a co-author of the ACS study.
"This discovery is even more significant because we recently found gigantopterid fossils in China with leaves and stems that are quite similar to modern flowering plants," Taylor notes -- further evidence that flowering plants and gigantopterids evolved together roughly 250 million years ago.
Moldowan and his colleagues point out that the chemical fossil record can be an important tool for studying the history of life on Earth.
"In our research we use molecular fossils, or biomarkers, such as oleanane to provide evolutionary and paleoenvironmental information from sediments and petroleum," he says.
Perhaps one day this
technique will help solve Darwin's "abominable mystery" once and
Gigantopterids, an extinct plant
group that lived some 250 million years ago, may be the oldest
relatives of roses, daffodils and other flowering plants.
Fossilized deposits of gigantopterids contain oleanane – an
organic chemical found in modern flowering plants but absent in
pines and other seed plants. This drawing is a reconstruction of
Vasovinea tianiia, a species of gigantopterid unearthered in China