Oily fossils provide clues to the evolution of flowers

Daffodils, tulips, roses and other flowers are so much a part of our daily lives that we take them for granted. Yet, how and when flowering plants appeared on Earth remains a mystery, a question that has gone unanswered by evolutionary scientists for more than a century.

Fossil Flower
A fossil flower. New evidence suggests that flowers may have evolved earlier than prevously thought.
Credit: Hanman’s Fossils

According to the fossil record, mosses were the first plants to emerge on land, some 425 million years ago, followed by ferns, firs, ginkgoes, conifers and several other varieties. Then, it seems, about 130 million years ago flowering plants abruptly appeared out of nowhere.

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 nineteenth-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 flowering plants may have evolved 250 million years ago – long before the first pollen grain appeared in the fossil record.

"Our research indicates that the ancestors 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, presented 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."

"This is important and exciting work," says Bruce Runnegar, professor of paleontology at UCLA and NAI member. "Although this is not the first time such a claim has been made, previous work has used details of plant anatomy rather than the biomarker molecules used here. I would imagine that the jury will remain out on this for some time, but this research does open up a new approach to solving this difficult problem."


The Chemical Stucture of Oleanane. Moldowan and his colleagues have detected Oleanane in oily rock deposits that are hundreds of millions of years old. Credit: IUPAC

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.

Archaefructus liaoningensis
Archaefructus liaoningensis. The leaf-like structures on the stem of this 140 million year old fossil are pods containing the seeds, a characteristic unique to flowering plants.
Credit: University of Florida

"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.

"One should be aware that tracing the ancestry of a group does not necessarily tell us how or why the group evolved," cautions Runnegar. "For example, the line leading to birds separated from other vertebrates, including mammals, back in the Paleozoic some 300-plus million years ago. At the time of the split the animals did not resemble either birds or mammals and there would be no way to predict the origin of birds at that time. So, even though the flowering plants can be traced to back to a Permian ancestor, it doesn’t mean that we know why they eventually appeared."

Molecular fossils

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.

Runnegar agrees, saying that this study should encourage astrobiologists to look for similar biomarkers for other significant evolutionary events.

Perhaps one day this technique will help solve Darwin’s "abominable mystery" once and for all.

Related Web Pages

Original Press Release (Stanford University)
Darwin’s ‘Abominable Mystery’ is closer to a solution, CU botanist says (Cornell University)
Oldest Known Flower in the World Found in Northeast China (Chinese Acedemy of Sciences)
International Organization of Palaeobotany Plant Fossil Record Database (IOP)