New Haven, Conn. — Flowering plants may have appeared on earth tens of millions of years earlier than the fossil record indicates, suggests a new analysis by researchers at Yale University and the National Evolutionary Synthesis Center, published the week of March 15 in the Proceedings of the National Academy of Sciences.
Flowering plants are the dominant plants on earth and also by far the most diverse with more than 250,000 species. But the question of when they first appeared has been controversial since the time of Charles Darwin, who dubbed the question “an abominable mystery.”
The first firm evidence of flowered plants in the fossil record occurred about 140 million years ago. Then, “with a big bang in the Cretaceous, some 120 million years ago, they took off,” said Michael J. Donoghue, the G. Evelyn Hutchinson Professor of Ecology and Evolutionary Biology and Vice President for West Campus Planning and Program Development at Yale.
Yet when scientists analyzed the DNA of many species of flowering plants, the results suggested that the plants evolved much earlier - some 140 to 190 million years ago. Donoghue and his co-authors, former graduate student Stephen Smith and current student Jeremy Beaulieu, reasoned that the problem was that the rate of genetic change had varied through the ages.
“There may be shifts in the rates of molecular evolution through time,” Donoghue said, in which case using a strict molecular clock would give the wrong answer.
Donoghue and his colleagues believed an analysis using a newly developed “relaxed molecular clock” method, which allows marked changes in the rate of molecular evolution, might bring estimates of the birth of flowering plants in line with the fossil record. To their surprise, their new estimates pushed the origin of flowering plants back even further in time, to some 215 million years ago, which coincides better with the fossil record of the major insect groups that depend today on flowering plants for their survival.
Donoghue said there are two explanations for the new result. One is that the first flowering plants were not very abundant or diverse, and thrived in areas where fossilization was unlikely. They radiated very rapidly beginning 120 million years ago. This pattern of delayed radiation is seen in other groups, such as the mammals that rose to dominance after the demise of the large dinosaurs.
The second explanation is that “there is something wrong with the methods we are using,” which, Donoghue noted, may still not completely account for very major shifts in the rate of molecular evolution.
“Like all good scientists, when you find an anomaly like this one, you need to work even harder to make sure you’re accounting for all of the relevant data and that your methods are absolutely the right ones for the job,” Donoghue said.
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