Flowering plants older than expected

According to a recent study by Yale researchers, flowering plants — the dominant type of plant on Earth — may have originated tens of millions of years earlier than previously thought.

Fossil evidence has suggested that flowering plants first appeared around 140 million years ago, and then began to rapidly spread across the globe around 120 million years ago. However, the results of a new technique for calculating how long a species has existed suggest that flowering plants may have originated about 215 million years ago, said ecology and evolutionary biology professor Michael Donoghue, who is also vice president for West Campus planning and program development.

The 75-million-year gap between the fossil record and the study’s findings has raised questions about which number is correct, Donoghue said.

“Either our methods are correct and the fossil record is way off, or the fossil record is good and our methods are not giving the right answers,” he said.

Scientists have long wondered why flowering plants not only appear so suddenly in the fossil record, but also how they came to be the dominant plants on Earth, Donoghue said. In order to answer those questions, Donoghue and his fellow researchers used a “relaxed molecular clock” technique, a variant of standard molecular clock techniques that measure the rate of change in DNA sequences and information gathered from fossils to determine the origin of a lineage. The relaxed molecular clock technique allows for variations in the rate of evolution, which can make it more accurate, Donoghue said.

The researchers had expected to find that flowering plants did not originate long before 140 million years ago, but the large discrepancy between their results and the fossil record sent the researchers searching for answers. One possible explanation is that flowering plants did indeed originate over 200 million years ago but were not particularly abundant or were found in areas where fossils did not often form, Donoghue said.

Another explanation is that the researchers’ methods simply yielded the wrong answers. Because the relaxed molecular clock technique relies on statistical inference, it can give inaccurate results, unlike the hard evidence of the fossil record.

“It’s possible that our results aren’t correct,” Donoghue said. “You can always go wrong with a method.”

For now, the riddle of flowering plants’ origins remains unsolved, Donoghue said. One certainty that did emerge from the study, he added, is the need for more advanced methods of dating. He said that new methods might allow for even bigger variations in the rate of molecular evolution.

“What this study highlights is that we have a serious problem that is only getting worse, which is the gap between molecular methods and the fossil record,” Donoghue said. “We need better methods, and that’s where I’m putting my energies now.”

The study was published Mar. 16 online in the journal Proceedings of the National Academy of Sciences.

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