When the first pioneering plants emerged from the earth's ancient seas and took up life on land, they turned a barren landscape green and paved the way for countless animals and other organisms to follow.
Biologists have long set the date for that momentous event somewhere around 450 million years ago, but a new study in the current issue of the journal Science suggests that plants escaped the oceans at least 700 million years ago, a radically earlier date.
In fact, the new work pushes the origin of land plants so far back in time that the authors say these plants may actually have touched off critical events that have long been thought to have predated them. One such event is the famous evolutionary proliferation of animal groups called the Cambrian explosion.
Published on Friday, the new date of 700 million years has already garnered great interest as well as healthy portions of head-shaking disbelief. The team of researchers, composed largely of undergraduates at Penn State, was led by Dr. Blair Hedges, a molecular evolutionist known for using molecular data to try to turn conventional wisdom about evolutionary history on its head.
"I thought it was very exciting," said Dr. Linda Graham, a plant evolutionary biologist at the University of Wisconsin who was not involved with the study. "It is believable, but we do need some additional corroborating evidence."
So far, the fossil record has not been obliging — the oldest fossils of land plants are around 450 million years old. One problem with the new date, researchers say, is that it means land plants would have existed on earth for 250 million years without ever leaving a fossil that scientists have been able to unearth.
"It doesn't fit," said Dr. Brent Mishler, a plant evolutionary biologist at the University of California at Berkeley. He described the methods of analysis used in the paper as outdated. The date is so unexpectedly early, he added, that "it's something that would make you suspicious."
Dr. Hedges acknowledged that the analyses required their share of corrections and estimations, but expressed confidence in the study. Speaking of the date, he said, "I think that's pretty secure."
The work is also of interest as the latest in what is becoming a long line of molecular studies that suggest ages that are inexplicably older than fossil data would suggest for various groups. Some scientists say that the molecular data are likely to be right and that paleontologists need to dig harder for those fossils. Others suggest that molecular data may be biased toward overestimating ages.
Researchers often estimate the ages of groups using what are called molecular clocks, as was done in the new study. The logic of molecular clocks is simple: when two groups split apart from each other — for example, when the human lineage split off from the chimpanzee lineage — the two groups begin to accumulate differences in their DNA and proteins. The longer the two groups are separated, the more differences they will accumulate between them.
If researchers know when two groups split, perhaps by using fossil evidence to date that split, they can then count how many differences accumulated in the groups' DNA over that number of years. By knowing how many differences accumulate, for example, every million years, researchers have calibrated their molecular clock. Then, simply by knowing the number of differences that have accumulated in the DNA of any other two groups, researchers can estimate how much time has passed since those lineages split.
But researchers are still arguing over whether molecular clocks tick regularly enough to provide accurate ages.
In their study, Dr. Hedges and colleagues analyzed more than 100 previously published protein sequences to examine the differences accumulated over time between a number of fungi and plants.
Plants are widely thought to have made the leap to land accompanied by fungi, like the fungi that today can be found living in the roots of most plants. Using a protein clock, the researchers estimated that the necessary fungi were around more than a billion years ago, setting the scene for the evolution of land plants arising at least 700 million years ago.
Dr. Blair said the work suggested that biologists might need to rethink the dating of some of the more modern groups of plants, like the flowering plants and species like corn and rice. "It could push these dates back also," he said, "and that probably will tick off a lot of people."
The broadest implication of the new study was the suggestion by the authors that these early land plants might be responsible for one of the most famous of evolutionary events, the so-called Cambrian explosion about 530 million years ago. In the past, some researchers had suggested that what touched off that rapid evolution of animal life forms was an increase in atmospheric oxygen. The authors on the new work suggest that land plants predated that major event and may well have been the source of that crucial oxygen.
Dr. John Taylor, a mycologist at the University of California at Berkeley whose own work on a more limited set of molecular data had earlier raised the possibility that land plants might be 600 million years old, urged researchers to consider the newest number, but cautiously.
"It's an estimate," he said, while praising the work, "and it has to be treated that way. Our paper wasn't the last word, and this one won't be either."
File Date: 08.14.01