News Notes

Paleobotany and climate
Lichens in the Precambrian

According to the fossil record, fungi and plants first colonized the land about 460 to 480 million years ago. Studies using molecular clocks — which use knowledge of gene mutations to determine when a species originated — had suggested an older date of 600 million years, reasonably close to what the fossil record indicates.

But in work published in the Aug. 10 Science, a team led by evolutionary biologist Blair Hedges at Pennsylvania State University suggest that fungi had colonized the land by about 1 billion years ago and that vascular plants followed at about 700 million years ago. It is the latest in a series of studies showing that molecular clocks can yield dramatically different evolutionary time scales than those suggested by the fossil record alone.

The finding also offers an explanation for the cause of two of the most dramatic episodes in Earth’s history: the global glaciation, or Snowball Earth, of the late Precambrian and the rapid diversification of animal groups known as the Cambrian Explosion.

Hedges and his research team evaluated scores of genes for use as molecular clocks and found 119 that could time fungi mutations, 48 for green algae and 54 for land plants. As with a radiometric clock for dating rocks, the individual events measured — mutations, in this case — are random. But in large numbers, they average to a constant rate for each gene, Hedges says. The team calibrated their estimates against well-documented events in the fossil record.

“Our research shows that land plants and fungi evolved much earlier than previously thought — before the Snowball Earth event and Cambrian Explosion events — suggesting their presence could have had a profound effect on the climate and the evolution of life on Earth,” Hedges says.

The Snowball Earth refers to drastic cold waves in the Proterozoic, or latter part of the Precambrian, in which glaciers were common even at low latitudes. The easiest way to explain the frigid conditions is through a decrease of atmospheric carbon dioxide.

Fungi increase weathering of rocks, thus helping reduce the amount of carbon dioxide in the atmosphere. Land plants and lichens — a living collaboration between fungi and photosynthetic bacteria or algae — use carbon dioxide and water to produce sugars during photosynthesis. A byproduct of photosynthesis is release of oxygen, which would have built up in the atmosphere as plants and fungi became more abundant. In turn, the oxygen buildup made it possible for animal life to evolve.

“If there was such an early evolution of fungi and lichens on land then it follows that there would be an associated oxygen buildup and CO2 drawn-down as they hypothesize,” says Mark Chandler, a geologist with NASA’s Goddard Institute of Space Studies in New York who models the climate of the Precambrian. “If you consider that estimates of atmospheric CO2 in the Proterozoic are thought to have been as high as four to nine times modern levels, then some mechanism of CO2 drawn-down is required to explain the glacial periods in the late Precambrian.”

Even though the Penn State research team calibrated their molecular clocks with the fossil record, the use of such clocks is controversial, particularly among paleontologists who are unable to find fossil evidence to back up the estimates.

“These sorts of studies can be useful, but the value of them largely is in the reliability of the analyses,” says Douglas Erwin, a paleontologist with the Smithsonian Institution’s National Museum of Natural History and an expert on the Cambrian Explosion.

Erwin is wary of the evolutionary lineages, or phylogenies, Hedges and his research team used to calibrate their time estimates. He adds that paleontologists have been searching for microfossil evidence of land plants for decades, yet have found nothing that would support the early colonization dates suggested by Hedges and his team.

“I think this is one of those molecular clock papers that demonstrates a significant bias toward overestimating the results,” Erwin says.

While Erwin is skeptical of the adequacy of molecular clocks, Hedges is critical of the adequacy of the fossil record.
 “The fossil record always underestimates time of divergence, because the common ancestor is never preserved; we only see the descendants after morphological change has occurred,” Hedges says. “Molecular clocks on the other hand measure from the moment of speciation [divergence of the two lineages].

“There are entire phyla of animals that certainly existed in the Cambrian … yet have no fossil record because they are small, microscopic, soft-bodied, or for some other reason have preserved poorly.”

Hedges realizes that his team’s findings will remain controversial for some time, but he feels that the research has already made a valuable contribution.

It “is only a hypothesis,” Hedges says. “However, because it provides a simple explanation for … seemingly unrelated — and unexplained — phenomena, I think that it is worth considering.”

David Lawrence, contributing writer

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