Did Plants Cool the Earth and Spark Explosion of Life?

Hillary Mayell
for National Geographic News
August 10, 2001
Plants colonized Earth much earlier than previously believed, giving a jump start to the huge proliferation of animal species that occurred hundreds of million years ago, say scientists at Pennsylvania State University.

Based on the fossil record, scientists have thought that land plants and fungi evolved around 480 million years ago. The researchers at Penn State propose much earlier dates based on molecular clock analysis: 700 million years ago for land plants, and 1.3 billion years ago for land fungi.

The earlier estimates suggest a radical new possibility that ties two phenomena—cooling of the Earth's temperature, known as Snowball Earth, and the accelerated pace of the evolution of animals, referred to as the Cambrian Explosion—to the early emergence of plants.

"No one has considered connecting Snowball Earth and the Cambrian Explosion to the emergence of terrestrial plant life because no one considered that plant life existed until after these two occurrences," said evolutionary biologist and co-author of the study Blair Hedges.

"The earlier dates add a possibility for a biological explanation for the changes that is radical compared to what is believed now," says Hedges.

Explosion of Life

Snowball Earth describes periods of intense glaciations that took place 700 to 580 million years ago.

"The oceans were frozen all the way to the Equator," said Hedges, "as opposed to more modern times, when the ice sheets only go as far as Canada or maybe northern Pennsylvania."

The Cambrian Period occurred 540 to 500 million years ago. Animal life on the planet exploded during this period; most of the major groups of animals first appear in the fossil record at this point in time.

What happened to precipitate this sudden proliferation of species—an evolutionary "Big Bang"—has been the focus of much study. There hasn't been a widely accepted theory on what sparked the Cambrian Explosion, said Hedges. Most proposed explanations, however, have focused on geological events—perhaps tied to the breakup of the ancient super continents or a reversal in Earth's magnetic polarity.

This study, published in the August 10 issue of Science, suggests a biological, rather than geological mechanism.

Plants and the Atmosphere

Before plants drifted in from the sea, the Earth was a fairly barren wasteland of rocks, sometimes buried in ice.

Hedges and his collaborators suggest that as plants colonized Earth, they changed the climate, making it cooler by removing carbon dioxide from the atmosphere.

"Lots of plants would remove carbon dioxide from the atmosphere and increase the amount of oxygen," said Hedges. "It's the reverse of the global warming we see today, when we're releasing more carbon dioxide into the atmosphere by burning fossil fuels."

At the same time, plants release more oxygen into the atmosphere. "One widely entertained idea is that the Cambrian Explosion had something to do with an increase in oxygen at the time," said Hedges. The lack of oxygen would limit animal size because the larger an animal is, the higher its requirement for oxygen.

"The plants conceivably boosted oxygen levels in the atmosphere high enough for animals to develop skeletons, grow larger, and diversify," said Hedges.

The theory is bound to engender controversy; there is some question as to whether there would be enough plants to have caused the changes proposed by Hedges and company. But Hedges, whose area of expertise is astrobiology—the study of the relationship between life and the environment—is excited about the possibility.

"This study suggests that there's a lot more synergy between biological evolution and the Earth's geologic history."

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