Plants detonated Cambrian explosion

JOHN WHITFIELD

Early lichen-like plants could have increased erosion. © GettyImages

The first land plants might have triggered a rush of animal evolution. German researchers are proposing a controversial theory that the plants cooled Earth, making it conducive to complex life¹.

The idea is a new twist on the Gaia hypothesis that living things influence the global environment. "During the evolution of the Earth there was a decrease in temperature, and higher life forms have lower temperature limits," says Werner von Bloh of the Potsdam Institute for Climate Impact Research.

Many researchers think that rising, not falling, temperatures lit the fuse for the Cambrian explosion, the biological Big Bang 540 million years ago when most modern animal groups appear in the fossil record.

There is genetic evidence that simple plants, such as algae and lichens, colonized the land 800 million years ago. But no plant fossils from this time have been found.

It's also unclear whether there was a slump in temperatures, and whether such a drop would have favoured complex life. Says palaeontologist Robert Riding of Cardiff University, UK: "Nobody knows what's right or wrong, but it sounds odd to me".

But the link between temperature and life deserves further investigation, argues geochemist David Schwartzman of Howard University in Washington DC. "The global emergence of certain types of organism could be tied to their optimum temperature," he says.

Chalk it up

Land plants increased erosion, argue von Bloh and his colleagues. Calcium in the newly exposed minerals reacted with carbon dioxide in the air to make calcium carbonate, or limestone. Rivers washed this to the sea, sealing the carbon in the ocean bed.

The loss of carbon dioxide weakened the greenhouse effect and cooled the Earth. Once the temperature fell below a certain threshold, a runaway process began, say von Bloh and his team. Once conditions became favourable for complex life, these organisms cooled the Earth yet further, and so on.

In the space of about 40 million years, the researchers believe, global temperatures dropped from an average of more than 30 ºC to less than 15 ºC. Today, only bacteria are found in habitats hotter than about 60 ?C.

Heated debate

"It's possible that some part of the temperature story is true," says Tim Lenton of the Centre for Ecology and Hydrology in Edinburgh, UK. "The Earth has a lot less carbon dioxide in its atmosphere now than before the Cambrian explosion."

But Lenton thinks instead that increasing oxygen levels sparked the Cambrian explosion. "I'm not convinced that excessive global warmth held back evolution," he says, pointing out that organisms could still have evolved at the planet's poles.

I'm not convinced that excessive warmth held back evolution Tim Lenton Centre for Ecology and Hydrology, Edinburgh

Ancient rocks seem to show that Earth went through a cold snap - perhaps freezing over completely - between 500 million and 800 million years ago. Riding reckons that the end of this period alone might have favoured the emergence of animals.

"Everyone agrees that the Cambrian explosion was waiting to happen once the right conditions came along," Riding says. "People argue over what those conditions were."

1. von Bloh, W., Bounama, C. & Franck, S. Cambrian explosion triggered by geosphere-biosphere feedbacks. Geophysical Research Letters, 30, 1963, doi:10.1029/2003GL017928, (2003). |Article|

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