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Cartoonists have found plenty of clever ways to give life to this bit of conventional wisdom - complex life evolved in the sea, then crawled onto land. But a provocative new study suggests that procession might be drawn in the wrong direction.
As NPR's Richard Harris reports, the earliest large life forms may have appeared on land long before the oceans filled with creatures that swam and crawled and burrowed in the mud.
RICHARD HARRIS, BYLINE: You may have heard of an extraordinary period in Earth history called the Cambrian Explosion, when complex life suddenly burst forth and filled the seas with a panoply of life forms. Well, it turns out that before that time, more than half a billion years ago, the Earth was home to a scattering of mysterious organisms called Ediacarans.
Greg Retallack, at the University of Oregon, says scientists have assumed Ediacrans were predecessors of jellyfish, worms and other invertebrates.
GREG RETALLACK: In recent years, we've looked at them more closely and they don't seem to grow like them, they don't seem to be preserved like them. And I had doubts all along.
HARRIS: Retallack has been building the case that Ediacrans weren't in fact animals, but actually more like fungi or lichens. And if that idea weren't enough of a departure from standard theory, he now argues in a paper in Nature that Ediacrans weren't even living in the sea, as everyone has assumed. He has reanalyzed some Australian rock where they're found and concluded that it's ancient soil, not marine mud. These early life forms were landlubbers.
RETALLACK: What I'm saying for the Ediacran is that the big forms were on land and life was actually quite a bit simpler in the ocean.
HARRIS: So are you suggesting that evolution started on land and moved into the ocean then?
RETALLACK: Yes, in a nutshell.
HARRIS: At least evolution of large and more complex life forms. This is a radical idea. But Retallack is not alone in entertaining this possibility. Paul Knauth at Arizona State University, has been pondering this same possibility.
PAUL KNAUTH: I don't have any problem with early evolution being primarily on land. I think you can make a pretty good argument for that, and that it came into the sea later. It's kind of a radical idea, but the fact is we don't know.
HARRIS: Knauth says it could help explain why the Cambrian Explosion appears to be so rapid. It's possible these many life forms gradually evolved on the land and then made a quick dash to the sea.
KNAUTH: And that means that the Earth was not a barren land surface until about 500 million years ago, as a lot of people have speculated.
HARRIS: The new analysis of the Ediacrans fossils is at least a hint that this could be right. But, of course, if you're a scientist making an extraordinary claim, you need to back it up with extraordinary evidence.
SHUHAI XIAO: To me, the evidence is not a slam-dunk.
HARRIS: Shuhai Xiao is at Virginia Tech. He argues, among other things, that the same species of Ediacrans found in what is arguably soil are also found in deposits that he says were ocean sediments. The same creature living in dramatically different conditions - that doesn't make a lot of sense to him.
XIAO: So it's pretty hard for the same species to be able to live in both environments.
HARRIS: So he wonders whether Retallack is really looking at fossils in terrestrial soil. And so begins a sharp academic debate. Xiao is far from alone in his skepticism. The current idea has many defenders. Greg Retallack at the University of Oregon seems to relish the controversy. He knows what he's in for.
RETALLACK: The idea that Ediacran fossils were marine invertebrates is so deeply entrenched, it's in all the textbooks. And it's been there for so long that somebody who comes along and says no, it's going to be greeted like a death in the family. It's going to go through all the phases of grief, starting with denial.
HARRIS: And it remains to be seen whether the story ends with acceptance of Retallack's provocative proposal.
Richard Harris, NPR News.