Life on land started millions of years earlier, claims scientist
A University of Oregon scientist has made the controversial claim that ancient multicellular fossils long thought to be ancestors of early marine life are in fact land-dwelling lichen or other microbial colonies.
Ediacaran fossils date to 542-635 million years ago, and have been identified as the remains of jellyfish, worms and sea pens.
But, says professor of geological sciences Gregory Retallack, they are in fact evidence that life appeared on land some 65 million years earlier than believed.
They've been preserved in different ways to marine invertebrate fossils, he says, and are found in iron-colored impressions similar to plant fossils and microbes.
Retallack examined ancient Ediacaran soils using an electron microprobe and scanning electron microscope. And, he says, the soils with fossils "are distinguished by a surface called 'old elephant skin,' which is best preserved under covering sandstone beds." The healed cracks and lumpy appearance of the soil are most like the surface of microbial soil crusts in modern deserts.
"This discovery has implications for the tree of life, because it removes Ediacaran fossils from the ancestry of animals," says Retallack.
"These fossils have been a first-class scientific mystery. They are the oldest large multicellular fossils. They lived immediately before the Cambrian evolutionary explosion that gave rise to familiar modern groups of animals."
Retallack identifies them as lichens, other microbial consortia, fungal fruiting bodies, slime molds, flanged pedestals of biological soil crusts, and even casts of needle ice.
And, he says, they represent an independent evolutionary branch of life on land that preceded by at least 20 million years the Cambrian evolutionary explosion of animals in the sea.
"The key evidence for this new view is that the beds immediately below the cover sandstones in which they are preserved were fossil soils," he says. "In other words the fossils were covered by sand in life position at the top of the soils in which they grew. In addition, frost features and chemical composition of the fossil soils are evidence that they grew in cold dry soils, like lichens in tundra today, rather than in tropical marine lagoons."
The research appears in Nature - but is disputed by Virginia Tech paleontologist Shuhai Xiao in an accompanying article. Many of the species identified by Retallack as land-dwelling are found elsewhere, he says, in soils that are clearly formed by marine sediments. "The evidence is unconvincing," he writes.