Old open air voyagers
Freshwater eukaryotes may have ventured onto land nearly 500 million years earlier than fossil evidence previously suggested
Among a heap of recently unearthed microscopic fossils dating to around 1 billion years ago are the preserved organic bodies of lake-dwelling eukaryotes that may have lived, at least part of the time, above the surface of the water.
|This "ball of cells" is about 30 micrometers in |
diameter and represents a level of complexity that
was previously unknown in organisms thought to
inhabit freshwater settings a billion years ago.
Image: Courtesy of Paul Strother
The finding, published this week in __Nature__, not only suggests that freshwater environments at this time rivaled the oceans as significant hotspots of eukaryotic evolution, but also provides compelling evidence that eukaryotes may have adapted to life on land nearly 500 million years earlier than the current fossil record suggests.
"This is very exciting," said paleontologist linkurl:Susannah Porter,;http://www.geol.ucsb.edu/faculty/porter/ who did not participate in the research. "Going back around 2.5 billion years ago, we probably had bacterial mats on land in near shore areas. This paper now tells us that in addition to bacteria, there were also diverse eukaryotes on land by 1 billion years ago."
The earliest uncontested evidence for life on land is fossilized spores from plants that lived near the water edge around 460 to 470 million years ago. Fossilized tracks from 530 million years ago, presumably left behind by an amphibious creature at low tide, suggest that animals may have found the shore nearly 60 million years earlier.
But going further back in time, "there really hasn't been any real direct evidence of anything eukaryotic living on land," said linkurl:Paul Strother,;http://www.bc.edu/schools/cas/geo/people/faculty/strother.html a paleobotanist at Boston College and first author of the paper.
|This tri-lobed thallus or "black blob" has a |
morphology that is mimicking a morphology that
exists today in certain kinds of plants.
Image: Courtesy of Paul Strother
In 2007, Strother and a colleague came across a dusty museum collection of microfossil slides recovered from an outcrop of the oldest sedimentary rocks in the British Isles: the Torridonian sequence. Sections of these rocks date between 1 to 1.2 billion years, and have preserved sediments that were once part of freshwater lakes formed in the depressions of an ancient land surface.
To Strother's amazement, the collection contained what appeared to be fossils of multicellular organisms. "We were finding clusters of cells as well things that looked like little pieces of tissue," he said. This came as a surprise given that multicellular organisms of that time period were believed to exclusively inhabit the oceans.
A more comprehensive fossil survey of the Torridonian sequence led by Strother revealed hundreds of thousands of microfossils representing at the very least around 50 different species.
Some of the rocks encasing the fossils had desiccation cracks in them, which formed as a result of the rocks drying up and cracking during their formation. It's the tell-tale sign that the ancient lake in which the fossils were found had dried up, at least in part, during the time the fossilized organisms were living there.
The finding suggests that the organisms must have been adapted for such periodic dessication, allowing them to live, at least for brief periods, in subaerial habitats, Strother explained.
"Perhaps these organisms were living in mud flats close to lakes," Porter said.
Also among the specimens, were several more hints of multicellularity, including balls of cells enclosed by what appears to be a cell wall and what Strother calls a "three-lobed black blob" that, at almost 1 millimeter wide, is a staggering size for a single cell.
"We've been thinking about the evolution of life in the Precambrian as primarily happening in the oceans," Strother said, "but 1 billion years ago, fresh water environments were really teeming with life."
P.K. Strother et al., "Earth's earliest non-marine eukaryotes," Nature, doi:10.1038/nature09943, 2011.
Correction (April 13): The original version of this story incorrectly quoted Susannah Porter as saying that there were bacterial mats on land 2.5 million ago. The date was corrected to 2.5 billion years ago. The Scientist regrets the error.
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[21st May 2009]