Thursday, July 01, 2010

earlier start to multicellular life

The Scientist | Newly uncovered fossils hint that multicellular life may have evolved more than 2 billion years ago -- some 200 million years earlier than previously expected, according to a study published this week in Nature.

The fossils are "not really [what] you expect to find in the rock record 2 billion years before present," said paleontologist Philip Donoghue of the University of Bristol, who was not involved in the research. "These fossils are centimeters in size" and "relatively thick" -- too large to be just a single cell, he said.

The once-biological shapes carved out of black shale formations in Africa outdate the next oldest example of what may have been multicellular life by about 200 million years. Unfortunately, "there's nothing preserved inside," said Donoghue, who wrote an accompanying perspective. "You can't demonstrate [for sure] that it was multicellular [because] you can't see component cells."

Sedimentologist Abderrazak El Albani of the University of Poitiers in France and his colleagues discovered the amorphous fossils in the black shale formations of the Francevillian Basin in Gabon, Africa. The team found more than 250 specimens at the site, all dating to approximately 2.1 billion years ago, and ranging up to 12 centimeters in length. Chemical analyses confirmed the biological origin of the fossils, which are now composed of the iron-sulfide mineral pyrite that replaced the organic tissue as the organism decomposed. And their large and complex structures, as revealed through X-ray microtomography, are indicative of cell-to-cell signaling and coordinated growth between cells, El Albani said.

Specifically, the fossils display scalloped edges with radiating slits, and many have a central structure, not unlike the overall structure of a jellyfish medusa. "This organism, in my opinion, was something very light, very gentle, very soft," El Albani speculated. Given the ubiquity of the radial structures among the highly diverse specimens, "I am sure that this radial fabric has some functionality for these specimens," he said, possibly for movement or fixation to the sediment, but "we have a lot of work [to do]" to determine what that function truly was. Still, the complexity and organization of their structure "shows clearly that [these organisms were] multicellular," he insisted.

But to call these fossils multicellular, it's important to first define multicellularity, Donoghue told The Scientist. "There are a great number of definitions, some of which are very restrictive and others which are all encompassing." Part of the difficulty in defining the term, he added, is that "much of the molecular machinery that is necessary for cell-to-cell communication is" found even in more primitive organisms, such as bacterial colonies.

Interestingly, these fossils appear just a couple million years after the Great Oxidation Event, when oxygen became more widely available in the atmosphere and in the shallow oceans. This may have facilitated the evolution of a thicker organism, where "it becomes more difficult for the cells in the middle to obtain that oxygen if it's only at trace levels in the atmosphere," Donoghue said.