By Amina Khan
An international team of paleontologists has uncovered the earliest known multicellular fossils, pushing back the fossil record for such life forms to 2.1 billion years ago and suggesting that they lived 200 million years earlier than scientists had thought.
Since most fossils in that period were microscopic and single-celled, finding fossils that stretched as long as 4.75 inches was "like ordering an hors d'oeuvre and some gigantic thick-crust pizza turning up," said Philip Donoghue, a paleontologist at the University of Bristol, who co-wrote a commentary on the finding. The report detailing the fossils, along with the commentary, was published online Wednesday in the journal Nature.
The organisms, which don't resemble modern-day living things, existed when Earth's atmosphere would have been uninhabitable for today's plants and animals.
Their fossils provide "the first record of that fundamental threshold in organismal complexity being surpassed," Donoghue said. "To put it into context, the godfather of evolutionary biology, John Maynard Smith, identified eight major events in evolutionary history; achieving multicellularity was one of these."
"I was astonished.... It's not the sort of thing you expect to see in rocks of that age," Donoghue said.
Paleontologist Abderrazak El Albani, the report's lead author, said the macroscopic fossils, which are visible to the naked eye, turned up at a point during the Paleoproterozoic era when life was thought to exist on a purely microbial level.
Learning about how and under what conditions that turning point was reached — it has happened at least 17 times in lineages that are still living, Donoghue said — could reveal much about how life developed.
The finding also dovetails with theories describing what Earth's environment must have been like at the time, illuminating how the changing atmosphere may have played a role in the development of life.
About 2.4 billion years ago, scientists say, oxygen began to build up dramatically in the environment. Though the element would have amounted to only a fraction of current levels, it may have been sufficient to allow some creatures to begin developing into multicellular organisms, the researchers theorize.
Given that the record for such ancient forms of multicellular life is spotty at best, Donoghue said, "we're certainly hungry for fossils in those intervals of our history."
Study coauthor Stefan Bengtson, a paleozoologist with the Swedish Museum of Natural History, pointed out that multicellularity was a key development in life on Earth because "once you start building things with smaller things, you can start building new structures, like a Lego game."
But there's still a long evolutionary road between these creatures and those whose genetics were complex enough to develop different tissues and organs within a single body, Bengtson said.
El Albani, of the University of Poitiers in France, said his team had been looking to study the sediments at the black shale formations in Gabon when they came across the fossils.
They weren't much to look at: lumpy and doughy-looking, though hard, of course. But a scan using X-ray tomography revealed complex organization and folding.
"It's spectacular.... It's really something," El Albani said of the structure.
The team also had to prove that the structures had been organic in nature.
The fossils were found to contain tiny grains of pyrite, or fool's gold. Such grains would have been created by sulfur-breathing bacteria munching away at organic matter, in this case the soon-to-be fossils' dead tissues, the scientists said.
El Albani said the next step would be to examine the fossils to learn more about how they lived and what their surroundings were like.
"I want to understand if this organism is moving or not; I want to understand the paleological ecosystem," he said.