A newly discovered group of 2.1-billion-year-old fossil organisms may be the earliest known example of complex life on Earth. They could help scientists understand not just when higher life forms evolved, but why.
The fossils -- flat discs almost 5 inches across, with scalloped edges and radial slits -- were either complex colonies of single-celled organisms, or early animals.
Either way, they represent an early crossing of a critical evolutionary threshold, and suggest that the crossing was made necessary by radical changes in Earth's atmosphere.
"There is clearly a relationship between the concentration of oxygen and multicellularity," said Abderrazak El Albani, a paleobiologist at France's University of Poitiers. The fossils are described in the July 1 issue of Nature.
Single-celled organisms emerged from the primordial soup about 3.4 billion years ago. Almost immediately, some gathered in mats. But it was another 1.4 billion years before the first truly multicellular organism, called Grypania spiralis, appears in the fossil record.
Grypania may have been either a bacterial colony or a eukaryote -- an organism with specialized cells, enclosed in a membrane. Whatever Grypania was, it was one of the few known examples of complex life until about 550 million years ago, when the fossil record explodes in diversity.
The newly described fossils, which have yet to be given a species name, make Grypania less solitary. They lived at roughly the same time -- Grypania in what is now the northern United States, the new fossils in Gabon. By raising the possiblity that multicellularity was a trend rather than an aberration, they also hint at an answer to the question of why complex life evolved, not just when.
Just a few million years before Grypania and the newly discovered fossils appear in the fossil record, Earth experienced what's called the Great Oxidation Event. The sudden evolution of photosynthesizing bacteria radically changed Earth's atmosphere, kick-starting its transformation from nearly oxygen-free into today's breathable air.
"The bacterial world was undergoing the greatest episode of climate change in the history of the climate," wrote University of Bristol paleobiologists Phil Donoghue and Jonathan Antcliffe in a commentary accompanying the findings. "The proximity in the age of these fossils to the timing of the Great Oxidation Event fits elegantly" with the notion that changing ocean chemistry fueled the evolution of complex life.
Bacteria possess chemical signaling systems, and many researchers now see their colonies -- which can stretch for centimeters, numbering millions of individuals -- as collective organisms, with different individuals having specialized body types and tasks.
Growth patterns seen in the new fossils fit with those found in multicellular organisms capable of complex signaling and coordinated responses. Earth's suddenly fluctuating climate would have favored communication.
"When bacteria are under stress, it triggers their cooperation," said biophysicist Eshel Ben-Jacob of Tel Aviv University. "Those that have to cope with a more complex environment show higher complexity."
"You have multicellular organization during the first upswelling of oyxgen in the atmosphere," said El Abani. He said multicellular organisms likely evolved in many places, but the fossils haven't yet been found.
"All life on the earth had to change," said Ben-Jacob.
Images: 1) Virtual reconstruction of outer and inner structure of fossil specimen./Abderrazak El Albani and Arnaud Mazurier. 2) Fossil remains./Abderrazak El Albani.
See Also:
- Microbe May Answer Mystery of Multicellular Life
- First Animals Found That Live Without Oxygen
- Life's Complexity Began With Poop
- Viral Missing Link Caught on Film
- Complex Life Traced to Ancient Gene Parasites
Citations: "Large colonial organisms with coordinated growth in oxygenated environments 2.1 Gyr ago." By Abderrazak El Albani, Stefan Bengtson, Donald E. Canfield, Andrey Bekker, Roberto Macchiarelli, Arnaud Mazurier, Emma U. Hammarlund, Philippe Boulvais, Jean-Jacques Dupuy, Claude Fontaine, Franz T.Fursich, Francois Gauthier-Lafaye, Philippe Janvier, Emmanuelle Javaux, Frantz Ossa Ossa, Anne-Catherine Pierso. Nature, Vol. 466, No. 7302, July 1, 2010.
"Origins of Multicellularity." By Philip Donoghue and Jonathan Antclifee. Nature, Vol. 466, No. 7302, July 1, 2010.
Brandon Keim's Twitter stream and reportorial outtakes; Wired Science on Twitter. Brandon is currently working on a book about ecological tipping points.
