To a human, two billion years is an unfathomable interval. But that, a team of European, Gabonese, and American scientists now say, is how long ago a recently discovered hoard of fossils suggests Earth's first big life evolved -- large enough to see with the naked eye, and in a spectrum of forms that tease and bewilder. What do their shapes mean? Would the life that existed on 2-billion-year-old Earth have given us any clues to the Earth we see today? Just what were these things?
To put the unexpectedness of this fossil cache in perspective, consider that Earth condensed out of the planet-forming gas disk that shrouded our young sun about 4.5 billion years ago. Life evolved surprisingly swiftly after Earth's birth, probably by 3.5 billion years ago and perhaps more than 4 billion years ago. But the ancestors of the large life forms we see today did not appear until the Cambrian Explosion, about 550 million years ago, and the first large life, the Ediacaran Biota, appeared just 50 million years before that, ca. 600 million years ago (relatively simple but macroscopic branching algae may, however, have evolved as early as 700-800 million years ago).
These new fossils nearly split the difference and represent a mind-boggling 1.5 billion year leap back in time. They appear to have been a surprising punctuation mark in an otherwise monotonous microbial interlude. Of course, the fossil record being as fragmentary as it is, this is not to say it was the only one. Just the only one we know about so far.
Take a look for yourself and see if you can imagine what the creatures that left these remains behind would have looked like while alive. And remember: these fossils are over 2 BILLION years old.
Here are some more of the flat disks with the ruffled edges you saw in the image at the top of this post. 3D X-ray reconstructions at right.
This one looks a bit like one of the flat disks attached to one of the beaded strings. Are they parts of the same organism?
This fossil is evocative of a gelatinous sand dollar or some sort of delicious, crispy snack food. Check out that ruffle at right.
On the right below is one half of an impression fossil of something truly bizarre. It is reminiscent of another unexplained life form that exists both in real life and in fossils all the way back to the Cambrian: Paleodictyon nodosum. This phenomenon is a hexagonal grid of interlocking burrows on the seabed, the occupant and/or creator of which has never been discovered. The New York Times had a fascinating article about this unexplained biological phenomenon a few years ago that you can read here. But as the authors of this paper point out, whatever this Francevillian fossil was differs in several ways -- its individual units are both irregularly sized and distributed, unlike the highly regular burrows of whatever Paleodictyon is.
Here's the other half of the fossil (the rock was split in half to reveal it), flipped and tilted slightly from above. Notice that the grouping of the dots is itself a circle. This distinguishes it from similar fossils found in the Ediacaran called Nemiana and Beltanelloides.
This new collection of fossils (a "lagerstätte" in the bio-lingo) is called the "Francevillian Biota" and hails from the west African nation of Gabon. So far scientists have collected over 400 fossils in scores of new types from the 2.1 billion-year-old deposit, including several new types not shown in their original 2010 Nature paper describing the cache, according to a new paper published last summer in PLoS ONE. "[These fossils] appear to represent a first experiment in megascopic multicellularity," the authors suggest. And by megascopic, they mean mega -- the largest specimen is 17 cm (nearly 7 inches) long.
The researchers who've studied the remains of these creatures know a few things about them -- they lived on the quiet seabed of an oxygenated, shallow ocean -- but beyond that, clues are few. After their death, they were buried by sediment and often preserved in the sparkly mineral pyrite (also called fool's gold; I wrote about some similarly fossilized crustaceans here) by the action of bacteria in what would ultimately become a dense black shale. In fact, it was the combination of these organisms' quiet neighborhood and conditions that fostered fast fossilization by pyrite to which the authors attribute their remarkable preservation.
What makes these fossils extra-special is not that they were multicellular organisms, as they almost certainly were. It's that they are visible to the naked eye. Multicellularity has evolved many times in Earth's history, and this was likely not the first time. But it is the first that we know of that was big enough for human eyes to see unaided.
So what might have encouraged this early flirtation with bigness?
Immediately prior to their evolution, atmospheric oxygen spiked during the Great Oxygenation Event of 2.3 billion years ago, a result of the invention of photosynthesis by cyanobacteria and the running out of iron deposits with which to sop all the resulting oxygen up by rusting out (the genesis of the famous banded iron formations in Minnesota and Western Australia). This is consistent, the authors say, with the idea that more fuel in the form of oxygen allowed the evolution of bigger life forms. Aerobic respiration of the type made possible by oxygen yields 15 times more power than anaerobic respiration, and all this extra power could fuel bigger bodies, some have hypothesized.
Intriguingly, these creatures disappeared around the time atmospheric oxygen levels nosedived, where the oxygen concentration stayed for the remainder of the Proterozoic Era up to about 800 million years ago, just before big life went mainstream in the Ediacaran/Cambrian. Although the idea that oxygen levels determine how big and complex life can be is "not universally accepted as a major driver for the evolution and complexification of multicellular life", this evidence tends to support the idea, these scientists say.
Proterozoic, incidentally, means "before animals". These creatures, whatever they were, were probably not animals, not plants, and not anything we know today. What they were is tantalizingly unknowable, but also good food for a lively imagination. For us, their distant kin, they can be whatever we might dream them to be.
Abderrazak El Albani, Stefan Bengtson, Donald E. Canfield, Armelle Riboulleau, Claire Rollion Bard, Roberto Macchiarelli, Lauriss Ngombi Pemba, Emma Hammarlund, Alain Meunier, Idalina Moubiya Mouele, Karim Benzerara,Sylvain Bernard, Philippe Boulvais, Marc Chaussidon, Christian Cesari, Claude Fontaine, Ernest Chi-Fru, Juan Manuel Garcia Ruiz, François Gauthier-Lafaye, Arnaud Mazurier, Anne Catherine Pierson-Wickmann, Olivier Rouxel, Alain Trentesaux, Marco Vecoli, Gerard J.M. Versteegh, Lee White, Martin Whitehouse & Andrey Bekker. (2014). The 2.1 Ga Old Francevillian Biota: Biogenicity, Taphonomy and Biodiversity, PLoS ONE, 9 (6) e99438. DOI: http://dx.doi.org/10.1371/journal.pone.0099438
Baderrazak 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. Fürsich, François Gauthier-Lafaye, Philippe Janvier, Emmanuelle Javaux, Frantz Ossa Ossa, Anne Catherine Pierson-Wickmann, Armelle Riboulleau, Paul Sardini, Daniel Vachard, Martin Whitehouse & Alan Meunier. (2010). Large colonial organisms with coordinated growth in oxygenated environments 2.1 Gyr ago, Nature, 466 (7302) 100-104. DOI: http://dx.doi.org/10.1038/nature09166