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July 5, 2010

The Jump to Multicellular

How life on this planet began is something I've discussed multiple times in the past, for instance here. This wasn't the only major milestone in the evolution of life as we know it, however. The step from unicellular creatures to multicellular ones is on par with the development of life itself, both in importance and in how little we understand it. Obviously, by the Cambrian explosion, life had made the jump from single-celled creatures to multicellular animals: this was about 550 million years ago. However, this was most certainly not the first time multicellular creatures appeared on the young planet Earth. A new discovery sets the date far back, to about 2.1 billion years ago.

The new species, found in Gabon
At a first glance, these new fossils don't seem that impressive. They look a bit like a squashed seashell; each is flat disc, around 5 inches long, with small slits and ridges along the edges. However, only a few other complex organisms, including Grypania spiralis, have been found at such an age (Grypania is about 2.0 billion years old). This new fossil demonstrates that Grypania was not as much of an exception as previously though. If other multicellular species existed around the 2 billion year old mark, then perhaps it wasn't so much a random accident as an evolutionary trend, arising from changing conditions on planet Earth. This is known as convergent evolution. In similar environments, filling a similar role in the ecosystem, two unrelated species may develop very similarly, as that is the ideal form to fill that role. Dolphins and sharks could be an example of this today.

For the first complex creatures, 2 billion years ago, the factor that seems to have changed is oxygen. A few million years prior (barely any time at all in geologic history), oxygen levels spiked. The development of photosynthesis proved highly successful, and the content of the atmosphere changed as single-celled, algae-like organisms began consuming carbon dioxide and releasing oxygen. This in turn changed the temperature of the planet, the chemistry of the air and ocean, and created a lot of evolutionary pressure. In modern environments, when bacteria are under these sorts of stressors, they tend to clump together, communicating and sharing resources. Paleontologists suspect that something similar happened 2 billion years ago, to the point where the bacteria became so interdependent that they became a single organism, instead of many separate ones. If this is true, then they expect that more of these little disc-shaped creatures will be found in ancient rocks around the globe. It's certainly a viable explanation, and takes us one step closer to solving a great mystery in the evolution of life as we know it.

Source: Wired Science- 2-Billion-Year-Old Fossils May be Earliest Known Multicellular Life