|The new species, found in Gabon|
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