In the largest study to date that does not focus on vertebrates, researchers from Pennsylvania State University used molecular dating methods to create a new timeline of eukaryotic evolution. By adding information about the numbers of different cell types possessed by each group of organisms, the researchers reconstructed how the complexity of life has increased over time. The study shows that organisms containing more varied cell types evolved following increases in atmospheric oxygen.
Professor Blair Hedges, who led the research team said: "To build a complex multicellular organism, with all the communication and signalling between cells it entails, you need energy. With no oxygen or mitochondria, complex organisms couldn't get enough of this energy to develop."
The study showed that organisms containing more than two or three different cell types appeared soon after the surface environment became oxygenated around 2,300 million years ago. This was around the same time that cells became able to extract the energy from oxygen, thanks to the emergence of mitochondria.
Life forms became even more complex following the evolution of organelles able to produce oxygen. Plastids, such as chloroplasts found in plants, evolved around 1,500 million years ago. During the following 500 million years, organisms that contained up to 50 different cell types evolved. These more complex organisms included algae, which would have benefited directly from being able to produce their own oxygen, and early animals and fungi, which could use this extra oxygen to provide energy for their development.