Oxygen triggered the evolution of complex life forms

Oxygen played a key role in the evolution of complex organisms, according to new research published in BMC Evolutionary Biology. The study shows that the complexity of life forms increased earlier than was thought, and in parallel with the availability of oxygen as an energy source.

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.

The authors of the study write: “The results support a deep history for complex multicellular eukaryotes, and implicate oxygen as a possible trigger for the rise in complex life.”

To calculate when the different groups of organisms diverged, the researchers compared the sequences of nuclear proteins from a wide range of different organisms using all the available molecular dating methods. All the methods gave similar results.

The pattern and timing of the rise of complex multicellular life during the history of the Earth has not been firmly established. There are large differences between the history suggested by the fossil record, and that estimated using DNA and protein sequence data.

Molecular dating has some obvious advantages over the fossils, however. Hedges said: “This type of information is very difficult to obtain from the fossil record of early life. However the genomes of organisms are packed with millions of bits of data that biologists are now beginning to decipher, and some of those data can be used to tell time.”