Gassing up: oxygen's rise may have promoted complex life.

Life's long journey from single-celled organisms to multicellular plants and animals may have depended upon the increasing presence of oxygen in the atmosphere, a new study suggests.

S. Blair Hedges of Pennsylvania State University in State College and his colleagues reached this conclusion after correlating the number of cell types in dozens of classes of eukaryotes with the amount of oxygen in the atmosphere when those classes emerged in evolution. Eukaryotes include animals, plants, and other organisms that, unlike bacteria, store their DNA inside a cell nucleus.

Hedges and his colleagues used what scientists call the molecular-clock method to date the origin of each eukaryotic class. That is, knowing how quickly proteins typically evolve, the team used the current differences in related proteins between groups of organisms to estimate how long it's been since the groups shared an ancestor. For example, the researchers date the split between plants and animals at 1.6 billion years ago.

Early in the history of the planet, when only single-celled life forms such as bacteria were around, there was little oxygen in the air. About halfway through Earth's 4.6 billion years, oxygen began to rapidly accumulate in the atmosphere. The first eukaryotes with several different types of cells emerged at the same time, Hedges and his colleagues report in an article posted Jan. 28 to the online journal BMC Evolutionary Biology.

Driving this boost in complexity may have been mitochondria, which are components of eukaryotic cells and use oxygen to create energy. Mitochondria originated between 2.3 billion and 1.8 billion years ago, not long after the rise in oxygen began, the researchers say.

Hedges suggests that the development of mitochondria was the catalyst for the increase in cell types among eukaryotes. "It makes a lot of sense, because to have complex organisms, you need energy," he says.

From the dates of origin the scientists assigned to a wide range eukaryotes, Hedges' team also concludes that ones with up to 10 cell types had evolved by 1.5 billion years ago and those with up to 50 cell types had emerged by 1 billion years ago. Plants and animals have more than 100 cell types.

What prompted the leap in complexity starting 1.5 billion years ago? Hedges and his colleagues estimate that plastids, the components of cells that enable plants to produce oxygen through photosynthesis, originated around that time. This innovation quickly began pumping more oxygen into the atmosphere.

Fossil evidence has suggested that there was a major jump in eukaryotic complexity some 600 million years ago, during a period dubbed the Cambrian explosion. Hedges argues that his team's data indicate that this complexity boost occurred much earlier, soon after plastids emerged.

The new study is "an important next step in the study of complexity," says evolutionary biologist Daniel W. McShea of Duke University in Durham, N.C.