The energy drives biochemical reactions including the combination of water and carbon dioxide to make organic matter.

Scientists became convinced that chloroplasts (below right), like mitochondria, evolved from symbiotic bacteria — specifically, that they descended from cyanobacteria (above right), the light-harnessing small organisms that abound in oceans and fresh water.

When one of her professors saw DNA inside chloroplasts, Margulis was not surprised.

In 1970 she published her argument in The Origin of Eukaryotic Cells.

In the 1970s scientists developed new tools and methods for comparing genes from different species.

In the late 1960s Margulis (left) studied the structure of cells.

Mitochondria, for example, are wriggly bodies that generate the energy required for metabolism. She knew that scientists had been struck by the similarity ever since the discovery of mitochondria at the end of the 1800s.

The DNA in mitochondria, meanwhile, resembles that within a group of bacteria that includes the type of bacteria that causes typhus (see photos, right).

Margulis has maintained that earlier symbioses helped to build nucleated cells.

“I'm joking about that, of course, but the point is that it's a relatively easy thing to fix.” The EDF, a Texas-based non-profit organisation, said the gas industry must be alert to leaks.