One of the reasons that hydrogenosomes are of interest is because of the light they can cast on how life on earth may have evolved. Most life on earth is single celled, like Bacteria and Archaea. By contrast, plants and animals are made up of many cells. Crucially, each cell of a plant or animal is organised very differently from a bacterial cell. Amongst other things, animal and plant cells (which are special cases of what is known as 'eukaryotic' cells) contain mitochondria.