In addition to mitochondria, cells of algae and green plants also contain plastids, the organelles responsible for photosynthesis. These, too, were once independent organisms, and now researchers believe they have identified the host and symbiont that are the ancestors of all species of algae and plants. It's a hugely exciting piece of work, with equally huge implications. DNA sequencing shows that the plastid of a species of glaucophyte, a small group of obscure, microscopic blue-green algae, retains genes associated with early cyanobacteria, the photosynthetic bacteria from which plastids are believed to have evolved. Comparison with the gene maps of a variety of plastids suggests not only that all algae and plants evolved from a single symbiotic event, but also that another organism was involved: 'the DNA includes genes similar to those from ancient bacteria similar to the Chlamydiae bacteria.' If the hypothesis is correct, the bacteria (which were probably some kind of parasite) have all but vanished, leaving only a few of their genes in plastids, a little like words from the languages of long-vanished civilisations that live on in English and other modern languages. It isn't a unique phenomenon - one of the more unexpected results of the human genome project was the discovery that genes from retroviruses make up something like 8 per cent of the human genome. We are the expression of texts from many sources.