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Partners in Slime

The first eukaryotic cells--the complex cells dotted with organelles that make up all "higher" organisms--may have arisen from bacteria with an appetite for the waste products of their neighbors. The new hypothesis, reported this week in Nature, suggests that eukaryotic organisms arose not from a haphazard accident but from a beneficial relationship that was a matter of survival.

Most textbooks say that the first complex cell was a predator, which evolved the ability to eat other bacteria. It gained its organelles--like the energy-producing mitochondria--when some of its prey happened to escape digestion and took up permanent residence. But recent analysis of the genes of certain single-celled eukaryotes called protists hint that eukaryotic cells might have acquired their mitochondria before they had evolved the ability to engulf other cells.

A possible explanation came to biochemist William Martin of Technische Universität in Braunschweig, Germany, one evening when he looked at a picture of a protist called Plagiopyla. These one-celled eukaryotes play host to hydrogen-eating bacteria called methanogens. The methanogens cluster near hydrogen-producing organelles called hydrogenosomes, which are thought to be related to mitochondria. Martin realized that what he saw inside the protist--the partnership of the organelle and the methanogens--might be similar to the union that had led to the first eukaryote.

He consulted with Miklós Müller of Rockefeller University in New York City and they concluded that a partnership between an ancestral methanogen and a hydrogen-producing eubacterium could have led to the first complex cell. They propose that the relationship started casually, in an oxygen-free, hydrogen-rich environment. The microbial pair--one partially surrounded by the other--later found itself far from that original environment, where the methanogen could not survive alone. Then, Martin and Müller suggest, an exchange of genes cemented the partnership, allowing the host bacterium to enclose its guest completely. The new genes enabled the hydrogen-dependent methanogen to import small molecules, make sugars, and break them down into food for the enclosed hydrogen-producer.

The new idea "is elegantly argued," says Michael Gray of Dalhousie University in Halifax, Nova Scotia, but he and others say they're not ready to abandon the classic theory. The debate might be resolved if researchers can find direct descendants of the earliest eukaryotes, say Martin and Müller. The best place to look for what might be the ancestor of us all, says Müller, are environments that are "foul-smelling, muddy, or inside of a digestive tract."