Meet our long-lost cousin, whom we lost sight of billions of years ago. The family resemblance is far from obvious. For years, in fact, scientists doubted our kinship with this one-celled marine organism, which lives at depths of thousands of feet in scalding seafloor hot springs, feeding on hydrogen and giving off methane. They thought Methanococcus jannaschii and other archaea--microbes with similarly exotic habitats and tastes, such as a love of sulfur or salt--were just quirky bacteria. And bacteria are about as unrelated to us as living things can be. They make up their own branch of the family tree of life, a world apart from the branch that is home to more complex, eukaryotic cells--the ones that have a nucleus and that make up plants, animals, and people.
But the DNA evidence is in, and the relationship can’t be denied any longer. As a few researchers had been arguing ever since microbial evolutionist Carl Woese of the University of Illinois first suggested it in the 1970s, archaea may look like bacteria, but they actually belong on a third branch of the tree of life. And this branch may share a common origin with the eukaryotes.
The clinching evidence came this past year, when biologist Carol Bult and her colleagues at the Institute for Genomic Research in Rockville, Maryland, deciphered the microbe’s complete genetic sequence, consisting of 1.7 million genetic letters in its DNA. So far researchers have sequenced the genomes of three other organisms: two kinds of bacteria and a yeast, which is a eukaryote. But as soon as Bult and her colleagues began picking out individual genes from M. jannaschii’s string of letters, they knew they had their hands on something completely different. More than half the genes were unlike any seen before.
The microbe’s DNA, however, does show a few telling similarities to those of eukaryotes, says Woese, who worked with Bult to understand the new sequence. For example, the genes the microbe uses to copy genetic information from DNA and translate it into proteins are very similar to the ones we use. Says Bult, who is now at the University of Maine at Orono, You see very clear cases where these genes in archaea are much closer to eukaryotes.
If so, the first great evolutionary split, billions of years ago, must have been the one dividing bacteria and archaea. Only later did eukaryotes say good-bye to their strange relatives and set off on their own evolutionary path.