Microscopic Life Unlike Any Other
EVOLUTION OF THE SPECIES
BOSTON — It thrives in near-boiling temperatures and at extreme pressures. It eats hydrogen, nitrogen, and heavy metals in concentrations that would be fatal to most organisms. Yet oxygen is deadly to its system.
Scientists have now confirmed that this single-cell creature that was discovered 20 years ago, Methanococcos jannaschii, represents a group of microbes unlike any other life form on the planet.
Like architects unrolling a set of drawings, scientists have laid out the first complete genetic blueprint for this tiny, primitive creature. The gene-map proves, researchers say, that out of the Earth's primordial soup, there arose three, not two, branches of one-cell organisms that developed into all life on the planet.
This finding may not only help scientists trying to trace the evolution of life, including humans', to a "universal ancestor," but may also point to new clues in the search for life elsewhere in the universe. And it may lead to breakthroughs in areas ranging from renewable energy and pollution cleanup to chemical manufacturing.
The results, announced yesterday at a press conference in Washington and published in today's edition of the journal Science, are "like discovering a new continent," says J. Craig Venter, president of The Institute for Genome Research (TIGR), which spearheaded the effort. "It shows how little we really know about the diversity of life on Earth."
The subject of the excitement is a creature that thrives deep beneath the ocean near hydrothermal vents. There, mineral-laden water wells up from deep inside Earth's crust. Physically, its profile resembles a microscopic squid some 6-millionths of a meter long. Genetically, however, "two-thirds of the genes are unlike anything we have seen in biology," Dr. Venter says, referring to the molecules that carry instructions governing the organism's form and functions.
The large number of unknown genes settles a long-standing debate about whether a universal ancestor split into two or three branches around 3 billion years ago.
Initially, biologists divided sin-gle-celled organisms into two broad domains: those that have no central core, or nucleus, to harbor their DNA; and those that do. The coreless are classified as bacteria. Those with nuclei are classified as eukaryotes and form the basis for organisms ranging from protozoa and fungi to plants and humans.
In 1977, University of Illinois microbiologists Carl Woese and George Fox reported the discovery of a single-celled, methane-producing organism that, even by the more primitive analytical techniques of the day, looked genetically unique.
"I was blown out of my mind," Dr. Woese recalls, noting that the harder the two looked for genetic similarities with other bacteria, the fewer they found. This led the two to propose a third grouping of primitive single-celled organisms, known as archaea.
"Our proposal was rejected out of hand by most people," says Woese, who also is part of the research team that sequenced M. jannaschii's genome. Because archaea have no nuclei, many thought they were merely a bacterial oddity. Although his ideas have gradually gained wider acceptance as more archaea have been discovered, he says this most recent work "is the definitive test of our theory. And it passed. Period."
As a group, archaea are estimated to be as much as 50 percent of Earth's biomass. They thrive in a range of environments untenable for higher life forms. Archaea have been found in salt deposits, bogs, and volcanoes, for example, as well as deep undersea.
The particular species the TIGR team sequenced was discovered in 1983 as microbiologist John Leigh, then at the University of Illinois, analyzed material plucked from a deep-sea vent off Mexico.
Throughout 1995, the 36-member research team, which included scientists from TIGR, as well as from Johns Hopkins University and the University of Illinois, identified each gene's sequence of the four chemical bases that make up the DNA molecule encoding their unique sets of instructions.
Known as gene sequencing, the process gives researchers a detailed look at an organism's genetic blueprint.
As they compared M. jannaschii's genome with that of fully sequenced bacteria, they found striking similarities in the genes governing energy production and the use of nitrogen. When comparing it with sequences for Eukaryotes, M. jannaschii's genes governing expression of a gene's instructions and secretion "found their best match in genes from humans," Venter says.
Indeed, molecular biologists say this type of whole-genome comparison across the three major domains will allow them to track the evolutionary development of life at a molecular level just as fossils have allowed anthropologists and paleontologists to track that development on a larger scale.
The US Department of Energy, which funded this research effort, is interested in such organisms as renewable sources of natural gas. Because these organisms can "digest" heavy metals and convert them into other compounds, researchers are interested in them as potential pollution eaters.