Evolutionary Theory Continues to Evolve, Fueled by Scientists' New
The world of undersea volcanoes could be the setting for a 1950s science fiction movie.
As deep as three miles beneath the ocean's surface, superheated liquid gushes from deep volcanic caverns and slams into the frigid waters near the ocean floor.
The collision creates thick, dense clouds of precipitated minerals called "black smokers."
It's hard to believe anything could live in such surroundings. But for newly discovered hyperthermophilic bacteria, it's the perfect home.
They flourish in the darkness of volcanic vents and in other steaming hot places devoid of oxygen. In fact, they can't even grow in the cooler temperatures where all other life thrives.
Extremely hot habitats harbor more than the mysteries of life at high temperatures. They may reveal secrets about the origin and evolution of life itself.
These climates are thought to closely resemble the formative environments where life on this planet first arose. The more than 20 species of hyperthermophilic bacteria collected and identified in the past decade are probably direct descendants of the early cellular organisms that inhabited the earth some three to four billion years ago.
"These hyperthermophiles are providing new insights into the classification of life on this planet," said UGA biochemist Mike Adams.
Scientific findings about these unique life forms are changing the way we understand and classify the world around us. In time, the textbooks we took for granted will be obsolete, replaced by the new theories scientists glean from research on hyperthermophiles.
What each of us learned about the tree of life depends in part on when we attended school. If you went to school during World War II you probably learned the "two-kingdom" theory: Living things were either animals or plants.
By the time the Vietnam war was underway, scientific researchers devised a new theory based on what life forms look like and how they grow. The result was that the tree of life sprouted three additional branches. Science instructors began teaching five kingdoms -- the monera, which are bacteria; the protista or protozoan-like organisms, the fungi, the plants and the animals.
Post-Cold War era students, however, will probably learn a system similiar to one devised by Carl Woese, a scientist at the University of Illinois who proposed dramatic changes to the classification system. In the late '70s, Woese devised a system that groups life into only three "domains" -- one of them brand-new.
All but one of the hyperthermophilic bacteria fall into Woese's new group. Originally named Achaeabacteria for "old bacteria," Woese changed the name to Archaea in 1990. The second group, Bacteria, includes those organisms, with few exceptions, that we commonly know to be "conventional" bacteria. The third group, Eucarya, encompasses all other life forms-- from club mosses and oak trees to amoebae, earthworms and humans.
Such a grouping may seem strange at first. But the old "plant-animal kingdom" idea encompassed a hodgepodge of vastly different organisms with few similarities. Yet, at the cellular level, all life forms except "bacteria" are more or less identical.
Since the invention of the microscope, scientists have known that two basic kinds of cells exist-- the "haves," like ferns, frogs and protozoa, whose complex and specialized cells contain genetic material enveloped in a membrane; and the "have-nots," the microscopic, one-celled bacteria that lack a defined nucleus.
Woese's proposed three-domain classification system recognizes this difference between the "haves" and "have-nots." And it is based not on the cells of an organism but on the nature of the molecules within the cells. For instance, the molecules of ferns, frogs and protozoa "are essentially identical yet very different from the molecules of conventional bacteria," Adams said.
Perhaps more importantly, the new system also recognizes that "bacteria" are of two distinct types: the conventional Bacteria and the unconventional Archaea, which includes the hyperthermophiles. This new system places hyperthermophilic organisms near the root of the tree of life, with Bacteria branching out in one direction and Eucarya sprouting more recently in another direction.
"These hyperthermophiles are the closest organisms we know of at present to some universal ancestor of all life," Adams said.
There is strong evidence for the new three-group theory. For example, of the thousands of species of living organisms, only a handful are known to use tungsten, rather than molybdenum, as an ingredient in some protein molecules.
"So far we have looked at five of the 20 or so genera of hyperthermophilic organisms that are known and all five use tungsten," said Adams. But tungsten has been found in only one "conventional" bacterium. The tungsten similarity could be just an anomaly of nature - or it could be an evolutionary link. No one knows for sure.
"If you look at the biochemistry and the physiology of the Archaea, they are very different from conventional bacteria," Adams said. "They have different cell walls, different genetic machinery, different fats and a range of unique enzymes.
"In essence, the only thing they have in common is that neither has a nucleus," he said.
And important similarities have been found between Archaea and Eucarya.
"The genetic machinery of the Archaea, the way they make their proteins and the structure of many of their molecules, is more similar to how we do it than how the Bacteria do it," he said.
One thing is for certain: Each new discovery from the science fiction frontier of undersea volcanoes adds a new piece to the puzzling theories of the origins and evolution of life.