SCIENCE / BIOLOGY : New Data Backs Disputed Theory on Origin of Life
A controversial UCLA molecular biologist has discovered new evidence supporting his long-held theory that all life on Earth descended from bizarre heat-loving, sulfur-eating bacteria that lived 3.5 billion years ago and emitted a stench like rotten eggs.
Those organisms, close relatives of which survive in hot water vents on the ocean’s floor and in geysers at Yellowstone National Park, are the closest living bacterial relatives of humans and other animals and plants, James A. Lake says today in the journal Science.
The evidence, based on analysis of the structure of a key protein, provides strong support for Lake’s unorthodox view that all life on Earth should be divided into four major groups rather than the three groups that are accepted today.
Lake and others are studying the relationship between bacteria and other forms of life because the results could offer insight into the processes by which life began on Earth and by which it evolved into today’s complex ecology.
Lake’s discovery “makes a new split in the tree of life,” said UCLA paleontologist Bruce Runnegar.
Other researchers dispute his results. “The weight of the evidence supports an alternative” to Lake’s view, said microbiologist Gary Olsen of the University of Illinois in Champaign-Urbana. “His statistics show that his results could have occurred by chance alone, that it was just a fluke.”
Molecular biologist Norman Pace of Indiana University in Bloomington is more blunt. “Jim takes idiosyncrasies in a much larger body of data and weaves tales on those idiosyncrasies. . . . That kind of flaky data is nothing to root the big tree of life on.”
But Lake’s results will not be dismissed easily. Molecular evolutionist Jeffrey Palmer of the University of Illinois said Thursday that he will soon report much more substantial data that supports Lake’s view of the linkage between the sulfur-loving bacteria and plants and animals. He notes that his and Lake’s evidence are “completely opposite” from what other researchers have found.
“There is a direct and fairly striking conflict” in the data, Palmer said, “that is not readily resolved. It’s going to take a lot more effort before we know exactly what is going on.”
Conventional wisdom holds that the earliest bacteria on Earth grew in cool pond water or tidal pools and drew energy from a rich soup of carbon compounds created by lightning and other natural phenomena. But Lake and a minority of other researchers believe that life originated in pools of near-boiling water and drew its energy from sulfur-containing compounds.
Conventional wisdom also divides all life into three major groups. The largest group is the eukaryotes, which are organisms, including all plants and animals, whose cells have nuclei from which all life processes are controlled.
The second group is the prokaryotes, which are organisms, primarily bacteria, that do not have nuclei.
The third group, discovered about 25 years ago, is the archaebacteria. This group includes three major types of bacteria: those that convert organic compounds into methane (methanogens), those that live in salty environments such as marshes (halobacteria), and those that eat sulfur and love heat (eocytes, a name meaning “dawn cells”).
Lake argues that the eocytes, because of their apparent close relationship to eukaryotes, should be considered a class by themselves. Lake’s work, conducted with graduate student Maria C. Rivera, suggests that prokaryotes and other archaebacteria branched off from the tree of life long before eukaryotes and eocytes diverged from a common ancestor.
Lake and Rivera focused on a protein, called Ef-Tu, that is present in virtually all species. Proteins are composed of about 20 simple chemicals, called amino acids, that are put together in strict order to form proteins in the same way that the 26 letters of the alphabet are put together to form words.
The researchers found that, at one crucial location, samples of Ef-Tu from eocytes and from all eukaryotes that they have studied contain a similar sequence of 11 amino acids. The same was not true for the other life forms. The inescapable conclusion, Lake says, is that “eocytes are the closest relative to eukaryotes. . . . The evidence is black and white.”
“If you go back 3 million generations,” he said, “we are distantly related to eocytes in the same way that we are related to our parents and grandparents.”
