All Life Descended From Sulfur-Eating Bacteria, Biologist Says

All life on Earth descended from sulfur-eating bacteria that lived 3.5 billion years ago in near-boiling water and emitted a stench like rotten eggs, according to a new analysis by a controversial UCLA molecular biologist.

The bacteria, which were very similar to those that now proliferate around hot-water vents on the ocean floor, were not the first form of life to appear on Earth, James A. Lake reported in a paper in this week’s Nature magazine. But “it’s as far back as we can trace our common ancestry,” he said. “There’s a little bit of (that bacteria) in everything alive today.”

Lake thus joins a small but growing number of researchers, led by Wolfram Zillig of the Max Planck Institute in Munich, West Germany, who have departed from the long-held belief that the earliest bacteria grew in much cooler pond water and drew energy not from sulfur but from a rich soup of carbon compounds created by lightning and other natural phenomena.

Scientific Controversy

But Lake’s report has created a storm among microbiologists because his analysis also indicates that all life on Earth should be divided into five major groups rather than the three groups commonly accepted by scientists today.

Even those researchers who agree with Lake that the most recent ancestor of all life on Earth was a heat-loving, sulfur-eating bacterium contend that the methods he used to reach that conclusion are flawed and that he has not proved his case.

And they strongly reject Lake’s contention that life should be divided into five groups.

“I just don’t find his arguments particularly credible,” said molecular biologist Norman Pace of Indiana University. “He is the only one who wishes to do this (break life into five groups),” said microbiologist Carl Woese of the University of Illinois at Champaign-Urbana.

Lake reached his conclusions by studying evolutionary changes in cellular structures called ribosomes, which manufacture proteins under instructions from the cell’s genetic blueprint.

Although the primary genetic blueprint of the cell is composed of deoxyribonucleic acid (DNA), the blueprint for the ribosome is composed of ribonucleic acid (RNA) that is separate from the rest of the cell’s genetic material. Because the ribosome has the same function in all living organisms, scientists believe that it has changed less over time than has the DNA.

That change occurs when individual components of RNA or DNA, called nucleotides, are mutated (altered) by sunlight, chemicals or mistakes in copying. If two species of bacteria split apart from a single ancestor a million years ago, for example, the composition of their RNA would be expected to be quite similar.

But if the two bacteria split apart hundreds of millions of years ago, each would have had much more time to mutate independently, and the compositions of their RNAs would be much different.

Geneticists and anthropologists have concluded that such differences in composition provide a valuable way to tell how long ago two different animals--or two different species--had a common ancestor.

A year ago, for example, geneticist Allan Wilson of the University of California at Berkeley reported that he had used the same technique on deoxyribonucleic acid (DNA) from a structure in women’s cells to suggest that all humans were descended from a common ancestor who lived in northern Africa about 200,000 years ago. He has also shown that human and chimpanzee lineages split apart from that of baboons 30 million years ago, while humans and chimps split apart only 5 million years ago.

But Lake is trying to show relationships that existed 3.5 billion years ago rather than a few million, and that is more difficult. One problem is that the rate of mutation of the ribosomes is different in different species--for reasons that are yet unknown.

Lake developed a sophisticated computer program that, he said, eliminates the bias caused by different mutation rates.

Lake compared the already-known compositions of ribosomal RNA from 32 different modern-day species, including bacteria, humans, toads, corn, yeast and rats. The computer made more than 1.4 million comparisons of RNA to produce a family tree of life, showing approximately when various families branched off. At the base of the tree was the heat-loving, sulfur-eating bacteria.

But critics object to these results because the analysis also divides all life into five major groups, whereas the vast majority of biologists recognize only three. The largest group is the eukaryotes, which are organisms, including all plants and animals, whose cells have nuclei from which life processes are controlled.

The second group is the prokaryotes, which are organisms, primarily bacteria, that do not have nuclei.

The third group, discovered only about 20 years ago, is the archaebacteria. This group includes three major types of bacteria: those that convert organic compounds into methane, those that live in salty environments such as marshes and those that eat sulfur and love heat.

Three-Way Division

Lake has argued for several years that the last group should be divided into three separate families, which he calls methanogens, halobacteria and eocytes (a term coined by Lake, meaning “dawn cells”). He believes that eocytes are much more closely related to eukaryotes than to other archaebacteria.

The conclusion that eocytes are closely related to eukaryotes is “absolutely wrong,” Woese said. “The vast bulk of scientific evidence shows exactly the opposite,” he added.

In an editorial in the same issue of Nature, zoologist David Penney of Massey University in New Zealand conceded that more work is needed to validate Lake’s technique. “The prize is not whose (evolutionary) tree is correct,” he wrote, “but a better understanding of the last common ancestor of all living organisms and, by implication, elucidation of the origin of life.”