The common evolutionary ancestor of all species living today may have been a bacterium that dwelt in boiling-hot springs and fed on dissolved sulfur compounds, a biologist has proposed after comparing certain attributes of today's species.

The suggestion, reported in last week's Nature, contrasts with the widely, but not universally, held supposition that the earliest life forms inhabited more moderate climates and fed on a rich broth of organic compounds.

The ancient ancestor, according to James A. Lake of the University of California at Los Angeles, may have resembled organisms called eocytes, which means "dawn cells," bacteria that today live in hot sulfur springs.

Lake reached his conclusions after studying a form of nucleic acid, ribosomal RNA, that is part of an intracellular structure called the ribosome. All cells, from the most primitive bacteria to human cells, have ribosomes. Their function is to assemble protein molecules according to instructions encoded in the cell's genes.

Like a gene, ribosomal RNA is made of a particular sequence of smaller units. In closely related species, this sequence is highly similar. But as the evolutionary distance between species grows, so do the differences among ribosomal RNA sequences.

Lake obtained the sequences from 32 different species and worked out a family tree that showed varying degrees of closeness of evolutionary relationship. At the base of the family tree were the eocytes, microbes whose ribosomal RNA sequences appear close to what must have been the ancestral form.