Some people look to the stars. Some look to themselves. I’ve always been interested in the cosmic goings on of DNA and RNA and their minuscule hosts.
According to science writer legend Nicholas Wade of The New York Times life first emerged on Earth via a single-cell, bacterium-like organism, known as Luca, the Last Universal Common Ancestor, and is estimated to have lived some four billion years ago, when Earth was a mere 560 million years old.
The new finding sharpens the debate between those who believe life began in some extreme environment, such as in deep sea vents or the flanks of volcanoes, and others who favor more normal settings, such as the “warm little pond” proposed by Darwin.
The nature of the earliest ancestor of all living things has long been uncertain because the three great domains of life seemed to have no common point of origin. The domains are those of the bacteria, the archaea and the eukaryotes. Archaea are bacteria-like organisms but with a different metabolism, and the eukaryotes include all plants and animals.
Specialists have recently come to believe that the bacteria and archaea were the two earliest domains, with the eukaryotes emerging later. That opened the way for a group of evolutionary biologists, led by William F. Martin of Heinrich Heine University in Düsseldorf, Germany, to try to discern the nature of the organism from which the bacterial and archaeal domains emerged.
Their starting point was the known protein-coding genes of bacteria and archaea. Some six million such genes have accumulated over the last 20 years in DNA databanks as scientists with the new decoding machines have deposited gene sequences from thousands of microbes.
Genes that do the same thing in a human and a mouse are generally related by common descent from an ancestral gene in the first mammal. So by comparing their sequence of DNA letters, genes can be arranged in evolutionary family trees, a property that enabled Dr. Martin and his colleagues to assign the six million genes to a much smaller number of gene families. Of these, only 355 met their criteria for having probably originated in Luca, the joint ancestor of bacteria and archaea.
Genes are adapted to an organism’s environment. So Dr. Martin hoped that by pinpointing the genes likely to have been present in Luca, he would also get a glimpse of where and how Luca lived. “I was flabbergasted at the result, I couldn’t believe it,” he said.
The 355 genes pointed quite precisely to an organism that lived in the conditions found in deep sea vents, the gassy, metal-laden, intensely hot plumes caused by seawater interacting with magma erupting through the ocean floor.