The nature of the gut tribes is heavily influenced by diet, according to a research team led by Ruth E. Ley and Dr. Jeffrey I. Gordon of the Washington University School of Medicine in St. Louis. With the help of colleagues at the San Diego and St. Louis Zoos, Dr. Ley and Dr. Gordon scanned the gut microbes in the feces of people and 59 other species of mammal, including meat eaters, plant eaters and omnivores. Each of the three groups has a distinctive set of bacteria, they report Friday in Science, with the gut flora of people grouping with other omnivores.

Despite the vast changes that people have made to their diet through cooking and agriculture, their gut bacteria “don’t dramatically depart in composition from those of other omnivorous primates,” Dr. Gordon said.

This new view of people as superorganisms has emerged from the cheap methods of decoding DNA that are now available. Previously it was hard to study bacteria without growing them up into large colonies. But most bacteria are difficult to culture, so microbiologists could see only a small fraction of those present. Analyzing the total DNA in a microbial community sidesteps this problem and samples the genes of all bacterial species that are present.

The goals of the human microbiome project include analyzing the normal makeup of bacterial species in each niche on the human body. “The focus in microbiology has been on pathogenic bacteria, but we are trying to identify the commensal bacteria so that we can begin to understand what proteins they make and how they contribute to our health,” Dr. Segre said.

Another goal is to understand how pathogenic bacteria manage to usurp power from the tribes of beneficial commensals in the skin or gut, causing disease.

The lifetime of an individual bacterium in the human superorganism may be short, since millions are shed each day from the skin or gut. But the colonies may survive for a long time, cloning themselves briskly to replace members that are sacrificed. Just where these colonies come from and how long they last is not yet known. Dr. David A. Relman of Stanford University has tracked the gut flora of infants and finds their first colonists come from their mother. But after a few weeks, the babies acquired distinctive individual sets of bacteria, all except a pair of twins who had the same set. Dr. Relman said he was now trying to ascertain if the first colonists remain with an individual for many years.

Taking a broad spectrum antibiotic presumably wreaks devastation on one’s companion microbiome. If the microbiome is essential to survival, it is perhaps surprising that the drugs do not make more people ill. Dr. Relman said that perhaps there were subtle long-term consequences that had not yet been identified. Much the same set of bacteria recolonize the gut after a course of antibiotics, he said, suggesting that the makeup of the colony is important and that the body has ways of reconstituting it as before.