Dogs may have started as freeloaders at the edge of human camps
Commensalism, a word rooted in the Latin for sharing a table, describes a partnership where one species gains and the other barely notices. Many researchers think the dog began exactly that way: bolder, calmer wolves scavenging around hunter-gatherer camps long before anyone set out to tame them.
The Belgian zoologist Pierre-Joseph van Beneden coined the term in 1876. It sits alongside related arrangements: in mutualism both partners profit, in parasitism one gains at the other's expense, and in amensalism one suffers while the other is untouched. In the commensal case the beneficiary typically is the smaller animal, often heavily adapted, while its host stays unchanged. Remoras cling to sharks and eat their droppings, pilot fish pick at leftover scraps, and birds snap up insects flushed by grazing herds.
The dog was the first animal humans domesticated, well before farming. A dog buried at Bonn-Oberkassel about 14,000 years ago supports the idea that this happened near the last glacial peak, when people hunted megafauna. The wolves likeliest to hang around camps were the less aggressive, lower-ranking ones, slower to flee and less stressed by people. Such animals could have fed on leftover carcasses, and the first physical signs of domestication, shorter snouts, crowded and smaller teeth and more juvenile skulls, may have appeared during that freeloading stage, before humans took an active role. How far wolves relied on this life without being fed deliberately is still argued over. Cats may have gone further, living fully off human settlements by hunting the rats and mice drawn to them.
Our bodies host commensals too. Staphylococcus aureus, better known for its dangerous strains, lives harmlessly on the skin of roughly 20 to 30 percent of people, settling in the nose, mouth and ear canal. The hardy fungus Aspergillus can survive in the harsh chemistry of the upper gut.
Microbes also live off each other's waste. In nitrification, bacteria such as Nitrosomonas turn ammonium into nitrite, and Nitrobacter then draws energy by converting that nitrite into nitrate. Spoiling milk shows a similar relay, as acid made by early fermenters lets more acid-tolerant species take over.
Source: Commensalism
