Is an ant colony one creature or thousands? Biologists still argue
A siphonophore looks and behaves like a single jellyfish, yet it is built from many semi-independent units called zooids. An ant nest has workers that never breed, much like body cells. Saying what counts as one organism turns out to be surprisingly hard, and some biologists wonder whether the concept is needed at all.
The word organism comes from the Greek organon, meaning tool or instrument, and entered English in the 1660s, first meaning any organized structure. Immanuel Kant, in his Critique of Judgment of 1790, described an organism as a being both organized and self-organizing. The simplest modern test is that an organism reproduces, grows and runs its own metabolism. That rule leaves out viruses, which evolve but borrow a host cell's machinery to copy themselves.
Other proposed tests each run into trouble. Richard Dawkins suggested an organism is varied enough in form that cutting it in half would stop it working, yet many creatures regrow whole from pieces. Some point to genetic uniqueness, an immune response that marks self from other, or Erwin Schrödinger's idea of life as something that maintains order against decay. One view says these traits should be scored separately, giving several kinds of biological individual instead of one.
Evolutionary biologists David Queller and Joan Strassmann propose cooperation as the defining trait: organisms arose as simpler units, from cells upward, learned to work together without conflict. By that measure a lichen, a partnership of fungus and alga or cyanobacteria that can thrive on bare rock where neither partner could alone, qualifies as an organism. So does the permanent mating pair of an anglerfish. In 2016 Samuel Díaz-Muñoz and colleagues added that organism status then depends on context and can shift over evolutionary time.
The philosopher Jack Wilson lists sponges, slime moulds and colonies of naked mole-rats among the cases sitting between colony and individual. Engineers now add new puzzles, building chimaeras from the cells of several species and hybrots that combine living tissue with electronics.
Source: Organism
