What Is an Individual? Biology Seeks Clues in Information Theory.

To recognize strange extraterrestrial life and solve biological mysteries on this planet, scientists are searching for an objective definition for life’s basic units.

More than half a billion years ago, during the Ediacaran Period, a surreal world of life overran the ocean floor. Its bizarre, soft-bodied animals had physical forms that defy the imagination: quilted blobs and ribbed discs, segmented tubes and upturned bells, tapered spindles and slender cones. They were perhaps the planet’s first large multicellular organisms — but they soon went extinct without leaving behind any modern descendants; trace fossils in ancient slabs of sandstone and quartzite are all that remain of those utterly weird and fantastical creatures.

Because of that weirdness, paleontologists still debate even the most basic questions about them: how they developed, how they ate and reproduced, even where one fossilized individual leaves off and another begins. Were those animals single organisms or colonies of smaller individuals, akin to the Portuguese man-of-war? Where did their jellylike bodies end and their environment begin?

The task of distinguishing individuals can be difficult — and not just for scientists aiming to make sense of a fragmented fossil record. Researchers searching for life on other planets or moons are bound to face the same problem. Even on Earth today, it’s clear that nature has a sloppy disregard for boundaries: Viruses rely on host cells to make copies of themselves. Bacteria share and swap genes, while higher-order species hybridize. Thousands of slime mold amoebas cooperatively assemble into towers to spread their spores. Worker ants and bees can be nonreproductive members of social-colony “superorganisms.” Lichens are symbiotic composites of fungi and algae or cyanobacteria. Even humans contain at least as many bacterial cells as “self” cells, the microbes in our gut inextricably linked with our development, physiology and survival.

Fossil of a rangeomorph. Rangeomorphs are odd sea animals that lived during the Ediacaran Period. Studies that sought to determine what guided the growth of their fernlike bodies drew on ideas similar to recent mathematical theories about biological individuality.

J. Hoyal Cuthill These organisms are “so intimately connected sometimes that it’s unclear whether you should talk about one or two or many,” said John Dupré, a philosopher of science at the University of Exeter and director of Egenis, the Center for the Study of Life Sciences.