Biology: What is an individual?

Quanta Magazine said:

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.

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.

Yet being able to make such distinctions is extremely important to scientists. Ecologists need to recognize individuals when disentangling the complex symbioses and relationships that define a community. Evolutionary biologists, who study natural selection and how it chooses individuals for reproductive success, need to figure out what constitutes the individual being selected.

The same applies in fields of biology dealing with more abstract concepts of the individual — entities that emerge as distinct patterns within larger schemes of behavior or activity. Molecular biologists must pinpoint which genes out of many thousands interact as a discrete network to produce a given trait. Neuroscientists must determine when clusters of neurons in the brain act as one cohesive entity to represent a stimulus.

“In a way, [biology] is a science of individuality,” said Melanie Mitchell, a computer scientist at the Santa Fe Institute.

And yet, the notion of what it means to be an individual often gets glossed over. “So far we have a concept of ‘individual’ that’s very much like the concept of ‘pile,’” said Maxwell Ramstead, a postdoctoral researcher at McGill University. “If there’s a pile of sand, you intuitively know this is a pile of sand. But a pile is not a precisely defined thing. It’s not like after 13 grains, it moves from a collection to a pile.”

Such a fundamental definition is absent in part because “biology as a field is completely under-theorized,” said Manfred Laubichler, a theoretical biologist at Arizona State University. “It’s very much still an empirically driven discipline.”

Now, a few groups of scientists are setting out to change that — and they’re beginning by formalizing the concept of the individual according to a set of principles and measurements that they hope will guide biology into a new era.

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