What is a wasp?” might seem like an overly simple question for a Ph.D. biologist to be asking. “What is a human?” Even more so.

But these are strange times in the life sciences. Seth Bordenstein of Vanderbilt University in Nashville now embraces the notion that each wasp he studies, each squirrel darting around campus — not to mention himself, every reader of science magazines and every other representative of see-it-without-a-microscope life on Earth — is really a blend of one big organism and a lot of little ones.

In recent years, research has shown that what people commonly think of as “their” bodies contain roughly 10 microbial cells for each genetically human one. The microbial mass in and on a person may amount to just a few pounds, but in terms of genetic diversity these fellow travelers overwhelm their hosts, with 400 genes for every human one. And a decent share of the metabolites sluicing through human veins originates from some microbe. By these measures, humanity is microbial.

But numbers are just the beginning.

The evolutionary impact of animals’ microbial denizens can be substantial. Adult wasps of the genus Nasonia are only about 30 percent microbial, Bordenstein estimates. But those microbes keep two species apart that could otherwise interbreed.

Some researchers think of these microbes as just another part of a plant or animal’s environment, like a mountain range that keeps two related species separate. But, with a squint and a slap to the worldview, researchers like Bordenstein are exploring whether a body’s microbes are so intimate that they’re part of the organism itself. Or, if you prefer, the metaorganism.

“Ecosystem” is the word that 26 scientists used in a call for new thinking about animal-bacteria interactions that was published in February by the Proceedings of the National Academy of Sciences. The recent accumulation of knowledge about bacteria vis à vis their animal hosts “is fundamentally altering our understanding of animal biology,” the group declared.

Why would biologists get so excited about teeming microorganisms now? Even someone who missed the earliest fiddling with magnifying lenses has had 330 years to catch up on volume 14 of the Royal Society’s Philosophical Transactions, wherein merchant microscopist Antonie van Leeuwenhoek reported “to my great surprise,” that watered-down scrapings from his teeth revealed “very many small living Animals, which moved themselves very extravagantly.”