How did the division of labor emerge in animals and humans? Little-known organisms hold clues

Banner image: A colony of fluted bryozoans (Hippodiplosia insculpta) off the coast of California. (Credit: CC image by Ed Bierman via Wikimedia Commons)

Human societies have long depended on the division of labor to thrive-walk into any town, and you can probably find bakers baking bread, truck drivers driving that bread to market and grocers selling it.

A new fossil study from paleobiologists at the CU Museum of Natural History offers insight on how some colony-forming animals, like ants or bees, may have evolved their own system for divvying up work millions of years ago. The team's findings suggest that when colonies share resources like food efficiently, it can free up some individuals to evolve never-before-seen traits.

The research taps into a universal drive for both tiny animals and human beings, said Sarah Leventhal, lead author of the new study published last month in the journal PNAS Nexus.

"If we look around us, this division of labor exists almost everywhere," said Leventhal, who earned her PhD in geological sciences from CU Boulder this year. "But before this study, it hadn't really been investigated using the fossil record."

Floating time capsules

The study casts a spotlight on a little-known group of aquatic animals called bryozoans. They tend to be small and sometimes look like globs of mucus floating in the water. They also grow in thin sheets over surfaces like corals and piers. They're not really individuals: Bryozoans are made up of dozens to thousands of microscopic "zooids" that live together in a single, cohesive colony.

In the new research, the team assembled fossils from a short but critical period in the evolution of bryozoans belonging to the genus Wilbertopora-roughly from 103 to 96 million years ago, or during the reign of dinosaurs.

The researchers discovered that during that window, some zooids in bryozoan colonies began to lose the ability to feed themselves. But they didn't die off. Instead, other members of the colony likely shared resources to keep those evolutionary oddballs alive. In the process, these new zooids were able to evolve a wide range of never-before-seen abilities.

"Despite this loss in feeding, those non-feeding members could still be supported by the colony without costing it much," Leventhal said. "This gave them the freedom to explore all those different functions."

Specialized workers

Leventhal noted that, among modern bryozoans, most zooids in a colony, which are genetically identical, tend to look the same-they're shaped like tiny shoe boxes with a round hole at the top. But a few strange zooids, known as "avicularia," don't fit the mold.

In some species, for example, avicularia have evolved appendages that look a bit like the head of a flamingo. They use them to poke at animals that get too close to their colonies.

"You can find some Caribbean colonies that, to avoid getting buried by sand, use their avicularia to crawl over the sea floor," Leventhal said.

To date, however, scientists have struggled to explain how such a division of labor could emerge in natural populations. One popular explanation borrows from human economics. In early human societies, the thinking goes, every individual did a little bit of everything: They cooked. They cleaned. They hunted and gathered, and on and on. But over time, some individuals became specialists in one job (say, hunting), while others did something else (say, cooking). In the same way, ancient animals may have evolved distinct jobs by becoming specialists at tasks their ancestors were already doing.

In their new research, Leventhal and her colleagues decided to put that idea to the test.

The group assembled roughly 120 fossils of bryozoan colonies from the Smithsonian Institution's National Museum of Natural History. The Smithsonian's Stewart Edie was also a co-author of the new study.

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Leventhal explained that bryozoans began to evolve avicularia roughly 103 million years ago. But in the blink of an eye-evolutionarily speaking-the holes at the top of those zooids began to take on shapes and sizes unlike anything seen in their ancestors. (Some, for example, became spatula-shaped, not round). Many of these new zooids also lost the ability to feed.

Put differently, these organisms weren't just specializing like the conventional wisdom might suggest. They were becoming something entirely new.

"It's like, all of a sudden, they discovered these new possibilities, and they just ran with them," said study co-author Carl Simpson, curator of invertebrate paleontology at the CU Museum of Natural History and assistant professor in the Department of Geological Sciences.

To find out how they managed to survive, study co-author Rebecca Morrison, assistant professor of computer science at CU Boulder, developed a series of mathematical simulations. They mimicked how nutrients and other resources may have cycled through these ancient bryozoan colonies. The group discovered that normal zooids could easily have collected enough nutrients to sustain their entire colonies, including the non-feeding avicularia. Those avicularia, in turn, may have played a critical role in keeping food moving through the colony's digestive system.

The researchers can't be sure whether other colony-forming animals, such as ants and bees, underwent similar evolutionary pathways. Simpson, for his part, recently teamed up with a group of archaeologists to study how the division of labor may have emerged in a completely different group of animals: humans.

"We get together and say, 'We can't believe we're studying the same problem,'" Simpson said.

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