In the guts of vertebrates, a thin layer of mucous protects the rest of the body from harmful bacteria. Most invertebrates have an intestinal membrane composed of chitin.

Both strategies guard against infection, but scientists have struggled to determine which mechanism evolved first and how they relate to each other. For clues, researchers looked to tunicates, a group of filter-feeding marine invertebrates.

Tunicates, like sea squirts, deploy both a layer of chitin and mucous.

"We noticed that tunicates seemed to have an intermediate kind of gut lining that could show that mammalian and invertebrate gut linings shared an evolutionary link," Keisuke Nakashima, marine biologist at the Okinawa Institute of Science, said in a news release.

A survey of intestinal defense strategies showed lancelets, jawless fish and a few bony fish species are capable of producing chitin in their guts. Scientists determined only mammals are incapable of producing chitin.

After cellulose, chitin is the most abundant biomaterial. Inside the guts of the tunicate species Ciona intestinalis, chitin takes the form of nanoscale fibers. Analysis of the sea squirt guts showed chitin fibers don't operate entirely on their own. They're rooted in a surface mixture featuring gel-forming mucin. Scientists determined mammalian gut mucous is a derivative of the primitive mucin.

Scientists used their study of sea squirt guts to develop a new evolutionary model. The comparative analyses of intestinal defenses in chordates showed species have deployed different combinations of chitin and mucous over time.

The loss of chitin in more advanced vertebrates allowed mucous to develop its role as the primary defense against microbes in the guts of mammals. Mucous works like a decoy, allowing microbes to colonize the protective layer, leaving the cell walls to function infection-free.

Researchers described their findings Friday in the journal Nature Communications.

"These data suggest that chitin-based barrier immunity is an ancient system, the loss of which, at least in mammals, provided mucus layers as a novel niche for microbial colonization," researchers wrote. "These findings provide a missing link for intestinal immune systems in animals, revealing disparate mucosal environment in model organisms and highlighting the loss of a proven system as innovation."

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