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It's responsible for the properties of materials like wood and cotton and is the primary component of dietary fiber, so it's hard to overstate its importance to humanity.
Given its ubiquity and the fact that it's composed of a bunch of sugar molecules linked together, its toughness makes it very difficult to use as a food source.
The animals that manage to extract significant calories from cellulose typically do so via specialized digestive tracts that provide a home for symbiotic bacteria—think of the extra stomachs of cows and other ruminants.
"Our evolutionary analysis strongly suggests that [these strains] likely originated in the ruminant gut and later transferred to humans, possibly during domestication," the researchers conclude.
For example, the strains found in primate guts often had genes that enabled the digestion of chitin, a different polymer that is a major component of insect exoskeletons.
By contrast, some human-specific strains had enzymes that can efficiently break down the cell walls of plants like rice, wheat, and corn.
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