You may know that ruminants (e.g. cows, sheep) have a diet which is
heavy in cellulose, the tough material that makes up the cell walls of
plants. They can’t actually digest this material on their own;
instead, they have colonies of bacteria living in their guts, that are
capable of breaking this down into something the animals’ own
digestive systems can handle.
You can start to see why these herbivores have such complicated
digestive systems.
The assumption was, humans haven’t got anything like this. But in
2003, it was discovered that there are some kinds of
cellulose-digesting bacteria living in our own guts. Obviously not to
the same degree as in herbivores, of course. Some of these might have
crossed over to us from the animals we domesticated. And it seems like
urban dwellers have less of them, compared to rural/agrarian ones.
<
https://arstechnica.com/science/2024/03/human-gut-bacteria-that-can-digest-plant-matter-probably-came-from-cows/>
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