Lignin is a good invention. It protects against harmful UV radiation, and provides rigidity and structure to plant tissue. Those first, noble plants that rose up out of the muck and gave life on land a go needed lignin, and so, somewhere between 400 and 500 million years ago, they evolved it. Plants came to dominate the land. Lignin, a key to their terrestrial success, came from phenylalanine, that familiar component of proteins. Take Phe, decarboxylate it, maybe hydroxylate it, maybe methylate it, and then polymerize it, and you have lignin.
Animals also evolved that lived on land and ate the plants that had come to live there. Lignin became a fundamental component of the tissue that formed the plants they ate — it was a fundamental part of their diet. Despite that, animals never figured out a good biochemical strategy to digest lignin.
It wasn’t for lack of trying. Today’s molecule, enterodiol, is derived from lignans, which (more or less) are precursors to lignin. We eat these lignans, and they go to our gut, where our microbial symbionts then take over.
They can metabolize these lignans to compounds like enterodiol. All it takes is an ancient biochemical innovation (lignin), the co-evolution of animal life forms in the context of this evolution, and the ability of symbiotic microorganisms that evolved in the gut of the animals’ digestive systems to complete the transformations.
Phenylpropanoids. Enterodiol. Easy.