Question

How can you explain human's inability to digest cellulose?

Answer 1

Hint: The inability of humans to digest cellulose is related to evolution and the concept of vestigial substances. Appendix was used for this purpose.

Complete answer:
1)Cellulose is one of the major components of the plants due to which it forms the major component of the human diet but humans cannot digest cellulose because they do not have enzyme cellulase that is required for digestion of cellulose. The cellulase enzyme is required to break the beta acetyl bonds present in the cellulose. Humans have many enzymes that facilitate digestion but cellulase enzyme is absent in humans.

2)Many years back, humans used to eat only raw vegetables as fire and cooking were not discovered and this raw vegetable have a lot of cellulose, so they had cellulase enzyme in their body to digest the cellulose properly but in the course of evolution, humans started consuming cooked food, and hence the cellulase enzyme became vestigial and less useful and now humans do not have cellulase enzyme at all.

Note: Although cellulose cannot be digested by humans, still the presence of undigested cellulose in humans is helpful because it makes the process of digestion much easier and efficient, This is due to the presence of fibers in the cellulose. Animals like cows, goats mainly feed on different plants and grasses that contain a lot of cellulose, hence they have cellulase enzymes to facilitate the digestion of cellulose.