Humans are only walking digestive tracts. From an evolutionary perspective, all other organs originated from a primitive gut and have only one purpose: to serve it.
(credits to /u/susieq0044@sh.itjust.works who phrased it so well in this post, even though i had the thought already years ago)
There was an article in JIR a while back pointing out that all animals were just the reproductive stage of primordial ooze.