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)
not reproductive tissue, and also nervous tissue. primitive animals only have ganglia, ganlions, or nerve nets as thier sole “Brain” reproductive come from something more basal, if you look at sponges.
if you want to go into protosomes and deuterosomes its gets wierder, and pseudoscoeolmates and acoelomates.