Sometimes a species evolves by losing their ancestors traits, rather than gaining new ones. We can see this in the flightless wings of the kiwi and emu or a whale’s virtually nonexistent legs. But the platypus hasn’t just lost a limb, it’s totally forgone the use of an organ most species would find vital. You see, the platypus has no stomach. Their food goes from their esophagus straight into the small intestine. The stomach, a large sack of acids and enzymes, is missing.
It doesn’t seem to hurt the platypus any, in fact they get along just fine without one. But why have they lost it? It turns out that nearly all the genes responsible for making a stomach really work have been disabled. Large retrovirus-like sections of unrelated DNA have forced themselves into the genes responsible for making both stomach acid and the digestive enzymes, effectively rendering them mute. These genes have been conserved from the earliest vertebrate ancestors, so their total inactivation is somewhat astounding. Only the echidna has a similar inactivation.
It’s likely that without the powerful stomach juices the platypus’ stomach became nothing more than an empty sack halfway down the digestive tract and was likely removed by natural selection (why spend energy growing a stomach that doesn’t do anything)? Nevertheless, this total deviation from normal vertebrates marks yet another evolutionary mystery surrounding the platypus.