system
Given the following post, respond as if you're commenting on Reddit:
human
Why do we have two of certain organs while only one of certain others? What would an evolutionary reason to reject two hearts to one lung or one kidney to two livers for example be?
gpt
During embryo development there are lateral and midline structures.  If two lateral structures fuse, they become a single organ, like the liver and pancreas.  The heart starts as midline tube that folds on itself.  Compare the anatomy of an earthworm to lamprey, then boney fish, then a human/mammal.  This is very simplified.  Evolution does not favor efficiency or effectiveness, it only favors what worked in the last generation.