Trantis asks:

>Why did animals develop two Kidneys and two testicles/ovaries?

Once evolution has settled on a bilaterally symmetric design plan during
embryogenesis, duplicating any structure simultaneously on each side of the
midline is probably the simplest design possible. The same question could be
asked of why do we have two eyes, two ears, two arms, two lungs, two brain
lobes or two nares?

The more difficult question to answer is why do we have only one heart, one
liver, one pancreas, one stomach, one gut, each asymmetrically placed against
the midline?

Wirt Atmar
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