Genuine Puzzles In Biology?

Interesting topic.Three things come to my mind right now.1. Why do we have 2 kidneys when we can very well live a normal life even with one functioning kidney.2. Why do we have a liver so big when we can very well live a normal life even with a liver half as big?3. Why do we have a brain so big? I don't remember how much of the brain we use, but it is a pretty small percent.

I tried to post this as a new topic. Because it is sitting idle in the proposed topics category, I thought may be I should post it here.1. How does thought originate?2. How do you recall something from your memory?Probably these two questions are similar. But I think they are different. May be their mechanism of origin is similar.If you believe in mind body dualism, then, they may have an easy answer. But is the mind different from brain? Or is it a 'metaphor' used to signify the collective out put of the brain?