Couple of verses

I read these two chapters of Genesis and these are the verses that stuck in my mind.

Gensis 1:30...And to every beast of the earth, and to every fowl of the air and to everything that creepeth upon the earth, wherin there is life, I have given green herb for meat: and it was so...

The transformation from herbivore to carnivore is interesting. The fall of Adam and Eve made quite a physical transformation on the initial creation plan, (shaping of teeth, digestive tracts, etc...)

Genesis 2:17 But of the tree of the knowledge of good and evil, thou shalt not eat of it: for in the day that thou eatest thereof thou shalt surely die.

This is curious to me. Why does this tree exist???