Of course we are to observe and reason. However the tree presented differing choices of what we would choose to believe. There is nothing intrinsically wrong with believing in evolution, but, if it leads you away from the knowledge of God one might conclude that is part of the 'knowledge of evil'. Jes' sayin'.
Recall that the root words contained in the two opposing bodies of knowledge are 'good', from 'God', and 'evil', from 'devil'? Note that the Hebrew root for serpent embodies the scientific method of observation and reason, whereas the 'knowledge of (God)' comes by revelation. Most of man's problems come from 'leaning on his own understanding', and by rejecting revelation.