After Adam and Eve ate from the forbidden tree, they realized they were naked. Is it not that some kind of intelligence that was revealed in their mind? I am saying this because chimpanzees and other animals move around nude without realizing they are naked.
Intelligence was already shown in the complexity of the commands given to Adam and Eve.
"And the LORD God commanded the man, saying, "Of every tree of the garden you may freely eat; but of the tree of the knowledge of good and evil you shall not eat, for in the day that you eat of it you shall surely die" (Genesis 2:16-17).
Rules like this would not make sense to an animal.
We can also see intelligence in Adam's speech. He named the animals (Genesis 2:20). And when Eve was presented to him, "And Adam said: "This is now bone of my bones and flesh of my flesh; she shall be called Woman, because she was taken out of Man" (Genesis 2:23). Animals don't create poetry.
So what eating the fruit did to the man was more than just give him intelligence. In fact, if he had truly lacked intelligence, Adam and Eve could not be held responsible for their actions. It is because they knew God's law and broke it that they were held accountable.