Genesis 1:29
So the change from that diet in creation week to after the flood pretty much tells me that no man nor living thing ate any other for meat other than plant life and herbs for meat. The global flood had changed that diet significantly in scripture.
The Bible says nothing about eating meat during the time of Cain and Abel. So the next place one would look, is the Book of Jasher, which is used in Scripture as an authority:
"Is not this written in the Book of Jasher?"--Joshua, 10-13.
"Behold it is written in the Book of Jasher."--II Samuel, 1-18
And the Book of Jasher gives us the answer! Yep! He confirms that they ate meat!
18 And Cain approached his brother Abel in anger, and he said unto him, What is there between me and thee, that thou comest to dwell and bring thy flock to feed in my land?
19 And Abel answered his brother Cain and said unto him, What is there between me and thee, that
thou shalt eat the flesh of my flock and clothe thyself with their wool?
20 And now therefore, put off the wool of my sheep with which thou hast clothed thyself, and recompense me for their fruit
and flesh which thou hast eaten, and when thou shalt have done this, I will then go from thy land as thou hast said? Jasher 1:18-20
Moral: Don't argue from silence!