Genesis 29&30:
Then God said, “I give you every seed-bearing plant on the face of the whole earth and every tree that has fruit with seed in it. They will be yours for food. And to all the beasts of the earth and all the birds in the sky and all the creatures that move along the ground—everything that has the breath of life in it—I give every green plant for food.” And it was so.
Animal meat as food for humans was not introduce in the Bible until the Flood. That is when life spans decrease.
To feel pain a central nervous system and a brain need to be present.
The Gen 1 account was when God had only created Adam, was before Eve was created. After they were removed from the Garden for disobedience, they had children. In Gen 4:3 After Cain and Abel were born, Cain presented Fruits of the Soil as an offering, while Abel presented Fat Portions of some of this flock. God did not accept the fruit offering, but favored the Fat portions of his flock. This is the first mention of the concept of an offering to God.
"In the course of time Cain brought some of the fruits of the soil as an offering to the Lord. 4 And Abel also brought an offering—fat portions from some of the firstborn of his flock. The Lord looked with favor on Abel and his offering, 5 but on Cain and his offering he did not look with favor. "
I tend to believe (my opinion), they ate meat since the time they were offering Fat portions, even if it was not authorized. My opinion is based on the fact that humans became so overcome with evil so quickly, that God decided to wipe them out.
In Gen 6:3, Before the flood, God reduced the lifetime of humans. It was not the result of eating meat, but rather because of their inclination of doing evil.
In Gen 9:3 after the flood, God authorized all food. It does not say this is when humans began eating meat, but that it was now authorized.