The gemara learns out from the pasuk that was said to Adam, all the 7 Noachide mitzvos, implying that these mitzvos applied to Adam as well. At the bottom of the page the gemara quotes the opinion of R. Yehuda who says that Adam was only commanded against Avoda Zara, and others add cursing Hashem and dinim, implying that the first opinion holds that Adam had to keep all 7. The gemara 59b says that Adam wasn't allowed to eat meat, he was only allowed to eat vegetation, until after the mabul when Hashem allowed Noach to consume the animals. Tosafos has 2 questions on that gemara. First, why is the prohibition against eating meat not counted as one of the 7 mitzvos given to Adam? Tosafos answers that commands such as this and eating from the eitz hada'as which only applied to Adam and not for future generations don't count. Tosafos also asks, how can there be an issur of 'ever min ha'chai', that Adam shouldn't eat a limb of an animal that was separated when it was alive, since it was forbidden for him to eat any meat? Tosafos answers that Adam was allowed to eat meat. He wasn't allowed to kill animals in order to eat them, but was able to eat animals that were found dead. Therefore, the issur of ever min ha'achai was to forbid even limbs that were severed by themselves, which isn't included in the issur of eating meat.
The Rambam (Hil. Melachim 9:1) answers Tosafos question by saying that Adam was commanded on 6 of the 7 mitzvos, but ever min ha'chai wasn't introduced until Noach after the mabul. The Maharatz Chiyus points out that the Rambam is saying this to deal with Tosafos' question.
Rashi 59b seems to hold that it was forbidden for Adam to eat meat until Noach, not just assur to kill the animals as Tosafos says. Rashi 57a clearly writes that Adam wasn't allowed to eat any meat. Rashi is therefore bothered why Adam had to be commanded in ever min ha'chai since he couldn't eat any meat, to which rashi says that a limb that was severed by itself wasn't included in the issur to eat meat, but is included in ever min ha'chai. The maharsha explains that Tosafos holds that if an animal that died by itself was assur, then even a limb that was severed by itself should be assur. Tosafos had to be mechadesh that if it died by itself it was mutar, and therefore when it is severed by itself it is mutar, if not for the issur of ever min ha'chai. Whereas Rashi seems to hold that even if an animal that dies by itself is included in the issur of eating meat, a limb that was severed by itself would not be included in the issur.
The Ramban (Breishis 1:29) quotes Rashi in chumash and in our gemara that says that Adam was supposed to share all plants and vegetation with the animals, but not to eat the animals. However, the Ramban himself holds that Adam was allowed to eat fruits and even seeds, but the animals were only allowed to eat the vegetation. He agrees with Rashi that Adam wasn't allowed to consume any meat, even if it died by itself, against Tosafos. The Ramban seems to agree with the Rambam that ever min ha'chai was only introduced to Noach. The Ramban writes that when Hashem permitted Noach to kill animals and eat them, He maintained the issur on eiver min ha'chai because Adam had permission to eat the animals after they were killed, but didn't have the right to eat the nefesh itself.
The Ramban concludes with cryptic words: וזה טעם השחיטה, ומה שאמרו צער בעלי חיים דאורייתא, וזו ברכתנו שמברך אשר קדשנו במצותיו וצונו על השחיטה
The Ramban holds that the issur of צער בעלי חיים and the mitzvah of shechita are all a result of the limitation of control on the nefesh of the animal.