Thurs Nazir 54

The Gemara has a Drasha to include graves before Matan Torah also becomes Tamai by touching (but not by Ohel.)

Tosfos explains: this has nothing to do with the argument (between R' Shimon and Chachumim) whether a Goy's grave is Mitameh in an Ohel. Over here, without the Drasha, we would assume to all that there is no Tumah in such a grave. Since the Torah starts the Parsha of Tumah "if someone dies" which connotes only future deaths,and not ones before the Torah was given.

Tosfos asks: the gemara in BB says that R' Banah was marking the graves for the Maaras Hamachpeila. We only mark graves that are mitamai for Ohel. Since they died before Matan Torah, why was it needed to be marked?

Tosfos answers: they're the exception. The Torah says when an "Odom" dies." Therefore we would assume it includes Odom Harishon, since he's definitely included in the term "Odom." Even Avraham is called "Odom HAgadol."

Alternatively, he held that our Drasha to include the dead before Matan Torah is Mitamah even with an Ohel. Therefore they're more stringent than Goyim's graves (according to R' Shimon) after Matan Torah.