For the source text click/tap here: Zevachim 115
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According to the Mishna (112b) until the Tabernacle was established in the desert, sacrifices were brought on bamot – private altars – and the sacrificial service was performed by the bekhorim, the firstborn.
After the Tabernacle was erected sacrifices could only be brought there and the sacrificial service was performed by the kohanim – descendants of Aharon HaKohen.
According to the Mishna, the commandment to sanctify the firstborn was given at the time of the Exodus from Egypt (see Ex 13:2), and the firstborn redeemed themselves from Temple service later on, as described in Num (3:12).
According to the Talmud Yerushalmi, the reason for this was the participation of the firstborn in sacrificing to the Golden Calf, something that the members of the Tribe of Levi did not do.
We explore the transition from Firstborns to priesthood.