Chabad.org — English translation + Rashi
Torah Temimah continues the firstborn theme: the dual category (human and animal) underscores the breadth of the original consecration that the Levites now replace.
English: “Both man and beast” — the original consecration was comprehensive; the Levites inherit that comprehensive role in the service of the sanctuary.
Scope of the original claim — the verse’s pairing of human and animal firstborn defines the magnitude of what is being transferred.
By taking the Levites in place of every firstborn, Israel is relieved of a claim that once covered both family and flock.