Primates include not just tailless apes but also humans.

Literal

Primates [topic-は] tail [of-の] not-having monkeys [only-だけでなく] humans [too-も] include.

~だけでなく~も ('not only X but also Y') is a common additive-expansion pattern. 尾の無い is a relative clause modifying サル — 'tail-lacking monkeys,' meaning apes. 含む ('to include, contain') is a formal verb common in definitions and classifications.