Simplify the following text for a Grade 2 native English-speaking person.
Original: When Tarzan was a year old , his mother died of natural causes , and his father was killed by Kerchak , leader of the ape tribe into which Tarzan was adopted . Tarzan 's tribe of apes is known as the Mangani , Great Apes of a species unknown to science .
Simplified: The book was called Tarzan of the Apes .