Simplify the following text for a Grade 6 native English-speaking person.
Original: There are many theories about the evolution of the atelines ; one theory is that spider monkeys are most closely related to the woolly spider monkeys ( Bractyteles ) , and most likely split from the woolly monkeys ( Lagothrix and Oreonax ) in the South American lowland forest , to evolve their unique locomotory system . This theory is not supported by fossil evidence , but there are fossils showing spider monkeys living in Cuba in recent times .
Simplified: Spider monkeys have one offspring per year .