Simplify the following text for a Grade 5 native English-speaking person.
Original: Two species are described in the literature : A. ramidus , which lived about 4.4 million years ago during the early Pliocene , and A. kadabba , dated to approximately 5.6 million years ago ( late Miocene ) .
Simplified: Two species are known : A. kadabba , dated to about 5.6 million years ago ( late Miocene ) , and A. ramidus , which lived about 4.4 million years ago during the early Pliocene .