Simplify the following text for a Grade 9 native English-speaking person.
Original: It is thought that the first passerines evolved in the southern continent of Gondwana in the Paleogene , maybe around the Upper Paleocene some 60 â `` 55 mya .
Simplified: It is now thought that the first passerines evolved in Gondwana at some time in the Paleogene , maybe around the Late Paleocene some 60 â `` 55 mya .