Simplify the following text for a Grade 7 native English-speaking person.
Original: While the ratites were traditionally thought of as an ancestrally flightless , monophyletic group , the branching of the tinamous within the ratite lineage suggests that ratites evolved flightlessness at least three times .
Simplified: Therefore , ratites are a group of medium to large birds , most of whom are flightless , with the exception of the tinamous . The ratites evolved flightlessness more than once .