Simplify the following text for a Grade 11 native English-speaking person.
Original: With the development of plate tectonics in the 1960s geologists like Arthur Holmes and John Tuzo Wilson concluded the Atlantic Ocean must have had a precursor before the time of Pangaea .
Simplified: With the development of plate tectonics in the 1960s geologists concluded the Atlantic Ocean must have had a precursor before the time of Pangaea .