Simplify the following text for a Grade 14 native English-speaking person.
Original: Notochords were advantageous to primitive fish-ancestors because they were a rigid structure for muscle attachment , yet flexible enough to allow more movement than , for example , the exoskeleton of the dominant animals of that time .
Simplified: Notochords were advantageous to primitive fish-ancestors because they were a rigid structure for muscle attachment , yet flexible enough to allow movement .