I had the same question. It is possible that the trait survived as a recessive one for a long time. In the same way that sickle-cell is only lethal if a person

I had the same question. It is possible that the trait survived as a recessive one for a long time. In the same way that sickle-cell is only lethal if a person has the defect in both sets of chromosomes (homozygous recessive), the weaker jaw muscles would only be a disadvantage for big-jawed primates if they were homozygous recessive. Later, when alleles for smaller jaws appeared, the weak-jaw-muscle allele had a higher chance of being propagated. Another possibility is that the myosin heavy chain, or a related regulatory function, induces larger jaws. Without this the jaw was not given enough stimulus to grow large enough.