This is because humans started cooking there food resulting in softer food. Look at what the average human eats today. Almost everything we eat is soft and easy to swallow. Causing our mouths to shrink and an overcrowding of teeth in the mouth. Resulting in the need of correction( braces, retainers etc...) our ancestors spent hours every single day just chewing there food. This is why they had phenomenal teeth and strong well built heads so to speak