What specifically causes the breakdown of teeth?

When you see images of skulls from 100,000 years ago, they have seemingly perfect teeth.

Today, modern humans have terrible dental health. I understand sugar causes a number of issues- the growth of bacteria which decays enamel.

What is the process teeth go through in the modern world to cause this? How quickly after consuming a sugary drink for example does this degradation start to occur?

And as a follow up, how does tooth paste help prevent this?