What did rosenberg and trevathan discover in their study comparing human and nonhuman primate birth?

When comparing the birth patterns of humans and non-human primate, Rosenberg and Trevathan discovered some very clear differences:

1. The human period of labour and child birth is exceptionally long and painful compared to other non-human primates.
E.g. a human labour can last anywhere from 10-20 hours while that of a chimpanzee lasts only 2 hours.

2. Humans have the ability to walk straight, the trade off however is that it is extremely difficult and painful to give birth to a big brained human baby.

Very hard to understand