Nursing and pregnancy are not a gender role, they are biological capabilities unique to the female sex.

What tasks remain for the males to do benefits from grip strength (and being stronger in general), meaning the ones who got stronger survived and reproduced more.

I'm not saying that males having increased grip strength over previous males does not provide a selective advantage - I'm saying that males having increased grip strength _over females_ provides a selective advantage only if the difference between the sexes is the cause of the advantage.

To put it another way: if the advantage is because of tool/weapon use, why wouldn't females also be stronger than previous generations?

Assuming we're using the evolutionary biology approach here (which I'm not convinced is appropriate), we should be trying to explain not why humans have increased grip strength over time but why female grip strength has not similarly increased. I see two options:

1) Females with grip strength equal to males successfully reproduce at a lower rate than those with less.

2) Males with grip strength greater than the females of their own species successfully reproduce at a higher rate.

My "just so" comment is pointing out that in this case, the poster seemed to be projecting their own biases on human evolution, and making assumptions. Among those assumptions are "men usually do the hunting". I'm pointing out that if that were the case then there would be no reason that females would be weaker. In the case of reproduction via rape - which has without question been common throughout the history of mankind - males being stronger than females would have a very direct correlation with their success in passing down their genetic makeup. Many children have been born with strong fathers and weak mothers.

Having more strength is not always an evolutionary benefit, for men or women. If it were we would all have the hormone levels of a professional body builder. Having more upper body strength and being generally bigger costs calories and there is an evolutionary incentive to be physically smaller (to a point). Also building strength is an adaption to stressors, by exercising the muscles. If it is the males who do the majority of the hunter/gathering then they will be getting more exercise and consequently growing stronger. It's not possible to consider male and female evolutionary pressures separately. If males have adaptive pressures to grow stronger and evolve stronger, then females would have some of the opposite pressures.