Just because we did something in the past doesn't mean we need to continue doing it now. There's no reason for adult humans to continue consuming another species' breast milk and ingest all the saturated fats and hormones that come along with it to damage our health.
edit: to add to that, there are horrible ethical violations in the dairy industry and it causes tremendous harm to the environment.
The damaging effects of consuming a food loaded with fat, protein, and hormones meant to grow a calf as fast as possible is not a good argument to stop consuming it?
> Do countries with high milk consumption have shorter life spans?
Total lifespan is dependent on a lot of factors. But YES moving to dairy as a population shortens your lifespan by greatly increasing a bunch of sicknesses. See "the china study" for more in-depth info.
It's diversification in your food ressources. In the ancient past that made it easier to survive.