Comment by gmunu

They aren't absolutely infertile, but in hunter gather societies this is handled by infanticide. When it isn't possible for a mother to support two children under the age of four, the only solution was to kill one, which is what they did.

In the _Wandering God_ by Morris Berman, there is a story of a story of a girl (maybe 19th or early 20th century?) in a nomadic society in Africa. Her mother got pregnant again and was going to kill the baby, but the three year old child protested and agreed to go with being breast fed to try to save her newborn sibling. It wasn't possible to breastfeed two children at once, so her mother was going to kill the infant.

Miraculously the child managed to survive, but it was by no means guaranteed and she had to figure out to be self sufficient to some degree. It was considered such an odd occurrence that the relevant anthropologist reported the story.

I think we forget that infanticide wasn't uncommon that long ago. It was one of the major 'moral' victories of Christianity to make infanticide uncommon. And it could only do that because it wasn't necessary anymore.