pelvis requires that babies leave the birth canal facing away from the mother, contrary to all other primate species. This makes it more difficult for the mother to clear the baby's breathing passageways, to make sure the umbilical cord isn't wrapped around the neck, and to pull the baby free without bending its body the wrong way. The human need to have a birth attendant also requires sociality. In order to guarantee the presence of a birth attendant, humans must aggregate in groups. It has been controversially claimed that humans have eusociality, like ants and bees, in which there is relatively high parental investment, cooperative care of young, and division of labor. It is unclear which evolved first; sociality, bipedalism, or birth attendance. Bonobos, our closest living relatives alongside chimpanzees, have high female sociality and births among bonobos are also social events. Sociality may have been a prerequisite for birth attendance, and bipedalism and birth attendance could have evolved as long as five million years ago. As female primates age, their ability to reproduce decreases. The grandmother hypothesis describes the evolution of menopause, which may or may not be unique to humans among primates. As women age, the costs of investing in additional reproduction increase and the benefits decrease. At menopause, it is more beneficial to stop reproduction and begin investing in grandchildren. Grandmothers are certain of their genetic relation to their grandchildren, especially the children of their daughters, because maternal certainty of their own children is high, and their daughters are certain of their maternity to their children as well. It has also been theorized that grandmothers preferentially invest in the daughters of their daughters because X chromosomes carry more DNA and their granddaughters are most closely related to them. Paternal investment As altriciality increased, investment from individuals other than the mother became more necessary. High sociality meant that female relatives were present to help the mother, but paternal investment increased as well. Paternal investment increases as it becomes more difficult to have additional children, and as the effects of investment on offspring fitness increase. Men are more likely than women to give no parental investment to their children, and the children of low-investing fathers are more likely to give less parental investment to their own children. Father absence is a risk factor for both early sexual activity and teenage pregnancy. Father absence raises children's stress levels, which are