The grandmother hypothesis suggests that humans have "given up" their reproductive potential in later years in order to invest in the children they already have as well as their grandchildren. Naturally, this is an unconscious, biological adaptation that emerges over many generations and is not the result of individual decision-making. For such a hypothesis to be confirmed it would have to be demonstrated that children are significantly more likely to survive when a grandmother is present than when she isn't.
Dr. Lummaa has done just that in her study published in the journal Nature, demonstrating that children are 12% more likely to survive to adulthood when they have a grandmother's support than when they don't.
Let's hear it for grandmas! Link