Grandparents, individually known as grandmother and grandfather, are the parents of a person's father or mother – paternal or maternal. Every sexually-reproducing living organism who is not a genetic chimera has a maximum of four genetic grandparents, eight genetic great-grandparents, sixteen genetic great-great-grandparents, thirty-two genetic great-great-great-grandparents, sixty-four genetic great-great-great-great grandparents, etc. In the history of modern humanity, around 30,000 years ago, the number of modern humans who lived to be a grandparent increased. It is not known for certain what spurred this increase in longevity, but it is generally believed that a key consequence of three generations being alive together was the preservation of information which could otherwise have been lost; an example of this important information might have been where to find water in times of drought. In cases where parents are unwilling or unable to provide adequate care for their children (e.g., financial obstacles, marriage problems, illness or death), grandparents often take on the role of primary caregivers. Even when this is not the case, and particularly in traditional cultures, grandparents often have a direct and clear role in relation to the raising, care and nurture of children. Grandparents are second-degree relatives to their grandchildren and share 25% genetic overlap. A step-grandparent can be the step-parent of the parent or the step-parent's parent or the step-parent's step-parent (though technically this might be called a step-step-grandparent). The various words for grandparents at times may also be used to refer to any elderly person, especially the terms gramps, granny, grandfather, granddad, grandmother, nan, maw-maw, paw-paw (and others which families make up themselves). Titles When used as a noun (e.g., "... a grandparent walked by"), grandfather and grandmother are usually used, although forms such as grandma/grandpa, granny/granddaddy or even nan/pop are sometimes used. When preceded by "my ..." (e.g., "... my grandpa walked by"), all forms are common (anywhere from "... my grandfather ..." to "... my Gramps ..."). All forms can be used in plural, but Gramps (plural Gramps) is rare. In writing, Grandfather and Grandmother are most common, but very rare as a form of address. In speech, Grandpa and Grandma are commonly used in the United States, Canada, and Australia. In Britain, Ireland, United States, Australia, New Zealand and, particularly prevalent in the Canadian province of Newfoundland and Labrador, Nan, Nana, Nanna, Nanny, Gran and Granny and other variations are often used for grandmother in both writing and