Grandparents are the parents of a person's father or mother – paternal or maternal. Every sexually-reproducing creature who is not a genetic chimera has a maximum of four genetic grandparents, eight genetic great-grandparents, sixteen genetic great-great-grandparents, 32 genetic great-great-great-grandparents, 64 genetic great-great-great-great-grandparents, etc., although the numbers will be lower in cases of pedigree collapse. In the history of modern humanity, around 30,000 years ago, the number of modern humans who lived to be grandparents increased exponentially. 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., death of the parents), 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 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, grandmother and even more types that most families make up themselves, like "coolma" for younger step-grandparents.
The youngest grandmother in the world is Rifca Stănescu, a Romani girl from Romania, who at the age of 12 gave birth to her first child, a girl called Maria. Maria, in her turn, gave birth to a child at the age of 11, thus making her mother Rifca - the youngest grandmother in the world at the age of just 23 years.