Grandparents are the parents of a person's own parent, whether that be a father or a mother. 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, etc. Rarely, such as in the case of sibling or half-sibling incest, these numbers are lower. In the history of modern humanity, around 30,000 years ago, the number of modern humans who lived to be grandparents began to skyrocket. It is not known for certain what spurred this increase in longevity. But it is believed that a key consequence of three generations being alive together was the facilitation of the passing along of information that prior to that point would 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.
One can also be a step-grandparent. A step-grandparent can be one's parent's step-parent or one's step-parent's parent. 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.