The explanation is pedigree collapse, or shared ancestors: when a set of parents somewhere in your tree are related to each other, there are fewer than the normal number of ancestors in an earlier generation, because more than one "slot" in the tree is filled by the same individual. For example, if two first cousins marry each other, their children will have only six great-grandparents instead of the expected eight. A few such cases in your family tree can drastically reduce the number of direct ancestors, and it was actually quite common in times when people seldom strayed far from their birthplaces and thus were likely to be related in some degree to just about every potential mate.
I have a classic case of pedigree collapse due to a marriage of first cousins in my Rand line. My fifth great-grandfather, Joshua Rand, was born, say 1715, probably in Rye, New Hampshire. He married Mary Moses ca 1740, and they had eight children. The oldest was John, born 15 Feb 1742, and the third was Joseph, born ca 1746. These two brothers were my fourth great-grandfathers.
Joseph Rand married Susannah Goss on 24 May 1764. They had seven children; the third was Joshua Rand, born 23 Aug 1779, in Rye, New Hampshire. Eight years later, on 4 Jun 1772, Joseph's elder brother John married Hannah Seavey. They had eight children; the oldest was Elizabeth, born 20 May 1773, probably also in Rye.
After the death of his first wife, Esther Marden, with whom he had two sons, Joshua Rand married Elizabeth Rand, his first cousin, on 29 Mar 1810. Joshua and Elizabeth had three sons, all born in Rye:
- Ezra Rand, b. 1810, d. 1827
- Nahum Alonzo Rand, b. 1813, married Dolly Brister
- Aaron Rand, b. 1816, married Elizabeth Yeaton
Their middle son, Nahum, left New Hampshire for Roxbury, Massachusetts, where he married Dolly Brister and subsequently removed to Andover, Maine. His pedigree chart below illustrates the pedigree collapse: Nahum Rand had only six great-grandparents, not eight.
|Pedigree collapse illustrated|
- Joshua Rand and Elizabeth Rand (third-great-grandparents)
- Nahum Alonzo Rand and Dolly Brister (great-great-grandparents)
- Kate Rand and Marchant Hodsdon (great-grandparents)
- Mary Milliken Hodsdon and Chester F. Kirk (my paternal grandparents)
(Note: This post is in response to Amy Johnson Crow's "52 Ancestors in 52 Weeks" challenge at No Story Too Small.)
- Langdon Brown Parsons, History of the town of Rye, New Hampshire: from its discovery and settlement to December 31, 1903 (Concord, N.H.: Rumford Printing Company, 1905), pp. 499-504; digital images, Google Books (http://books.google.com/books : accessed 28 Apr 2012).