According to Heinlein, the Howard Foundation was started in the 1800's by Ira Howard for the purpose of selectively breeding human beings to expand their lifespans. The novel Methusalehs Children is focused on the Howard Families, and their quest for acceptance on Earth and other planets.
The books Time Enough for Love and To Sail Beyond the Sunset tell the stories of two famous Howard Family members; Maureen Johnson and Lazarus Long.