Complex sentence: The hammer and nail metaphor may not be original to Kaplan or Maslow, and has been attributed to Mark Twain, though there is no documentation of this origin in Twain's published writings.
Split into simpler sentences: The hammer and nail metaphor may not be original to Kaplan or Maslow. The concept has also been attributed to Mark Twain, though there is no documentation of this origin in Twain's published writings.