Investigating an attack on his former archaeology professor, the Enterprise crew and their enemies begin a frenzied hunt for DNA samples which hold the key to unravelling an ancient genetic mystery.

Norman Lloyd (Professor Richard Galen)
Salome Jens (The Humanoid)
John Cothran, Jr. (Captain Nu'Daq)
Linda Thorson (Gul Ocett)
Maurice Roëves (Romulan Commander)

• This episode attempts to explain why virtually all of the species on Star Trek look like humans. The progenitors seeded the primordial oceans with DNA four billion years ago, creating life in their image and the program which created the holographic message.
  
• Salome Jens (The Humanoid) will go on to play the female Changeling on Deep Space Nine, and is pretty close in appearance to her character in this episode. (She first appears in the 4th season episode "Broken Link".)
  
• Gul Ocett marks the first appearance of an adult Cardassian female.
  
• Deep Space 4 is mentioned as being beyond the Romulan Empire, near the edge of explored space.
  
• This episode marks the first time that Klingons, Romulans, and Cardassians have appeared in the same episode.