In the episodes that feature the Face of Boe it is stated that he is the last of his kind, suggesting that there were other large head-like beings at some time.
For example, from New Earth:
HAME: The rest of Boe-kind became extinct long ago. He's the only one left.
And from Gridlock:
BOE: I have seen so much. Perhaps too much. I am the last of my kind, as you are the last of yours, Doctor.
If we take the suggestion in Last of the Time Lords that the Face of Boe is actually Jack Harkness, who wasn't a large head, as fact then where did the others like the Face of Boe come from?
Two possibilities suggest themselves to me:
- The other humans that died on Satellite 5 were also revived by Rose, and they too evolved oddly over the eons and became Boe-kind with Jack Harkness.
- Jack Harkness had children who gained a measure of his immortality, and consequently evolved similarly to him over the eons.
Is there any evidence or word from the writers/producers that support either option, or suggest some other source for the others who were like the Face of Boe?