In New Earth we see "The Sisterhood" (which are the future of human healing). This is about 50 billion years in the future, but a seemingly way better healing system was devised in only about 3000 years (5100); I'm referring to the nano-genes we see in The Empty Child and The Doctor Dances.
So my question is why don't future humans use nano-genes? Did they lose the technology due to the Doctor's "visit" there?
Humanity never really got the nano-genes technology to begin with, since the Doctor destroyed it before departing, as described in The Doctor Dances wikipedia page:
Meanwhile a German bomb falls onto the site, but Jack uses his ship to capture it and hauls it out into space. Then the Doctor sets the medical transport to explode once everyone is safely far from it, thus destroying the technology and matching the historical records of an explosion at the site.
This explain this apparent regression. If left intact, this technology would have modified the Earth's timeline quite heavily: we are talking about the Penicillin era :)
In the Doctor Who episode "Utopia" the Doctor refers to the human race periodically backsliding into physical forms that resemble modern humanity (in between their various attempts to become pure energy or thought, etc)
DOCTOR: Don't you see that? The ripe old smell of humans. You survived. Oh, you might have spent a million years evolving into clouds of gas, and another million as downloads, but you always revert to the same basic shape. The fundamental humans.
There's no immediate reason to assume that a technology that was designed to work on 50th Century humans would be effective on human beings from the 100000000000000th Century, merely because they look similar.
