How come every new race isn't destroyed by inbreeding?

From: pslant (pslant_at_yahoo.com)
Date: 12/28/04 

Date: Tue, 28 Dec 2004 06:26:23 +0000 (UTC)

If we all can be traced back to some common female ancestor (looking
at the maternal line), wouldnt that require extreme inbreeding ?
Is it correct to state
1) she was the first female human
2) she had children with a "non-human" , lets call that "race"
sub-humans.
3) for some reasom they (or some) shared her human characteristics
4) her female offspring were sired (is that the correct term?) by
other sub-humans.
at least some of those children ALSO had human characteristics

Wouldn't it be necessary to start breed between the human group fairly
soon in order to not dilute the "human" strain?

How can that small polulation produce over 20 billion people?

What's the error in my reasoning ?

If the answer is that "race" is a more continuous that step-wise
fenomenon,
how come "sub-humans" arent here now, or are they?