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carried a unique mitochondrial DNA lineage that has since been lost. This
lineage disappeared even though it was common in the DNA that has been
extracted from pre-Columbian dog remains. Jennifer Leonard of UCLA, who
discovered this lineage, suggests that perhaps these native dogs did not have
properties that the European colonists found useful.
My guess is that the human-dog bond is so logical, and so advantageous
to both sides, that it may easily have been forged more than once, perhaps in
India and Europe as well as East Asia. Subsequent loss of these breeds may
not seem so far-fetched when we consider how many domestic breeds of
other animals have been lost or nearly lost. We may only be starting to learn
about the complex history of dog domestication.
Revealing the dog genome
Artifi cial selection has produced an explosion of dif erent dog breeds.
Currently there are more than 400 recognized breeds, ranging from the
1.5-kilogram Chihuahua to the 100-kilogram Great Dane. A visitor from
Alpha Centauri would be astonished to learn that such obviously dif erent
animals belong to the same species, and that they are (mechanical dii culties
aside) fully interfertile.
As of this writing a complete DNA genome from the boxer breed has
been sequenced. Less complete sequences have been obtained for 24 other
breeds. We know from historical records that most current dog breeds
originated only a few centuries ago, and that they tend to be highly inbred.
Despite the inbreeding, the DNA evidence shows that the dogs of each breed
still preserve a great deal of the genetic variation that they inherited from
their wolf ancestors.
Both the maternal and the paternal chromosome sets of the boxer dog
have been sequenced. The two sets are separated by three-quarters of a mil-
lion single-base dif erences (out of about three billion), showing that there is
a substantial amount of genetic variation that still remains within the boxer
gene pool. Boxers and poodles dif er by one and a half million single-base
changes, and preliminary data from other breeds suggest comparable
