Technically, there is but one race of human....Homo sapiens.
That's not a race; it's a
species. The term "race" is out of favor, but it generally means a subgroup of an animal species
below the level of a subspecies. Subspecies are populations separated by geography or some other external force, which have evolved along slightly different genetic paths over many generations, but which still recognize each other as the same if they ever meet, for example not manifesting a preference in the choice of mates. Races may eventually diverge into true subspecies, but given their often-temporary separation, it's just as likely that they will merge after a few generations.
There are many variations of human skin colors, hair textures and colors, nose shapes, heights, etc., among populations and nationalities; but there is no fixed DNA difference that does not average out if cross-bred. The concept of race to distinguish groups with different appearances has social consequences, but virtually no genetically notable importance.
There were distinct populations of humans in the Paleolithic Era, when travel was limited to walking speed so hybridization across geographical boundaries was rare. But the invention of transportation technology changed all that, starting with boats capable of long-distance voyages, then large herbivores tamed for riding and finally wheeled vehicles and paved roadways to accommodate them. Even in the Bronze Age people traveled long distances between the various civilizations for trade and mere curiosity. The Silk Road, for example, was a major vector for the breakdown of the differences between human "races."
The full-rigged sailing ship increased the rate and volume of hybridization and extended it to previously isolated regions such as the Western Hemisphere. The motorized vehicles of the Industrial Era destroyed any remaining validity of the term "race" among humans.
It's ironic (and would be amusing except for the dire consequences) that the so-called human "races" are distinguished primarily by skin color. This is one of the most ephemeral of biological characteristics because it is controlled by only a few genes and is greatly affected by climate.
In the southern latitudes sunlight is intense and a bare-skinned species like ours needs a lot of melanin in its skin to protect against sunburn and melanoma. Darker-skinned individuals with more melanin will survive longer and outbreed the others.
But in the northern latitudes sunlight is less common and people need exposure to as much of it as possible in order to manufacture enough vitamin D to survive. Individuals with too much melanin will not live long and their lighter-skinned neighbors will outbreed them.
The light-skinned Latvians and the dark-skinned Bengalis are both descended from the Eastern branch of the Proto-Indo-European tribe. They are separated by a mere four thousand years of migration in different directions--only about 200 generations!
Or dogs. By the standards of us dog breeders,
all humans are "mongrels."