Well it depends if you mean in terms of genetics or in terms of culture. Genetically, humanity has been through a serious bottleneck. We are what biologists call (perhaps ironically) a "small species". Any two chimpanzees might be more different genetically than a Japanese, Italian, Scandinavian, Haitian etc. Confucius said that by nature men are very much the same, but in practice they are very far apart. I think he was on to something.
That all depends of which genetic evidence one would care to consult, if you are referring to Harvard professor Richard Lewontins estimate of genetic variation that he proposed in 1972, please do so with caution because he has been debunked to the point that his estimate of genetic variation that exist between human populations is known as the Lewontin fallacy. His calculation was that only 15 percent of genetic variation exist between human populations, whereas 85 percent is within populations.
Brilliant American evolutionist Vincent Sarich, in his work known as Race: The Reality of Human Differences cites a more accurate measurement of human variation that exist between human populations which comes 32.5 %, rather then 15 percent. The following is somewhat difficult to follow, but read it over and over until you understand it.
First is the 15 percent that is interpopulational. The other 85 percent will then split half and half (42.5 percent) between the intra- and interindividual within-population comparisons. The increase in variability in between-population comparisons is thus 15 percent against the 42.5 percent [not 85 percent] that is between-individual within-population. Thus, 15/42.5 = 32.5 percent [as opposed to 15/100 = 15 percent]
Page 169 of Race: The Reality of Human Differences
In any case, Richard Lewontin used the "Wright's fixation index" or FST to measure human variation, the creator of the FST genetic variation measurement system was a brilliant American geneticist named Sewall Green Wright who was one scientist along with R.A. Fisher and JBS Haldane, that laid the foundations for population genetics. He stated that "
if racial differences this large were seen in another species, they would be called subspecies." For more info on Sewall Green Wrights opinion on human diversity, read Evolution and the Genetics of Populations, 1978 and you will see that he is in favor of classifying humans into races.
If biological scientist of all ilks classify non-human species into subspecies based on certain criteria for genetic variation, then for scientific consistency, they should classify humans into subspecies as well if they share this level of genetic diversity. Vincent Sarich sums up this situation perfectly in this relevant quote.
"I am not aware of any other mammalian species where the constituent races are as strongly marked as they are in ours
except those few races heavily modified by recent human selection; in particular, dogs."
[ame]http://www.amazon.com/Race-Reality-Differences-Vincent-Sarich/dp/0813340861[/ame]