But if Earth is one unified government, then the human population should be about 1/4 Asian and 1/4 Arabic, no?
I don't see what one government has to do with racial/ethnic makeup of the planet.
Here's roughly how it breaks down today:
East Asian 24% (Korea, Mongolia ,China, Japan)
South Asian 21% (India, Sri Lanka, Pakistan, Bangladesh, Nepal)
Southeast Asian 9% (Cambodia, Bruma, Philippines, Malayasia)
Middle Eastern 8%
Unless population trends radically changed in the intervening centuries, Star Trek's representation of the human animal is basically backwards. We should see two Kaikos for every Miles and Benjamin.