First, researchers have discovered primate fossils in Yellowstone National Park that date back to way before the first undisputed primate (55 million years ago):

I was surprised that they found this in North America, but then I only know about human migrations, but honestly I have no idea where they’ve found other primate fossils, and they have done research that shows horses actually developed in North America and then moved into Europe and Asia, so migration among the continents seems common enough.

On to humans:
This article talks about how Indian women are “renting” their wombs to infertile couples who can get a better price in India (up to $5000, versus $10,000 in the U.S.): http://www.msnbc.msn.com/id/16988881/, http://www.latimes.com/news/nationworld/world/la-fg-surrogate19apr19,0,4100387.story?coll=la-home-headlines
These articles talk about the cultural implications, how it’s mostly lower-middle-class wives, are rich countries taking advantage of people in need, etc., which is all important, but my first thought was about the biology of it all.
More and more research is showing that the lifelong health and nutrition of the mother have a big effect on the fetus. I don’t mean to sound negative, but India is a poor country (hence why $5000 goes such a long way). The country doesn’t have a very good health care system, a lot of Indians probably don’t have very good nutrition, and living in a big Indian city like Mumbai or New Delhi is equivalent to smoking a pack of cigarettes a day! They said the majority of these women are living in Anand, which is India’s milk production capital according to Reuters, so the Anand women are possibly healthier overall because of easy access to the milk and they are possibly more prosperous than some other cities, but, bear with me here, I still think it fairly possible that by outsourcing fetus-growth services to a developing nation would result in a lesser-quality product (i.e. baby), as well as putting women at a greater risk for illness and death from the stress of carrying an extra child to term (all these surrogates already have at least one surviving child).
I have spent time in India, and have just spent months writing an article on entrepreneurship in India, and I know that in some ways India is ahead of the game, but in some ways they’re really far behind.
I could be delving into this way too much and I should also point out that so many U.S. women spend their lives drinking Coca-cola and eating McDonald’s, and their kids come out healthy (although they may not stay that way eating that crap), but it just made me stop and think.
Disclaimer: I am also not a mother, medical doctor, or even biological anthropologist, just your average cultural anthropologist who lives with an evolutionary biologist and in a community of hippies that talk about Chakras and don’t drink caffeine while pregnant (although doctors just recently decided a pregnant woman can drink up to three cups of coffee a day without potentially harming the fetus. Just shows that medicine isn’t exact).