Science doesn’t really think of people being different races. People who have ancestors from very different places may look very different though. They also may have very different languages. I am a biologist and we usually leave language study to professionals called linguists, but the two parts to your questions have one basic answer: change.

The people who were the ancestors of all people lived in Africa, but their descendants slowly spread all over the world. As groups traveled, they probably settled in clumps where the living was good. It may be hard to imagine how isolated groups of people used to be, now that you can have breakfast on one continent, lunch on another, and dinner on a third. But back then, each clump would have been pretty isolated from the other clumps. Walking was the only way to get around except for boats with no motors or navigation equipment. These groups would be sort of like islands, whether they were actual islands or not. The people in these clumps would probably only be having children with people from the same settlement or ones nearby. The random changes in genes (mutation) that happened in one place would be different than the mutations that happened in another group.

People also became adapted to their environments. As I said, mutation is random. You don’t get a mutation that protects you from skin cancer because you need it. But in an environment that kills a lot of people, the people lucky enough to get mutations that protect them from skin cancer are more likely to survive and reproduce. The same mutation, randomly occurring in a place where there is very little sunlight, might be a problem. It might prevent people from making enough vitamin D, leading to weak bones. The mutation would probably die out in that area. Red hair might be found in only 1 of 1,000 people in a big population, but if 10 people go to an island, and one has red hair, the frequency of red hair on the island is 10% to start with, and it may always be high. This is called the “founder effect.”

Another process that can happen in small populations is “genetic drift.” Imagine that 10% of the people in a big population carry a gene for blue eyes. 10 of 100 people who go to an isolated place have the gene for blue eyes, but for a variety of reasons, all 10 of them die before they have kids. None of their descendants will have the gene for blue eyes.

The ability to speak is genetic. It requires certain areas of the brain and the ability to use the tongue, lips, and throat in certain ways. But the different languages do not have a genetic basis. I was raised speaking English. If I had been raised in a family speaking Urdu, Swahili, or Korean, that language would be the one I spoke. We don’t know what language the first humans had. As the groups became isolated, their languages would have changed. We can actually trace how certain languages have changed over time. For example, English came from German. Even in one country, language changes over time. You might laugh at the slang your parents used when they were young. Someday, kids will laugh at words that you think are neat, the cat’s pajamas, tubular, rad, or ill now.

Do you think that modern transportation will influence our ideas about ethnicity in the future? To learn more about how groups of people came to be unique, you may want to study anthropology.

Race and language both evolve over time in a way that depends on the environment. For race, I'm mostly talking about climate--melanin (the pigment that makes skin tan or dark) is believed to reduce the risk of skin cancer, so peoples that deal with harsh sunlight most frequently tend to have darker skin. Culture plays a role too, because the current perception of beauty can affect choices of mates, which affects how often physical features are passed on to the next generation.

Language also evolves due to culture. Over time, people sometimes come up with new words, pronounce words differently, use existing words in different contexts, or take words from other languages. If it catches on, eventually the change becomes a part of the language. One example of this is the difference between American English and British English. In the beginning, Americans and Britons spoke the same way, but over time, because they talk among themselves so much more, their language evolved in different ways.

In the field of linguistics (the study of language), people trace the history of how every language evolved and keep track of common ancestry using language families.

The same reason there are many different languages and tribes within Africa: there is still expansion and isolation and drift, even within the same continent. Peoples from different parts of Africa all are dark-skinned due to natural selection for high melanin concentrations as a result of tropical sunlight; however, that does not mean that all Africans are the same.