Human Migration Patterns
Human Migration Patterns
One of the most heated debates in anthropology and archaeology involves the evolution of man and the subsequent migration of the species that led to humans populating the world. Scientists question whether humans evolved in Africa or somewhere else and if the human species did evolve in Africa, scientists have asked when they began migrating to other places. In addition, anthropologists wonder whether humans, as they began their migration, simply replaced pre-human species in a given location or interbred with them.
Traditionally, scientists trying to answer these types of questions have traveled throughout the world searching for the oldest human remains and artifacts in a given location. Then, using scientific dating techniques, such as carbon dating, they estimate when humans might have first lived in a location. This type of work is obviously painstaking and requires great amounts of experience. It is highly dependent on environmental factors that may or may not have preserved human remains in a condition that allows for proper dating. It also assumes that the materials found are actually the oldest.
Beginning in the 1990s anthropologists and archaeologists began using techniques similar to those used in forensic science to solve some of the questions relating to human migration patterns. Instead of collecting bones and artifacts, scientists began collecting DNA from people all over the world. The DNA contains information that can be used to determine when populations from different parts of the world arrived at their current locations.
The DNA found in human cells is an extremely long molecule that is made up of a sequence of four different nucleotides. Over time, small changes, called mutations, occur in the order of the nucleotides in this sequence. If a molecular biologist detects a particular mutation in the DNA of a population somewhere in the world and then detects the same mutation in another group, and this mutation is not found in any other populations, it can be assumed that the two groups are closely related. That is, they probably were at one time a single group and then one group migrated to a new location. By searching for mutations and mapping them to the locations of populations of people throughout the world, anthropologists can build a picture of human migration patterns. In addition, because the rate of mutations in DNA can be estimated, scientists can also estimate when the various waves of migrations took place.
Two major types of molecular analyses have been used to probe questions concerning human migration patterns. A large study of the Y chromosome, which is passed from father to son, shows that all humans share a common ancestor who lived in Africa about 60,000 years ago. Another study focused on mitochondrial DNA (mtDNA). Mitochondria, which are organelles that power the cell, contain DNA that is passed from mother to daughter. The mtDNA study agrees with the Y chromosome study in placing the origin of the human species in Africa, however it demonstrates that migrations began much earlier, around 150,000 years ago. Archaeological evidence suggests that a wave of migrations out of Africa and into the Middle East began around 90,000 years ago.
Evidence from studies of the DNA in the Y chromosome show that a second wave of migrations out of Africa began around 45,000 years ago. These people moved to the Middle East, India, and China. During a brief warming period between Ice Ages, humans migrated farther east, to Central Asia, 40,000 years ago and then a group of them reached Europe about 35,000 years ago. Somewhere around 20,000 years ago, a group from Central Asia migrated north toward Siberia and the Arctic Circle. At the end of an ice age around 15,000 years ago, a group from the Arctic Circle migrated across the Bering Strait and populated North America. While these patterns of migration generally agree with archeological data, the dates tend to be much more recent than fossils and artifacts suggest.
Similar to traditional archaeologists, researchers working with mtDNA believe migrations out of Africa occurred earlier than Y chromosome data suggest. However, mitochondrial DNA analysis indicates that a single wave of migration, rather than two major waves, left Africa about 80,000 years ago and moved through the Middle East and toward India and Asia. This research then indicates that humans populated Australia 60,000 years ago, and artifacts found there corroborate these findings. The mitochondrial DNA data also indicates that people reached Europe about 50,000 years ago. This means that they cohabitated with Neanderthals for about 10,000 years, but that there was no interbreeding between the two groups of hominids. Finally, about 25,000 to 20,000 years ago, mtDNA data indicates that people from Siberia crossed over a land bridge to populate North America. Tools dating to 16,000 years old have been found in current-day Pennsylvania in North America.
Although there are discrepancies between the results from the two molecular techniques and that of archaeological data, most scientists agree that as more disciplines become involved in answering the questions, new and better insight will arise. In particular, the types of investigations that allow forensic scientists to identify differences between individuals contribute greatly to the understanding of differences between populations throughout the world.
see also Anthropology; DNA sequences, unique; Genetic code.