Wednesday, August 13, 2014

Asian / European born people, forefather - Chenghiz Khan

Are you distantly related to Genghis Khan? If you have Asian and/or European ancestors, you just might be.

How did people figure out that the distant "father" of 1 in 200 males alive today lived 800-1200 years ago? Or that the father of all humans ("Adam") was alive 40-50,000 years ago? These estimates come from looking at differences in the Y chromosome over time.

But wait a minute you might be saying. You just said that we can use the Y chromosome for historical studies because it can't recombine and so stays constant.

Notice I said earlier that the Y chromosome passes from generation to generation virtually unchanged. It does change a little over time because of random mutations (nothing is perfect including copying DNA). It is these rare mutations that let scientists date when things have happened in the past.

A recent study was done to look at the Y chromosomes of 2,123 men across Asia. An astonishing 1 in 12 men shared the same Y chromosome. If this ratio holds up, that would mean 16 million males or 1 out of every 200 living males share this Y chromosome.

The study looked at blood samples collected over a period of ten years from more than 40 populations living in and around the former Mongol empire.

Geneticists use the Y-chromosome in population studies such as this because it doesn't recombine as other parts of the genome do. When it comes to eye color, or height, or resistance or susceptibility to particular diseases, each parent contributes half of a child's DNA, which join together to form a new genetic combination.

The Y-chromosome is passed on as a chunk of DNA from father to son, basically unchanged through generations except for random mutations.

Scientists use DNA to estimate when things have happened in the past by assuming a certain rate of mutations over time. Let's say that the Y chromosome gets 1 new mutation/generation (much higher than the actual rate). If this is the case, then if two people have 10 differences between them, then they are 10 generations apart.

The mutation rate scientists have used in the past was based on circumstantial evidence because there was just too much DNA to sequence. Until now. 

These random mutations, which happen naturally and are usually harmless, are called markers. Once the markers have been identified, geneticists can go back in time and trace them to the point at which they first occurred, defining a unique lineage of descent.

In this particular instance, the lineage originated 1,000 years ago. The authors aren't saying that the genetic mutations defining the lineage originated with Khan, who was born around 1162; they are more likely to have been passed on to him by a great great grandfather.

The lineage was found in only one population outside of the former Mongolian empire, in Pakistan.

"The Hazaras [of Pakistan] gave us our first clue to the connection with Genghis Khan," said Wells. "They have a long oral tradition that says they're his direct descendants."

Of course, the connection to Genghis Khan will never be a certainty unless his grave is found and his DNA could be extracted. Until then, geneticists will continue to seek out isolated populations in the hope of unraveling the mysteries of geographic origin and relatedness told by our genes.

For the first time, groups in Indiana and New Hampshire have figured out a mutation rate based on sequencing huge amounts of DNA from lots of the roundworm, C. elegans. How much DNA? An astonishing 4 million base pairs -- an impossible number just a few years ago.
What the researchers found was that the mutation rate was 10 times higher than previously believed or around 2 mutations/generation for C. elegans. There are possible reasons that given the way the experiment was done, the mutation rate might have been artificially high. But, if the new number is true, it calls into question all sorts of things.

For example, partly based on DNA evidence, scientists believed that chimps and humans separated about 5 million years ago. Was it actually 500,000 years ago? Humans began their migration out of Africa 100,000 years ago. Or was it 10,000? Did "Adam" live 50,000 or 5000 years ago?

Some of these new numbers are obviously wrong. From archeological digs, we know there were people in Europe, Asia, and even the Americas more than 10,000 years ago. From fossil evidence we know that chimps and humans separated more than 500,000 years ago.

Obviously, then, more work needs to be done to get at this critical number. And if the mutation rate isn't constant over time, we may never be able to get a good number. In the meantime, the work done here will need to be repeated lots more times before we begin rewriting history. With the advent of new sequencing technologies, we are now able to do experiments undreamed of just a few years ago.

Early results indicate that the distant "father" of all these men was from the Mongolia area around 800-1200 years ago. Who could have fathered all of these males? The most obvious candidate is Genghis Khan.

Genghis Khan lived from 1162-1227 and raped and pillaged from Mongolia to the gates of Vienna. Once he captured a village or town, he would essentially kill all the men and rape the women.

While deplorable from a social point of view, it seems to have been wildly successful from a biological point of view. As I said above, 1 in 200 males today may trace their lineage to Genghis and his sons.

Of course, this is a guess because we don't have Genghis Khan's DNA. His tomb remains hidden although the search is on to find it. Once we have his DNA, then we can determine if he really was as prolific as this data suggests.

To have such a startling impact on a population required a special set of circumstances, all of which are met by Genghis Khan and his male relatives, the authors note in the study published in the American Journal of Human Genetics.

Khan's empire at the time of his death extended across Asia, from the Pacific Ocean to the Caspian Sea. His military conquests were frequently characterized by the wholesale slaughter of the vanquished. His descendants extended the empire and maintained power in the region for several hundred years, in civilizations in which harems and concubines were the norm. And the males were markedly prolific.

Khan's eldest son, Tushi, is reported to have had 40 sons. Documents written during or just after Khan's reign say that after a conquest, looting, pillaging, and rape were the spoils of war for all soldiers, but that Khan got first pick of the beautiful women. His grandson, Kubilai Khan, who established the Yuan Dynasty in China, had 22 legitimate sons, and was reported to have added 30 virgins to his harem each year.

"The historically documented events accompanying the establishment of the Mongol empire would have contributed directly to the spread of this lineage," the authors conclude.