We are a commune of inquiring, skeptical, politically centrist, capitalist, anglophile, traditionalist New England Yankee humans, humanoids, and animals with many interests beyond and above politics. Each of us has had a high-school education (or GED), but all had ADD so didn't pay attention very well, especially the dogs. Each one of us does "try my best to be just like I am," and none of us enjoys working for others, including for Maggie, from whom we receive neither a nickel nor a dime. Freedom from nags, cranks, government, do-gooders, control-freaks and idiots is all that we ask for.
All humans are thought to be descendants of one woman, Mitochondrial Eve, who lived around 140,000 years ago - 4600 generations ago - in West Africa. Our cellular mitochondria follow the female genetic line.
Do the math as you count generations into the past. Two parents, four grandparents, eight great grandparents, sixteen great-great-grandparents, and so on through the centuries. Soon you reach a point where the number of ancestors in a given generation is larger than the population of the Earth at the time. (This is possible because as you move back, a single individual may occupy a large number of slots on your family tree, which is known as pedigree collapse.)
Pic is by Masolino, c 1426, in the Brancacci Chapel, Florence
Mother Nature keeps track all possible ways for genes to survive from one generation to the next, but it happens to be easiest for us right now to track mitochondrial DNA, which passes only from the mother. Mitochondria don't reproduce sexually and therefore don't swap genes, so their genome is very stable compared to ours. The slow, steady, natural rate of mutation gives us a pretty decent "clock" to measure how far back the lines split apart.
For some time I have read that we are all decended from the "Eve" in Africa, but have yet to see any convincing evidence. Given the age of civilization in China, my bet would be that there were humans there before the ones in Africa.
LomaAlta - no chance on the China hypothesis. The start of civilization in China is more than an order of magnitude more recent than humans leaving Africa.
As to the mathematics of a billion ancestors a thousand years ago, when there were not a billion people: Overlap means that the number of your ancestors alive at any given point in history maximizes at about 2 million. The phenomenon is called Pedigree Collapse, and I wrote about it last year. http://assistantvillageidiot.blogspot.com/2005/12/pedigree-collapse.html
Assistant Village Idiot
I forgot that I had commented the first time, and was about to write almost exactly the same thing. I had better come up with something new.
I recommend Cochrane and Harpending's anthropology blog West Hunter.
Assistant VIllage Idiot
Even more interesting than mitochondrial eve (definitionally go back to common female ancestor) is the idea of a much more recent 'window' where the modern human population collapsed. I think there is some evidence that all modern humans descended from one of something like 14 women, much, much more recently than mitochonrial eve. That is not to say that there were only 14 women alive at that time (other lines may have made it a while, but died out, other lines may have come through chromosonally through sons rather than daughters), but that we all descended from one of 14 women, and fairly recently in a genetic/geologic timeframe.
"This is possible because as you move back, a single individual may occupy a large number of slots on your family tree, which is known as pedigree collapse."
Anyone who has ever done is genealogy to before the invention of the railroad is familiar with this. James Burke, author of the "Connections" books and TV series, wrote that James Watt had actually caused THREE enormous changes in society. The first was the invention of the steam engine (or rather, made significant improvements to a predecessor device) which powered the First Industrial Revolution.
Second, the development of the steam engine made railroads possible. Before railroads, you generally lived in the small village that your family had lived in, going back generations - sometimes MANY generations. You married somebody else from the same village, who was almost inevitably SOME kind of cousin. And even in religions (such as Judaism) that imported brides from other villages, there was still a LOT of overlap. James Watt not only sparked the industrial revolution, he sparked a genetic revolution as well.
Go to the Tower of London, and look at the ancient armor. Henry the Eighth was a SHRIMP compared to modern man. In animal husbandry, we have a name for this - but it seems impertinent to use it for humans - but there it is. Modern Man is the product of "hybrid vigor".
In my own ancestry, there are a number of "knots" in the weave; generally second or third cousins.
James Watt's THIRD development? The soot scraped off the boilers of the steam engines got a new life; carbon paper. Watt also gave us the tool of the bureaucracy. Well, I guess he couldn't have foreseen THAT.