All humans can trace their family tree back to a surprisingly small group of common ancestors. Every person on Earth's most recent common ancestor might have died less than 2000 years ago.
A Question of Proper Breeding
When we get right down to it, we must face the truth that we're all hopelessly inbred. It's a question of basic mathematics - there simply aren't enough ancestors to go around. To understand what I mean, let's say you were born in 1975, your parents were both born in 1950, your four grandparents were born in 1925, your eight great-grandparents in 1900, and so on. In other words, your number of ancestors doubles every 25 years the further back in time you go.
If you take this back just 1,000 years, you'll find that you have well over 500 billion ancestors in a single generation. Considering there's fewer than seven billion people on this planet - and even that is far, far more than any other point in human history - there's something seriously wrong here. The solution, of course, is that you don't have 500 billion distinct ancestors, but rather a much, much smaller number of ancestors reappear over and over and over again in your family tree.
Figuring out just how many of your ancestors in a given generation were "real" and how many were duplicates can make for some fascinating statistics:
Demographer Kenneth Wachtel estimates that the typical English child born in 1947 would have had around 60,000 theoretical ancestors at the time of the discovery of America. Of this number, 95 percent would have been different individuals and 5 percent duplicates. (Sounds like Invasion of the Body Snatchers, but you know what I mean.) Twenty generations back the kid would have 600,000 ancestors, one-third of which would be duplicates. At the time of the Black Death, he'd have had 3.5 million - 30 percent real, 70 percent duplicates. The maximum number of "real" ancestors occurs around 1200 AD - 2 million, some 80 percent of the population of England.
Tangling the Family Trees
Exactly where this genealogical repetition kicks in can vary. Royal families are notorious for marrying off closely related relatives - which reached its most tragic extreme with Charles II of Spain, who actually would have been significantly less inbred if he was the son of a brother-sister pairing - creating some spectacularly tangled trees. Outside the aristocracy, there tends to be societal taboos against close intermarriage, but that can only be carried so far in small communities where people stay in the same place for their entire lives - which has been the norm for much of human history.
In those circumstances, it's quite common for people to have repeated great-grandparents or great-great-grandparents. The recent mass movement of humans in the last two centuries or so has scrambled this somewhat, so that people today tend to have far more diverse family trees than their ancestors of two hundred years ago, but this of course is just shunting all the inbreeding further back up the tree.
I should point out that, while this is inbreeding in the technical sense, the negative effects usually associated with the practice don't really apply because everyone involved is so distantly related. Indeed, since humans have 2^32 base pairs in their genomes, and only about half of a person's genes are passed on in reproduction, this means that a person's genes will be more or less completely flushed out of his descendants' genomes after 32 generations...or just about 1,000 years. So the fact that some geneticists believe we're all at least 50th cousins to everyone else on this planet thankfully doesn't mean we've completely corrupted our gene pool. Quite the opposite in fact.
A Most Recent Common Ancestor
While we don't really have 500 billion different ancestors, we can still look at the reverse of that idea: is there a single common ancestor that every person on Earth shares? The answer to that is a resounding "yes, of course" - indeed, the real question is figuring out which common ancestor you want to talk about. There are at least three different kinds of common ancestor that crop up in scientific discussion, and each lived at radically different parts of history.
The first type is simply known as the Most Recent Common Ancestor, or MRCA. The name says it all, really - this is simply the most recent person who, through any and all genetic lines, can be connected to every single person alive today. While it's fun to imagine a very small band of humans from which all humans are descended, the MRCA lived long, long after any such population bottleneck.
The MRCA is basically just a quirk of statistics, a random individual who happens to be the latest person who connects to everyone. He or she might have shared the planet with millions of other humans, all of whom left either no living descendants or are only related to some smaller subset of the people alive today. In fact, depending on who you believe, the MRCA might have lived just 2,000 years ago, so this definitely has nothing to do with being the world's only human.
Shifting Dates
What's particularly fascinating about this is that we in the present day can actually change who our most recent common ancestor was. After all, the estimate that the MRCA lived only two or three millennia ago, long after humans became isolated on far distant continents, only works because of the globalization of the last 500 years. The theory is that enough European explorers intermarried with the various indigenous populations of the places they colonized so that, over time, even the most isolated groups become linked into the overall family tree.
This is a controversial theory, particularly since there are still thought to be a handful of uncontacted groups in South America and southwest Asia. If these peoples - each group of which only numbers about two hundred or so - really have remained completely cut off from other humans for millennia, then that would force the most recent common ancestor back to the Upper Paleolithic, anywhere from 40,000 to 10,000 years ago.
We can at least say this: in 2011, it's possible but not proven that the MRCA dates back to a surprisingly recent date, anywhere from 8,000 to 2,000 years ago. In 1511, before European exploration had really begun in earnest, the MRCA was still unquestionably an individual who lived in the Upper Paleolithic. And, by 2511, the current trends in globalization suggest that everyone will definitely share a recent MRCA...and one that gets more recent with each passing generation as more and more lineages mix.
Mitochondrial Eve
So far, we've mostly discussed the MRCA in a purely statistical sense, without really bringing more tangible evidence to bear on the topic. There is some reason for that, and it goes right back to what we were discussing about long-term genetic inheritance. In sexual reproduction, a person will only pass on half of his or her genes, and their contribution to ensuing generations will continually divide by half. After a thousand years, even the MRCA's genetic contributions to humanity drop to virtually nil. That means the MRCA, who lived anywhere from 2,000 to 40,000 years ago, is more a statistical curiosity than a genetic benefactor, and he or she is related to all of us today in only the most technical of senses.
In order to find a common ancestor whose genetics have passed on, we need to look for things that are passed down from generation to generation with little or no alteration. Both genders pass along one thing that is unchanged during sexual reproduction. For women, this is the mitochondrial DNA, which is a distinct subset of genetic material found not in the cell nucleus but rather in the mitochondria, the power plants of the cell.
In most species, including humans, the female egg cells completely destroy the mitochondria in the male sperm cell shortly after fertilization, leaving only the female mitochondria behind. This is where we get the term "Mitochondrial Eve", made popular on shows like the new Battlestar Galactica. This individual passed down her mitochondria relatively unchanged to every human alive today, and all females will continue to pass down her mitochondria indefinitely.
By tracing the subtle mutations to mitochondrial DNA that have accumulated over the millennia, we can figure out which groups are most closely related, and ultimately fix the existence of Mitochondrial Eve to a fairly specific time in the past, which is currently estimated at about 200,000 years ago. As with the regular MRCA, Mitochondrial Eve would not have been exceptional during her own life. She certainly wasn't the only woman alive at the time, merely the only one who can trace descent to everyone alive right now.
Y-chromosomal Adam
Less well known than Mitochondrial Eve is her male counterpart. The science here is simple enough - since only men have a Y-chromosome, fathers pass it on more or less unchanged to their sons, which allows geneticists to trace patrilineal descent in much the same way that mitochondrial DNA allows us to trace matrilineal descent. Intriguingly, genetic evidence suggests that Y-chromosomal Adam lived about 90,000 to 60,000 years ago, long after Mitochondrial Eve.
Why is everyone's male ancestor so much more recent than everyone's female ancestor? It's all down to breeding patterns. In Paleolithic times, the general rule was that any fertile woman could expect to have a certain number of offspring, and this was fairly evenly distributed. Paleolithic men, on the other hand, might father many children by multiple mothers, or they might fail to father any children at all.
Of course, that's something of an oversimplification - anything describing tens of thousands of years of human activity inevitably will be - but it explains why our male ancestors cluster together more and why our common patrilineal ancestor is so much more recent than her matrilineal counterpart.
All of this speaks to how small our planet really is - and, indeed, always has been - a powerful reminder of how little real difference there is between us all. As Joseph T. Chang, Douglas L.T. Rhode, and Steve Olson observe in their 2004 paper on the MRCA, we're all shocking interrelated, and getting more so all the time:
"No matter the languages we speak or the color of our skin, we share ancestors who planted rice on the banks of the Yangtze, who first domesticated horses on the steppes of the Ukraine, who hunted giant sloths in the forests of North and South America, and who labored to build the Great Pyramid of Khufu. [And] within two thousand years, it is likely that everyone on earth will be descended from most of us."