Whoever it is probably lived somewhere in East Asia a few thousand years ago: Taiwan, Malaysia, and Siberia are likely locations.
He - or she - has done nothing more remarkable than be born, live, have children and die.
Yet this was the ancestor of all humans living on Earth today: the last person in history whose family tree branches out to touch all 6.5 billion people on the planet today.
This means that everyone on earth is descended from someone who existed recently, in the domain oftutanchamon, perhaps even during the Golden Age of Ancient Greece. There is even the possibility that our last common ancestor lived around the time of Christ.
"It's a mathematical certainty that this person existed," said science writer Steve Olson, whose 2002 book Mapping Human History traces the species' history from its origins in Africa more than 100,000 years ago.
It is part of human nature to care about our ancestors: who they were, where they lived, what they looked like. people follow their ownGenealogy, collecting antiques and visiting historical sites in the hope of catching a glimpse of those who have gone before to place themselves in history and position themselves in the web of human existence.
But few people realize how closely this web ties them not just to the people living on the planet today, but to everyone who ever has.
Using a statistician, a computer scientist, and a supercomputer, Olson calculated how interconnected the human family tree is.
You would only have to go back in time 2,000 to 5,000 years, and probably to the lower end of that range, to find someone who could count all humans alive today as descendants.
In addition, Olson and his colleagues found that going back a little further, to about 5,000 to 7,000 years ago, all humans alive today have exactly the same ancestors.
In other words, every person who lived then is an ancestor of the 6 billion people living today, or their lineage died out and they have no descendants left.
This revelation was "particularly startling," wrote statistician Jotun Hein of the University of Oxford in England, commenting on the research published by the journal.Nature.
"If around 3000 B.C. If you had walked into any town on earth, the first person you would have met would probably have been your ancestor," Hein marveled.
It also means that we all have ancestors of all colors and creeds. All Palestinian suicide bombers have a Jewish background. Every Sunni Muslim in Iraq is descended from at least one Shia. And every Klansman family has African roots.
How can it be?
It's simple math. Each person has two parents, four grandparents, and eight great-grandparents. Keep doubling the generations - 16, 32, 64, 128 - and within a few hundred years you will have thousands of ancestors.
It is nothing more than exponential growth combined with the facts of life. In the fifteenth century you have a million ancestors. On 13 you have a billion. In the 9th century, just 40 generations ago, the number reached one billion.
But wait. How can anyone, let alone everyone else, alive today have a billion ancestors who lived in the 9th century?
The answer is no. Imagine that 1,200 years ago there lived a man whose daughter was his mother's 36th great-grandmother and whose son was his father's 36th great-grandfather. This would place you on two branches of your family tree, one on your mother's side and one on your father's side.
In fact, most of the people who lived 1,200 years ago appear not twice but a thousand times on our family trees because at that time there were only 200 million people on earth.
Simple division (one billion divided by 200 million) shows that each person then appeared an average of 5,000 times in the family tree of all people alive today.
But things are never normal. Many of the people who lived in the year 800 never had children; They do not appear in any family tree. By now, the most productive members of society appeared more than 5,000 times in many people's family trees.
Go further back in time and there will be fewer and fewer people available to place more and more branches of the 6.5 billion family trees alive today. It is mathematically inevitable that a person will eventually appear at least once in every tree.
But don't stop there; it keeps coming back As the number of potential ancestors dwindles and the number of branches explodes, there comes a time when every person on earth is an ancestor of all of us, except those who never had children or whose lineage eventually died out.
And it's not that long ago. If you walk through an exhibition of ancient Egyptian art from the era of the pyramids, chances are that everything was created by one of your ancestors: every statue, every hieroglyph, every gold necklace.
If there is a mummy in the middle of the room, it is almost certain that this person was also your ancestor.
That is, if Muslims, Jews or Christians claim to be children of Abraham, they are all right.
“No matter what languages we speak or what color our skin is, we share ancestors who farmed rice on the banks of the Yangtze, who first domesticated horses on the Ukrainian steppes, who hunted giant sloths in the forests of the north and south. America, and that he worked on the construction of the Cheops Pyramid,” Olson and his colleagues wrote in the journal Nature.
How can you be so sure?
Seven years ago, one of Olson's colleagues, a Yale University statistician named Joseph Chang, began thinking about how to estimate when the last common ancestor of all humans lived on Earth today.
In an article published in the journal Advances in Applied Probability, Chang showed that there is a mathematical relationship between the size of a population and the number of generations from a common ancestor.
Plugging the current population of the planet into your equation gives you a little over 32 generations, or about 900 years.
Chang knew the answer was wrong because it was based on some common but imprecise assumptions that population geneticists often use to simplify difficult math problems.
For example, his analysis has claimed that the earth's population has always been as it is today. It has also been assumed that individuals choose their mates at random. And each generation had to procreate at the same time.
Chang's calculations essentially viewed the world as one large market where every man would have an equal chance of marrying any woman, whether he lived in the next town or on the other side of the world.
Chang was aware of the inaccuracy: Unless they go through an arranged marriage, people have to choose their partners from the pool of people they have actually met.
But even then, they are much more likely to mate with partners who live nearby. And that means that geography cannot be ignored if you want to determine the relatedness of the world's population.
A few years later, Olson contacted Chang, who began to think about the world while writing his book. They began to communicate via email and were soon involved in their deliberations.Douglas Rohde, a neuroscientist and computer expert at the Massachusetts Institute of Technology who now works for Google.
The researchers knew they had to consider geography to get a better picture of how the family tree converges as it moves further into the past. They decided to build a giant computer simulation that would essentially recreate human history as humans were born, moved from place to place, reproduced, and died.
Rohde created a program that placed an initial population on a world map sometime in the past, between 7,000 and 20,000 years ago. The program then allowed these early residents to go about their business.
It allowed them to expand their numbers according to accepted estimates of population growth in the past, but had to limit the expansion to 55 million people due to computational limitations.
Although unrealistic in some respects—55 million is far less than the 6.5 billion people currently living on Earth—he determined through trial and error that the limitation did not materially alter the outcome of common ancestry.
The model also had to account for migration, based on what historians, anthropologists and archaeologists know about how often populations in the past moved within and between continents.
Rohde, Chang, and Olson chose a range of migration rates, from a low level, where almost no one left their country of origin, to a much higher level, where up to 20% of the population resides in a city other than their home country. which is found live reproduced They were born, and one in 400 people moved to a foreign country.
Allowing for very little migration, Rohde's simulation gave a date of around 5000 BC. the youngest common ancestor of humanity. Assuming a higher but still realistic migration rate, a surprisingly new date emerged around AD 1.
Some people even suggest that the most recent common ancestor may have lived later.
"Several people have written to me that the simulations are too conservative," Rohde said.
Migration is the key. When a people have children far from their birthplaces, they essentially introduce their entire family lineage into their adoptive populations, giving their immediate descendants and all who come after them a set of distant ancestors.
People tend to think of pre-industrial societies as places where this rarely happened, where virtually everyone lived and died within a few miles of where they were born. But history is full of examples that refute this notion.
Take Alexander the Great, for example, who conquered all the lands between Greece and northern India and fathered two sons by Persian mothers.
Think of Prince Abd Al-Rahman, the son of a Syrian father and a Berber mother, who fled Damascus after the fall of his family's dynasty and founded a new one in Spain.
Vikings, Mongols, and Huns traveled thousands of miles to burn, loot, and, more relevant to genealogical considerations, rape more established populations.
The more peaceful people also moved. In the Middle Ages, gypsies traveled from northern India to Europe in stages. In the New World, the Navajo migrated from western Canada to their present-day homeland in the American Southwest. East Asian peoples spread across the islands of the South Pacific, and Eskimos often traveled back and forth across the Bering Sea from Siberia to Alaska.
"When these genealogical networks start to spread, they really can go almost anywhere," Olson said.
While people like to think of culture, language, and religion as barriers between groups, history is littered with religious conversions, intermarriage, illegitimate births, and adoptions across these boundaries.
Some historical times and places were particularly active crucibles, for example, medieval Spain, ancient Rome, and the Egypt of the pharaohs.
"And the problem is, you only need one," says Mark Humphrys, an amateur anthropologist and professor of computer science at Dublin City University.
An ancestral connection to another cultural group among your millions of ancestors, and you share ancestry with everyone in that group.
Anyone who has married someone born far from their homeland, any lost sailor, any young man who has set out to find his fortune, any woman who has married a merchant from a foreign land who has left, provided they have had children, you have helped weave the web of brotherhood that we all share.