Yale professor finds most common ancestor 3,000 years ago At some point in the distant and sparsely populated past, all humans were in the same small family.Now were all merely cousins.Homo sapiens appear to have originated in Africa and then radiated to every habitable spot on Earth.

Then as humans moved into Europe and Asia and trudged over the ice-bound Bering Straits into North America, families became increasingly tenuous.

Millennia elapsed and history happened, leaving us today with abbreviated genealogies.

Some families blessed with good record keeping can trace their ancestors back to Europe, Asia or Africa. What happened before that is lost.

Scientists traced present day mitochondrial DNA, which is only inherited from females, to a woman dubbed Mitochondrial Eve 100,000 to 200,000 years ago.

The idea is that all of us inherited mitochondrial DNA from our mothers, who got it from their mothers and so on, back to Mitochondrial Eve.

"Eve" is our most recent mitochondrial ancestor.

Joseph T. Chang, professor of statistics at Yale University, was intrigued.

Ancestors include both men and women, so our common ancestor was probably more recent, Chang reasoned  perhaps much more recent.

The question that interested Chang is how to mathematically estimate the date of our most recent common ancestor.

What is the typical time to the latest woman ancestor? The average is about twice the population size.

For example, in a population of 1 million reproducing at random, the mean time back to Mitochondrial Eve would be 2 million generations.

However, everyone has two parents. In a two-parent world of 1 million, the most recent common ancestor would be a mere 20 generations in the past, Chang said.

None of this is immediately obvious.

Chang, after a great deal of thought, proposed and proved two mathematical theorems on how to calculate the most recent common ancestor.

The theorems are elegant. They show that the time back to the most common ancestor equals the number of of years in a generation times the log (to the base 2) of the population.

The other theorem shows that at a certain time in the past, there will be two groups: In one group, everyone is your relative. The other contains no people related to you.

According to this model, all 6 billion of us currently alive could have a common ancestor less than 1,000 years ago  which doesnt seem right and isnt.

Thats because, in reality, everyone doesnt have an equal chance of mating.

The odds of someone in Australia mating with another Aussie are far greater than the probability that an Australian will couple with a person in Siberia.

So new mathematical and computer models had to take geography into effect, which adds another layer of complexity, Chang said.

People in close proximity are more likely to have kids than people on opposite sides of the country or at antipodes.

People also slowly move between continents and are more likely to leave at port cities.

Putting all of this together suggested that our most recent common ancestor should have been around about 3,000 years ago, Chang found.

"My co-authors and I tried to find a more realistic model of how people move. We started the simulations of the world population 20,000 years ago and ran them to the present," he said.

"We ran models under a number of scenarios  varying the probability of a person leaving his town, country and continent, how far they go and how many migrate per generation," Chang said.

The number still came out to 3,000 years. Thats the most recent common ancestor. As years recede back from 3,000, the number of common ancestors would increase.

"At a certain age back, everyone is a common ancestor or theyre unrelated to the current generation, extinct."

Chang calls these ancestors identical ancestors. Theyre at about 5,000 years before the Common Era.

If you somehow could go back to 5,000 BC, almost everyone you met would be a relative, no matter where you landed.

"5,000 BC seems very recent. The models may need to be changed. They have a lot of parameters," he said. Realistic values for these parameters are not known with precision.

There are also out of the way places on the planet where populations may have been isolated. Tasmania, for instance, may have received no visitors for thousands of years, meaning your most recentfull-blooded Tasmanian ancestor would have been thousands of years earlier. Meanwhile, 3,000 years is about 100 generations.

"Humanity is indeed one big family and were all 100th cousins," Chang said.

Chang said hes not sure whether his ancestry formulas will have other applications.

Even if they dont, he doesnt mind. His curiousity was satiated, at least temporarily.

Chang and co-authors had their paper accepted and published by Nature, one of the most prestigious scientific journals.