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Realistic calculation of the date of our most recent common ancestor

In the previous posting, I discussed the calculation of Joseph T. Chang in which he showed that the most recent common ancestor (MRCA) of all the people living today lived around 1100 CE, while around 400 CE everyone who lived then was either the ancestor of all of us or none of us. The date when this occurs is called the IA (identical ancestor) date.

Chang got these results assuming that the population is constant over time at some value N, that the generations (with each generation lasting 30 years) are discrete and non-overlapping (i.e. mating took place only between males and females of the same generation), and that mating was random (i.e., there was equal probability of any one male in a generation to breed with any female of that same generation.)

What happens to these dates if you relax these unrealistic assumptions? One practical difficulty of going to more realistic models is that exact mathematical calculations become impossible and one has to resort to computer simulations. This was done by Douglas L. T. Rohde, Steve Olson, and Joseph T. Chang and their results were published in the journal Nature (vol. 431, September 30, 2004, pages 562-566).

As a first improvement, they divided the world into ten population centers (or ‘nodes’): one each in North America, South America, Greenland, Australia, the Pacific Islands, and the Indonesian archipelago, and two nodes in Africa and in Asia. Within each subpopulation, they assumed random mating, but allowed for neighboring populations to exchange just one pair of migrants per generation. Their computer models found that the best way to accommodate varying populations was to take a fixed value N equal to the population at the time of the MRCA. They assumed N to be 250 million, which was approximately the global population in the year I CE.

Using this more realistic model, and a generation span of 30 years, they obtained the MRCA date as 300 BC and the IA date as about 3,000 BCE, both still surprisingly recent.

They then constructed an even more sophisticated and realistic model. They broke up the inhabited area into three levels of substructure: continents, countries, and towns. (These were not real places, of course, just models, but they used our knowledge of geography and migrations routes that existed before 1,500 CE to create their models.)

The model allowed for each person to have a single opportunity to migrate from his or her town of birth. Within a country, they could migrate to any other town. If the migrants went to another country, the probability of that occurring decreased with the distance to the new country. To go to another continent required them to go through certain ports, and so on. The model also incorporated our knowledge of the size of ports and when they opened up.

Generations could also overlap in this model and the birth rate of each continent was adjusted to match historical estimates.

After making all these sophisticated adjustments to make their model more realistic, they arrived at what they felt was a reasonable estimate for the MRCA and IA dates. It turns out that the MRCA lived around 55 CE and the IA date is about 2,000 BCE. They also found that our most recent common ancestor probably lived in eastern Asia, not Africa as had been commonly supposed.

So despite going to considerable lengths to simulate a realistic pattern of population growth, mating, and migration, the dates arrived at for the MRCA and the IA are still surprisingly recent.

(If the authors of the paper made their parameters very conservative, they pushed the date for the MRCA only as far back to 1,415 BCE and the IA date to 5,353 BCE.)

A little reflection should persuade anyone that this result that our most recent common ancestor lived as late as 55 CE and in just 2,000 BCE we had identical ancestors has profound implications for the way we view ourselves and our relationship with others. The authors capture the wonder of it all when they end their paper with the following comment:

[O]ur findings suggest a remarkable proposition: no matter the languages we speak or the colour 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 laboured to build the Great Pyramid of Khufu.

I find this amazing and remarkably encouraging. It should be more widely known. If more people realized how close we are to each other, perhaps we would stop killing one another and treat each other like the fairly close relatives we truly are.


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