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From:
Subject: Most recent common ancestors
Date: 15 Jan 2006 07:29:50 -0800


(Also published at http://www.livejournal.com/users/nhw/563512.html )

Doug Rohde's paper on the most recent common ancestor of all humanity
(http://tedlab.mit.edu/~dr/Papers/Rohde-MRCA-two.pdf) is, I think, well
known to most here. In my voiew it is by far the most interesting of
the numerous pieces of research cited by Mark Humphrys
(http://humphrysfamilytree.com/ca.html) on this topic. Rohde's computer
simulations gave results of between 3,000 and 5,000 years ago for the
lifetime of the most recent common ancestor of all humanity.

Reading through his paper, it seems clear to me that his conclusions
are too modest; that in fact it is entirely likely that the most recent
ancestor of all humanity lived around 2,000 years ago. I have several
reasons for thinking this:

1) Rohde admits that he is using unrealistically low rates of
inter-country migration, simply so as not to get results that are too
startling. If the real rate of migration between countries and
continents is higher than the one he used, the time to the most recent
common ancestor decreases.

2) His models assumes that women have an equal probability of bearing
children every year between the ages of 16 and 40, thus giving an
average age difference between mothers and their children of 28. I
reckon this flattens out the natural bump (!) at the lower end of that
age range, and my suspicion (without any proof) for most of human
history is that most children were born to women aged between 14 and
30. That too will decrease the time to our most recent common ancestor,
as the time between generations will be shorter.

(A digression: female-female lines are much harder to trace, which is
odd given that there is never any doubt about who a child's mother is.
For instance, little is known of Mary Garritt, the wife of Thomas Webb,
a surveyor in Stow-on-the-Wold in the mid-18th century. Her daughter
Frances (1775-1862) married Thomas Salisbury, landlord of Marshfield
House in Yorkshire. Their daughter Anne (1806-1881) married another
gentry type, Edwyn Burnaby of Baggrave Hall in Leicestershire. Their
daughter Caroline (1832-1918) married a widowed clergyman who was the
grandson of a duke. Their daughter Nina (1862-1938) managed to bag an
earl as her husband. Her daughter Elizabeth (1900-2002) did rather
better than a mere earl. Her daughter, another Elizabeth, was born in
1926 and is still alive; those of you in the UK and Canada will find
her depicted on certain useful everyday objects, ie money. But her
direct female line ancestry can be traced back only six generations
before it is lost in the Gloucestershire middle classes.)

3) Rohde leaves out the effect of occasional exceptional individuals
(what in homage to Asimov we might call the "Mule effect"), in this
case those with vast numbers of children all of whom produce
descendants, such as Genghis Khan. Zerjal et al demonstrated that
Genghis Khan's Y-chromosomes are present in large proportions of the
male population of his former empire.

That of course only measures the direct male-line descent of the
individuals concerned. It must be pretty certain that if you take all
lineages into account, Genghis Khan is an ancestor, quite likely the
most recent common ancestor himself, of everyone between the Aral Sea
and the Pacific north of the old boundary line. If he had not fathered
the immense number of children he appears to have done, that would
surely have added another couple of centuries to the time since the
most recent common ancestor of the people of the region.

I've argued at http://explorers.whyte.com/muhammad.htm that most of us
are descended from the Prophet Muhammad. Someone living in the first
few centuries AD, probably in East Asia, probably a man with children
by several different women (quite possibly in different places), is the
most recent person who is the ancestor of us all.

Of course, while this is a nice concept, it's not quite as strong as it
seems. Rohde points out one reason for this, which is that at the
distance of 50 generations the likelihood that we have inherited any
genetic material at all from this one particular ancestor is pretty
minimal unless you happen to be fairly close in geographical proximity
to them.

There is another reason as well, which is that family ties are not just
about genetics but are also about how you feel. By emphasising the
arrival of children in a family as the product of procreation between
married couples of opposite sexes, the Most Recent Common Ancestry
model leaves out all the messiness of real life - adoptions, most
obviously, but various other possibilities are all around us. It's an
attractive mathematical concept, but we have to bear in mind that it
isn't the whole story.

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