Common Ancestors of All Humans

Mark Humphrys - History - Family History - Our common ancestors - Royal - Royal Descents of
famous people - Common ancestors of all humans

Main Sources: 
Richard Dawkins (Foreword) and Steve Jones (p.320) in The Cambridge Encyclopedia of Human
Evolution, 1992. 
In the Blood: God, Genes and Destiny, Steve Jones, Harper Collins, 1996. 

Mitochondrial Eve: 
Ayala, F.J. (1995), The myth of Eve, Science 270, 1930-36. 

Y-chromosome Adam: 
Paabo, S. (1995), The Y chromosome and the origin of all of us (men). Science 268, 1141-42. 
The above is a commentary on (in the same issue): 
Dorit et al (1995), Absence of Polymorphism at the ZFY Locus on the Human Y
Chromosome, Science 268, 1183-5. 
There are further commentaries on this in: 
Various (1996), Estimating the age of the common ancestor of men from the ZFY intron.
Science, 272, 1356-62. 

Mathematical models: 
Chang, Joseph T. (1999), Recent common ancestors of all present-day individuals, Advances in
Applied Probability 31(4), 1002-26. Followed by discussion (including comments by Montgomery
Slatkin, W.J. Ewens, and J.F.C. Kingman) and author's reply, 1027-38. 

Neil O'Connell (formerly here) 
Branching and Inference in Population Genetics (1994) 
N. O'Connell. The genealogy of branching processes and the age of our most recent
common ancestor. Advances in Applied Probability, 27:418-42, 1995. 

Sources yet to be consulted: 
Mitochondrial Eve: 
Cann, R. L., Stoneking, M., and Wilson, A. C. (1987), Mitochondrial DNA and human evolution. Nature
325, 31-36. 
Vigilant, L., Stoneking, M., Harpending, H., Hawkes, K., and Wilson, A. C. (1991), African populations
and the evolution of human mitochondrial DNA. Science 253, 1503-07. 

Nei and Roychoudhury (1982), Evol. Biol., 14:1. 
Goldstein et al (1995), Proc. Natl. Acad. Sci. USA, 92:6723. 

Mathematical models: 
A.M. Zubkov. Limiting distributions for the distance to the closest mutual ancestor. Theory Probab. Appl.,
20(3):602-12, 1975. 
P. Jagers, O. Nerman, and Z. Taib. When did Joe's great ...grandfather live? Or on the timescale of
evolution. In I.V. Basawa and R.L. Taylor, editors, Selected Proceedings of the Sheffield Symposioum on
Applied Probability, volume 18 of IMS Lecture Notes-Monograph Series, 1991. 
Branching Processes 
V.A. Vatutin. Distance to the nearest common ancestor in Bellman-Harris branching processes.
Math. Notes, 25:378-87, 1979. 
Coalescent Theory 
Martin Moehle 

Common ancestor of all living humans ("CA"s) 

[O'Connell, 1995] is wrong to suggest there is any doubt about the existence of CAs. Their existence is certain, as is the fact
that they are human. The existence of genes for human abilities (e.g. language acquisition) in all humans is proof of this. 

Most recent female-female line ancestor of all living humans
("Mitochondrial Eve") 

Take all people alive today. Take all their mothers. This is a smaller set. Take all their mothers. This is a smaller set. And so
on, until you get to 1 person ([Slatkin, 1999] says you must get to 1 person since mathematically this is "a pure death process
that has an absorbing state at 1"). 

Our most recent female-female line ancestor is called "Mitochondrial Eve" since Mitochondrial DNA passes (almost) entirely
through the female line and so may be used to estimate a date for her. Contrary to a lot of confused discussion, e.g. [Ayala,
1995], Mitochondrial Eve's existence is not in doubt. We can work it out from our armchair. What is in dispute is the date,
which has been estimated at 100,000 to 200,000 years ago. 

Also contrary to much confused discussion by paleontologists, no date for Mitochondrial Eve implies any sort of population
bottleneck at that time. Mitochondrial Eve would have co-existed with huge numbers of male and female relations from whom
we also descend. Indeed, [Ayala, 1995] points out that our inheritance from Mitochondrial Eve would be only 1 part in
400,000 of our DNA. The rest we inherit from her contemporaries. But he still spends half the paper attacking the idea of a
small ancestral population - an idea that no one believes. 

Oxford Ancestors will tell you into which local European cluster your female-female ancestry belongs. See article and
New Scientist article. 
publications (and here) 
Report (and here) by the same Oxford group showing a close DNA relationship between some modern English
people and the native hunter-gatherer English of 7000 BC (not just pre-Norman, pre-Anglo-Saxon, pre-Celt, but

Most recent male-male ancestor ("Y chromosome Adam") 

Similarly, by studying male-only DNA, we can try to get an estimate for "Y chromosome Adam". Here there is little to no
variation, and much controversy about why. Estimates range from 15,000 to 270,000 years ago, depending on the model used.

Clearly if we had kept surnames since Y chromosome Adam, then we would all now bear his surname (despite him
having millions of male contemporaries of different surnames). As a result of thinking about Y chromosome Adam, we
can see that if surnames continue into the future, then not only will all hereditary titles die out, but all surnames except
one will die out too. 

Most recent common human ancestor in any line

Above we could only follow the female-female or male-male path, ignoring the billions of other ancestral paths and thus pushing
the common date much further back into the past. The most recent CA in any line, what [Chang, 1999] calls the "MRCA",
would be much more recent. 

[Jones, 1996, Ch.I] suggests as recent as 700 BC, but provides no real support for this since he does not estimate the intensity
of intermarriage. Apparently "anthropologists claim everyone on earth is a 40th cousin" (see also here) (i.e. any pair of 2 people
can find at least 1 common ancestor since about 1000 AD). 

Discussion of Chang's work 

[Chang, 1999] looks at mathematical models rather than DNA (above) or history (my Royal Descents page). What he
seems to do different from everyone else is build a 2-parent rather than 1-parent model - in pursuit of the real MRCA, rather
than just the female-female or male-male one. 

In his model, if we assume a constant population size, 2 parents per individual, and random mating, then we expect the MRCA
to be (log2 of the population size) generations in the past. This is incredibly recent. e.g. Take population size as (a generous)
500 million to estimate the world population over recent history. Then the MRCA is 29 generations ago - say around 1200
AD! ([Ewens, 1999] notes this is basically the reverse analogue of the fact that you only need to go back log2(n) generations to
need n separate ancestors.) 

Chang says this medieval MRCA is implausible (though as my Royal Descents page illustrates, it is not that implausible at
all) and notes that one problem with applying the model to humans is random mating. In reality, mating is of course local. The
model does allow for "unlucky" random mating which could push the MRCA back, but notes that it is very unlikely in a large
population that you get unlucky enough mating to push it even twice as far back. Perhaps local mating is just unlucky random
mating and makes little difference in the long run. But this needs to be proved (by constructing a local-mating mathematical
model). It may be that the pattern of local mating is extremely distorted by earth's specific geography, so perhaps only a
computer simulation (rather than a general mathematical model) can solve this issue. 

An extreme example of earth's geography would be total isolation. Many human populations, especially in Australia, the
Pacific, the Americas and the Arctic, seem to have been isolated from each other until modern times. If populations were truly
isolated, then the probability of 2 individuals mating either side of the barrier may truly have been zero for thousands of years. In
which case the MRCA for the world would be pushed back to thousands of years ago. Apparently [Nei and Roychoudhury,
1982] and [Goldstein et al, 1995] use DNA to estimate ages for the MRCA of 116,000 and 156,000 years ago. One wonders
if they are aware that DNA cannot be used to estimate the MRCA. 

Whatever about the world as a whole, Chang's model does suggest that the MRCA for Europe, where populations constantly
mixed, may be well within historical times. Quite likely (as is suggested by other independent evidence on my Royal Descents
page) the entire population of the West descends from Charlemagne. 

One wonders what Chang's model would predict for the most recent strict female-female or strict male-male ancestor.
Comparing this with the DNA figures might give us a handle on how unrealistic random mating is as a model. 

Chang's second result is that when you go far enough back, every individual is either an ancestor of the whole world today, or
else is an ancestor of no one alive today. In nature, it is obvious that this state must be reached as you go back, see [Dawkins,
1992] - just consider ancestral fish. If I am descended from a particular one, then so are all humans. 

In Chang's mathematical model this state is reached very quickly, within about 1.77 times the number of generations above. i.e.
With our estimate above we get: 

Before 700 AD, every single human is either ancestor of no one alive today, or ancestor of everyone alive today. So
the Islamic Muhammad, the Irish/Celtic Niall of the Nine Hostages, the English/Saxon Cerdic, and the
Continental/pre-Norman Charlemagne, are all ancestors of everyone alive today. In fact, the model predicts that 80
percent of the entire population at this time is an ancestor of everyone alive today. 
Between 700 AD and 1200 AD, every single human is either ancestor of no one alive today, ancestor of everyone
alive today, or ancestor of some people alive today. 
After 1200 AD, every single human is either ancestor of no one alive today, or ancestor of some people alive today. 

Accepting that it is wrong to draw the above conclusions with locally-mating humans - despite that, these figures are in fact
quite plausible (if restricted to the Western world at least). 

The MRCA will be much more recent than any DNA ancestors 

Chang also notes that the MRCA will be much more recent than any MRCA that could ever be found in DNA studies, even if
one were to study the ancestry of every single gene. The reason being that we are considering people who are simply
ancestors, through any route, whether or not any of their genes actually survived the journey. 

Actually, will it be much more recent, or just more recent? If you studied all genes in the genome, and found an MRCA
for each gene, would any of them be in recent history? Let us use the term "MMRCA" to describe the most recent MRCA of
any gene. Chang's result implies you would find actual CAs in recent history, but this is taking the most extreme definition of a
CA, including those who left no genes at all to the present day (let us call these "CA1s"). Now these CA1s outnumber the
"CA2s" who have left any genes in different places on the genome in a minority of their descendants, who themselves
outnumber the CA3s that have left the same gene in the same place in a minority of their descendants, who themselves
outnumber the CA4s who have left just one gene in the same place in all of their descendants. 

So the original CAs (the CA1s) outnumber the CAs of a gene (the CA4s), but do they vastly outnumber them? And what if
we are calculating the most recent MRCA of any gene (the MMRCA)? The answer seems to depend partly on the size of the
genome. As genome size tends to infinity, it becomes impossible, say, for one of your grandparents not to leave a gene to you
(if genes come randomly from each side). By extension, as genome size tends to infinity, it becomes impossible for an actual
ancestor (CA1) not to be at least a partial genetic ancestor (CA2) as well. So the difference between CA1 and CA2 breaks
down. Not just that, but as genome size tends to infinity, the number of genes a CA1 must pass to you goes to infinity, and (I
think) finding two in the same place in two descendants becomes more likely. So the difference between CA2 and CA3 breaks
down. And with a finite number of individuals, finding two in the same place for all of them becomes more likely. So the
difference between CA3 and CA4 breaks down. 

So the question is: As genome size tends to infinity, must the MMRCA be found in recent history as well? Note that almost all
MRCAs for all genes might be found much further back, but, as genome size tends to infinity, the MMRCA might become
simply as recent as the actual (Chang) MRCA. 

And the follow-on question is: How well does the human genome size correspond to "infinity", i.e. what would we expect for
the most recent MRCA for any human gene? 

The DNA ancestors will be much more recent than the fossil evidence
would suggest 

Even the MRCAs found in DNA studies will exist much more recently than the paleontologists might imagine looking at the
fossil evidence - for the simple reason that they are merely "statistical artefacts" of no real importance to the overall story of
human evolution. It would be totally wrong, for example, to imagine that the CA lived in an important or influential place or
culture. See [Dawkins and Jones, 1992]. 

For instance, [O'Connell, 1995] is confused about Mitochondrial Eve's relation to the fossil record - no date for Mitochondrial
Eve, no matter how recent, could possibly contradict the fossil record studied by the paleontologists. This is based on the error
of assuming that Mitochondrial Eve is important (see above). 

One could even say that genealogy is the pursuit of statistical artefacts. 

These CAs are all moving targets 

Finally, remember that all these CAs are defined relative to the world in the state it is in now, i.e. in 2000 AD. If we were
living in 1000 AD, we would be talking about a completely different MRCA. Similarly, someone alive today (maybe you,
maybe me) is an MRCA of the world at some future date. 

"Mitochondrial Eve" is only defined relative to AD 2000. In 1000 AD there was a different Mitochondrial Eve, and in 1000 BC
there was a different one still.