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The number of ancestors, what am I missing?

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LAN_403

Brett Clark

Thu Nov 26 2009, 12:00PM

Registered Member #482 Joined: Thu Jul 06 2006, 07:13PM
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Posts: 8

So you've got 2 parents, 4 grandparents, 8 great-grandparents, and so on. You see where this is going.

Assuming an estimate of 25 years per generation, each of us have 2^40 great*40 grandparents from only 1000 years ago.

1,099,511,627,776 great*40 grandparents.

That's only from the 1000 years.

I've read about http://en.wikipedia.org/wiki/Mitochondrial_eve and Y-Chromosomal Adam, but with numbers this large it would seem that humanity is seriously inbred.

I know this isn't a biology forum, but there are a lot of clever people here. What am I missing or what haven't I considered?

Proud Mary

Thu Nov 26 2009, 12:48PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

According to Bishop James Ussher, Anglican Primate of Armagh, in his Annales veteris testamenti, a prima mundi origine deducti the world was created in 4004 BC, a conclusion he reached by working backwards through the Old Testament genealogies from the present generation, until he arrived at our 'First Parents' in the Garden of Eden.

And who could doubt the weight of wisdom and erudition of a Bishop?

Sulaiman

Thu Nov 26 2009, 01:24PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Since at some time in the past there can have been only one modern human
(or a pair if you are of the Judeo-Christian-Islamic persuasion)
then we are all VERY inbred.
As population increased and diverged we got sort of less inbred

The answer to your conundrum is that there are comon ancestors to our ancestors
imagine a rural society where there is not much extermal gene exchange
everyone is quite closely related to everyone else,
this is why we can look at a person and know roughly where they are from.
Your initial assumption of UNIQUE ancestory is the problem - it's actually shared.

Bjørn

Thu Nov 26 2009, 02:59PM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

If we did not have common ancestors we would not all be humans but completely different species. It is the common ancestors that make us very similar to each other.

Noelle

Thu Nov 26 2009, 03:48PM

Napoleonic Powermonger
Registered Member #2 Joined: Thu Jan 26 2006, 05:10AM
Location: Meadville, PA
Posts: 70

Not even too long ago, it was a common practice to marry a family member such as a cousin. Nobody's family tree branches back that perfectly.. there are many crossovers and doubling back.

MinorityCarrier

Thu Nov 26 2009, 06:43PM

Registered Member #2123 Joined: Sat May 16 2009, 03:10AM
Location: Bend, Oregon
Posts: 312

In Marsing, Idaho, the family trees tend to have one branch.

But seriously, the "Common Ancestor" goes back way beyond humans. Mutations occur across a broad front with in a species population. There is research underway to see if there is Neandertal genes in Homo sapiens. It's erroneous to think of one individual as a common ancestor of all mankind.

The closest we as a species came to a singular linneage was circa 70,000 years ago (sorry Stella, Bishop Ussher didn't know squat about geologic history) when there was an apparent near-extinction of Homo sapiens in Africa.

Brett Clark

Fri Nov 27 2009, 08:00AM

Registered Member #482 Joined: Thu Jul 06 2006, 07:13PM
Location:
Posts: 8

To clarify, I'm not thinking about common ancestors of all humanity. The wiki article I linked in the OP discusses Mitochondrial Eve, our most recent common ancestor.

Each individual, 40 generations ago, has 1,099,511,627,776 ancestors. That's only an estimate for the last 1000 years. What about 2000 years ago? 2^80 is a tremendous number. If each one of those great*80 grandparents were a separate person, that would easily be more people than have ever existed.

What surprises me and leads me to believe I'm overlooking something is the fact that so many people in our individual family trees must have had "double, triple, quadruple, etc. duty." They must have been in multiple positions in our distinct lineage. Fathers who were also their sons' grandfathers, sisters who were also their brothers' mothers, and so on.

With numbers as large as 2^80 and beyond, the amount of incest happening just seems as if it would be staggering. Which is why I was skeptical about my numbers.

Bjørn

Fri Nov 27 2009, 11:05AM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

You are just confusing youself by going back more generations than you can keep track of. Nothing really interesting happens if you go more than one generation back, every generation is the same as all before.

Draw it on a piece of paper or make a computer simulation. Start with a population of 2 unrelated people and compare it to a population of 4 unrelated people and so on. Then it is easy to see that the larger the population the better is the DNA mixing. Also consider that the partner selection is not random.

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