"Ancestor's Tale" - Question(s)

[Darat]

I don't see why the concestor has to be just one individual - or even a sibling! Mutations may occur at any time. Let's say that a couple of rodents has only one offspring and that is when the essential change takes place!
I don't get the idea of siblings at all!
I also don't see why the concestor has to be just one individual! But that mutations always take place at the individual level is clear. Two individuals developing the exact same mutation sounds so unlikely that I'd consider it impossible.

I agree with you, but I don’t quite know why, I'm working through this idea at the moment. Three is something that either I'm not quite understanding (nothing new) or there is a mistake here (knowing me and Dawkins as I do I'm pretty certain who it is who is making the mistake.)

The point that I'm not getting my head around is that the fact we have a lot of ancestors. Each of us has two parents, four grandparents, eight great-grands, 16 great-great-grands and so on. Would "I" or rather my genetic make-up be that different if just one of my ancestors say to back just a hundred generations (i.e. to the year 0)? In other words would one change in 1267650600228229401496703205376 sets of genes really significantly alter my genetic make-up? (I know many of these are ancestors are shared with other people so the actual figure is a silliness but big figures look good!)

With this in mind I find it hard to see how we can have such an important singular individual that could make any real difference to my genetic make-up today, in other words there isn’t any one individual we could trace back to and say “look that’s where the tree branched”.

(And now I look forward to it being explained why I am being very silly and showing my immense ignorance – but as I’ve never considered this before I’m just mulling things over, please feel free to educate me!)

(Edited for words.)

 

[Dr Adequate]

Okay. This has to do with the sexes I suppose. Dawkins' concept of a concestor doesn't speak of the sex. So there would be the two concestors in the male and the female right off. Then their would be (X) the (at least 2 again) concestors of C and D. C and D having opposite sexes to A and B.
But could there be more than the 4 concestors we have now identified?

There are more elaborate schemes, yes.

But if you remove sex, by looking only at the female line, then there is indeed a unique female line (or "mitochondrial") concestor: that is, one creature the daughters of which of which fall into two non-empty sets: those that have only living human descendants in the female line, and those that only have living chimpanzee descendants in the female line, plus one possibly empy set --- those that have no living descendants in the female line. Moreover, the "human-ancestral" set will be ancestral to all humans, and the "chimp-ancestral" set will be ancestral to all chimps.

Whether or not she would still be a concestor using all and any criteria of descent (as I did originally) depends on whether she has descendants not in the strict female line, and also on who they happen to mate with.

 

[Donn]

Well - damn. It's looking like my pretty shaky groking of the premis of the book is now even more shaky...
It seems this is a bit of a fractal, the more we look at it, the more complex it becomes. I wish Dawkins had spent just a bit more time on the concestor-concept before launching into the book.

Now, I suppose that he said everything he needed to say - to one standing in his shoes - but from my shoes I must say that I've lost the plot.

It won't detract much from the book, I hope; overall it's painted in broad
strokes and I am learning as I go along.

Right, I'm off to re-re-read Dr A's last post!

 

[Dr Adequate]

With this in mind I find it hard to see how we can have such an important singular individual that could make any real difference to my genetic make-up today, in other words there isn’t any one individual we could trace back to and say “look that’s where the tree branched”.

* sobs, has nervous breakdown *

It's a question of where the family tree branched. The branches underwent change sufficient for speciation later, and gradually. Neither the concestor nor its offspring are remarkable --- except in retrospect. I've had a bit of a chance to think about it, so maybe I can make this a bit clearer. Otherwise, I fear a Monty Hall Lemon Jihad looming. So, those main points again.

If we look back over the joint family tree of humans and chimps, then it contains individuals with (1) only human descendants alive today; (2) only chimp descendants living today; (3) chimp descendants and human descendants living today. The fact that humans and chimps have common ancestry guarantees that set (3) is non-empty.

If you look at the last individual in set (3) ever to have lived (and there must have been a last one, because no such individual is living now) then just by virtue of his/her being the last, none of his/her children can fall into set (3). Hence those of his/her children who have living descendants today either fall into set (1) or set (2) --- and there must have been at least one of each, because otherwise the individual we're considering wouldn't fall into set (3).

That's just maths. You can't fight maths.

Biology comes into my claim that the "chimp-ancestral" (or "set (2)) offspring of this individual must be the ancestors of all chimps. For if not, there would have been, at that time, two (at least) seperate lineages of proto-apes both of which, remaining seperate ever since that time, have evolved into chimps. In the first place, that would be ridiculous, and in the second place, genetic analysis would tell us if it had happened. It didn't. The same reasoning applies to the set (1) offspring of the concestor.

 

[Donn]

If we look back over the joint family tree of humans and chimps, there it contains individuals with (1) only human descendants alive today; (2) only chimp descendants living today; (3) chimp descendants and human descendants living today. The fact that humans and chimps have common ancestry guarantees that set (3) is non-empty.

If you look at the last individual in set (3) ever to have lived (and there must have been a last one, because no such individual is living now) then just by virtue of his/her being the last, none of his/her children can fall into set (3). Hence those of his/her children who have living descendants today either fall into set (1) or set (2) --- and there must have been at least one of each, because otherwise the individual we're considering wouldn't fall into set (3).
That's just maths. You can't fight maths.

I gotta get that framed. Man that's purdy!

 

[Badger]

I agree with you, but I don’t quite know why, I'm working through this idea at the moment. Three is something that either I'm not quite understanding (nothing new) or there is a mistake here (knowing me and Dawkins as I do I'm pretty certain who it is who is making the mistake.)

The point that I'm not getting my head around is that the fact we have a lot of ancestors. Each of us has two parents, four grandparents, eight great-grands, 16 great-great-grands and so on. Would "I" or rather my genetic make-up be that different if just one of my ancestors say to back just a hundred generations (i.e. to the year 0)? In other words would one change in 1267650600228229401496703205376 sets of genes really significantly alter my genetic make-up? (I know many of these are ancestors are shared with other people so the actual figure is a silliness but big figures look good!)

With this in mind I find it hard to see how we can have such an important singular individual that could make any real difference to my genetic make-up today, in other words there isn’t any one individual we could trace back to and say “look that’s where the tree branched”.

(And now I look forward to it being explained why I am being very silly and showing my immense ignorance – but as I’ve never considered this before I’m just mulling things over, please feel free to educate me!)

(Edited for words.)

I'll offer this up:

Currently, anthropologists talk of an "Eve" that lived in Africa about 200 000 years ago. They have traced the time and place that she lived through mtDNA (Mitochondrial DNA that is passed down only from the female). "Eve" had some minor advantage that over the past 200 000 years has propagated itself through the whole human race.

There were other humans around when she was alive, and their genes intermingled with hers, and her progeny's over the milennia, but a turning point occurred with her.

With regard to the number of ancestors you mention a person having, there is a point where your ancestors "intermarry". At some point, your great(e6)grandfather married his (e9)cousin so the amount of ancestors one has is not really an exponential progression.

This goes back to the Eve example in that at some point all other lines had died off and there were nothing but cousins left to marry eachother.

Now, if we apply this to the branches of the hominid family tree, Chimp/Human Eve (a different Eve) has a couple of kids, one of which goes east and one goes west. Over the generations, the eastern Eve's kids genes propagate through the eastern tribe, and the western Eve's kids genes propagate through the western tribe. The eastern tribe lives in different conditions than the western tribe, and other adaptations make them good at living in their environment (same for the westerners). Due to some sort of separation, the tribes never have contact with eachother....never interbreed for eons. When they do come back into contact with eachother, they find that they have become different species (can't produce fertile offspring), but they remain successful in their respective environments, and differ by greater and greater amounts in response to the respective environmental pressures of their differing ranges, until today when we have Chimp and Human.

Did that help at all?

 

[Donn]

If you look at the last individual in set (3) ever to have lived (and there must have been a last one, because no such individual is living now) then just by virtue of his/her being the last, none of his/her children can fall into set (3). Hence those of his/her children who have living descendants today either fall into set (1) or set (2) --- and there must have been at least one of each, because otherwise the individual we're considering wouldn't fall into set (3).

Actually, as purdy as it is, I am stumped by this:
Does this not argue that there can only be one individual that is the concestor to the other two sets?

How would the set description be worded to cater for the other (many?) concestors? (We counted at least 4 earlier.)

 

[Donn]

Just because the concestor is the last common ancestor does not mean that the two species divurged in the following generation. It just means that siblings, cousins etc. in the two lines did not happen to inter-mate before the groups separated and evolutionary forces ensured that they would not. *

But Sibling A and Sibling B did mate with other members of the "parent" species, as did their children, etc during this period. Any given pair (one in each line) of those mates can also be traced to a common ancestor, a "concestor." This means that while there is a very latest concestor, which might be proved, as per Dr A to be common to all chimps and all humans, she is not unique, she is the last only because she was the youngest and/or longest-lived of a larger group for which the definition of concestor holds.

It seems, from this and other posts, to suggest that there is no real single concestor - it's a mix of matings and you can never really point to one pair and say "They are it!"

Has anyone read the book? Can anyone suggest a better way to picture these "concestors"?

 

[Dr Adequate]

Actually, as purdy as it is, I am stumped by this:
Does this not argue that there can only be one individual that is the concestor to the other two sets?

No. We picked out this one individual as being the last member of set (3) ever to be born, because having this property makes it easy to prove that this individual must be a concestor. The various scenarios I've laid out show how other members of set (3) can also be concestors --- but the last one born must be.

 

[Gwyn ap Nudd]

The concept of a "concestor" only applies as we look backward along the family tree. There is nothing special about the individual(s) marked off as concestors, except for which of her children happened to survive.

Consider a tribe of proto-chimps. They all have their own quirks and genetics, but what they share is more defining. They are one tribe and one species.

We determine that this tribe was the source of both modern chimps and modern man. Now consider each individual member. She is a proto-chimp just like all the others, her children are proto-chimps just like the rest of the tribe. No one is special, until we follow the path of their descendants. Then we can divide them into four different set based solely on what happened to their descendants. Some have had no offspring, or their line died off before the two modern species divurged (case 1); some have chimp descendants, but no human descendants (case 2); some have human descendants but no chimp descendants (case 3); and some have both human and chimp descendants (case4).

An individual in case 4 can be designated as a concestor (loose definition). Note again that the mutations and environmental/evolutionary pressures have not occured, and the offspring are still proto-chimps.

But each the offspring of each of the (loose definition) concestors falls into one of the four cases as well. Since we know that humans descend from them, at least one of those offspring must be case 3 or case 4. Likewise at least one must be case 2 or case 4. If the offspring are case 4, then they are (loose definition) concestors as well, and the process just repeats in the next generation. If none of the offspring offspring are case 4, then at least one is case 2 and one is case 3. These are the siblings A and B discussed earlier. Their parent is then a concestor (tight definition).

Notice that the concestors (both loose and tight) and their offspring are only singled out in retrospect. all of them are perfectly ordinary members of their tribe and proto-chimp species.

Since humans and chimps do not interbreed, and the proto-chimp species is extinct, there has to have been a last concestor.(By necessity, this would be a tight concestor.) This is the individual that Dawkins called the Concestor.

I agree with you, but I don’t quite know why, I'm working through this idea at the moment. Three is something that either I'm not quite understanding (nothing new) or there is a mistake here (knowing me and Dawkins as I do I'm pretty certain who it is who is making the mistake.)

The point that I'm not getting my head around is that the fact we have a lot of ancestors. Each of us has two parents, four grandparents, eight great-grands, 16 great-great-grands and so on. Would "I" or rather my genetic make-up be that different if just one of my ancestors say to back just a hundred generations (i.e. to the year 0)? In other words would one change in 1267650600228229401496703205376 sets of genes really significantly alter my genetic make-up? (I know many of these ancestors are shared with other people so the actual figure is a silliness but big figures look good!)

With this in mind I find it hard to see how we can have such an important singular individual that could make any real difference to my genetic make-up today, in other words there isn’t any one individual we could trace back to and say “look that’s where the tree branched”.

(And now I look forward to it being explained why I am being very silly and showing my immense ignorance – but as I’ve never considered this before I’m just mulling things over, please feel free to educate me!)

Just as there are two different individuals that we can point out as "the original mutant" for every new gene/trait in our evolution -- and the second might be the harder to pin down since more than one offspring inherits the gene,* The split in species occurs at two different points, the earlier one being the Concestor, the later one(s) being the ones subjected to mutation and survival of the fittest. So the tree did not branch becauseof the Concestor's genetic makeup, but because of the accident of her offspring. And not only can you not definitively point to one of your ancestor and say she was the first human, her mother would not be the Concestor, anyway.


*The one in whose gamete-producing stem cells the mutation first occurred, and the one in whom the effect of the new gene first manifested. Since most new genes are recessive to the "standard" version, it can be several generations before the effects of a mutation manifest.

 

[Donn]

Since humans and chimps do not interbreed, and the proto-chimp species is extinct, there has to have been a last concestor.(By necessity, this would be a tight concestor.) This is the individual that Dawkins called the Concestor.

Got it, and got Dr A's explanation too. I will go with the last concestor description as I read on.

 

[Dr Adequate]

Gwyn ap Nudd --- it might be helpful if you aligned the first three of your four cases with my three sets. We have enough sources of confusion.
____________________________

To phrase my last post even more clearly, the concestors are the members of set (3) which have no children in set (3). We don't know how many there were, but we know that there was at least one, namely the last member of set (3) to be born.

 

[Dr Adequate]

Got it, and got Dr A's explanation too. I will go with the last concestor description as I read on.

The last concestor is the only one that definitely exists, so if you want to think of a concestor, you should definitely think of that one.

In any case, the concept is more useful (we can use its consequences for dating the divergence of lineages) just looking along either exclusively the male or exclusively the female line, where we do have uniqueness with respect to those lines of descent, and don't have to worry about any fancy hypothetical family trees I might draw up.

 

[Gwyn ap Nudd]

Gwyn ap Nudd --- it might be helpful if you aligned the first three of your four cases with my three sets. We have enough sources of confusion.
____________________________

To phrase my last post even more clearly, the concestors are the members of set (3) which have no children in set (3). We don't know how many there were, but we know that there was at least one, namely the last member of set (3) to be born.

Yes, it would have.

Unfortunately I started my response before you numbered your "sets." (Your earlir posts referenced the "chimp-set" and the "human-set"). While it is true that before I posted, I refreshed the screen, and was able to pick up Darat's question and respond to it, changing my "cases" to match your "sets" seemed an unecessary burden.

For those of you who are concerned though: Dr A's set 1 (ancestors of only humans) is my case 3, his set 2 (ancestors of only chimps) is my case 2, and his set 3 (ancestors of both) is my case 4. Since the line decending from my case 1 died out without modern descendants, Dr A considered it irrelevant, so it does not match up to a set.