>Go back far enough and there is a set of individuals who were alive at a particular point in time that completely cover that forward tree.

So like Adam and Eve? :-)

Adam: https://en.wikipedia.org/wiki/Y-chromosomal_Adam

Eve: https://en.wikipedia.org/wiki/Mitochondrial_Eve

I'm trying to square those articles, which put Y-chromasomal Adam and Mitochondrial Eve >100kya with the Identical Ancestor Point, which is only 5000-15000ya.

I guess it's kind of like the difference between a permutation and a combination.

This is talked about in the article too: You do not have genes from all your ancestors, because any given person only contributes half their genes to their children. So you have to go much further back to find a point when someone's genes would be present in all modern-day humans.

In particular, if you have a male ancestor at the Identical Ancestor Point but the lineage from them to you passes through a woman at any point in the family tree, you would not get that male ancestor's Y-chromosome via that line of descent.

Or put yet another way, if you are male, you have 2^N (possibly overlapping) ancestors N generations back, but only _1_ ancestor N generations back that contributed your Y-chromosome. Comparing to a different male, the chance that you share Y-chromosomes is naively something like 1/P where P is the size of the male population at the time, while the chance that you share your ancestor sets is a much more complicated calculation, and the article claims it's ~1.

Simple explanation. It is the difference between "things have mixed so much that everyone is your ancestor somehow" and "on a strict matrilineal or patrilineal descent, everyone winds up at the same thing".

In the first the path goes through both fathers and mothers. This is easier to achieve, and so you get to it more recently. The second is harder and so happened longer ago.

Adam and Eve are along the male (or female) line, whereas the common ancestor could be anywhere in your family tree. So 10 generations back you only have a single male line ancestor but 1024 ancestors total.

Mhitochrondrial Eve and Y-Chromosome Adam likely could not have met.

There was too much time between them.

We focus too much on these two inputs, and forget the other (20) pairs of our nuclei.

To play with the river analogy: and then imagine that it flows next to a dye factory, and every now and then a droplet of dye finds its way to the stream, creating a colorful streak. That's to account for random mutations.

And then what would an analogy look for prokaryotes - bacteria, archaea? If my understanding of biology is correct, it would be like an infinitely tall, densely tiered waterfall. There's some forward generational movement, but there's also plenty of lateral ones - exchanging gene packets "at runtime".

To continue playing with our river analogy: consider a man fishing beside the river, who catches a large salmon. The man is ecstatic, for he is poor, and his family is hungry. Just as he draws his knife to gut it, the fish speaks! It says "Please do not kill me. Throw me back into the river, for I am the Fish King, and I will grant you 3 wishes. You can have whatever your heart desires." Astonished, the man agrees, and throws the talking fish back into the water. He speaks his 3 wishes into the air: to be rich, to be respected, and to be young again. Time passes. The man goes home to his hungry wife and child, hands empty. A day passes. A week. Then a year, and youth, wealth and respect remain elusive. A year after that, the man is fishing by the same riverbank - and remember, this is an analogous riverbank - an analogy for our ancestry - and the man catches the same fish! Or is it? It looks the same, but it does not speak. There is no recognition in its fishy eyes. It just lays on the shore, gasping, flopping. The man decides it's just a normal fish after all.

The metaphor appears to have run away with you somewhat.

It is true. It has run away with us all

What are you actually talking about, though? Veiled and gnomic proclamations can be fun, but sooner or later you have to show your hand.

The fish represents the lost dreams of our ancestry, if not the fishy ancestors themselves. The man is us, all of us. The hungry family is our hungry family. Or perhaps our hunger for connection to our past. The story as a whole is meaningless, and in being so, represents the randomness and chaos in which we find ourselves. Is the universe beneficent, malevolent, indifferent? We cannot know for sure. We can only stare into the dying eyes that stare at us in turn, from the mirror, and guess.

I do. This was quite entertaining.

Nobody likes a smartass.

This is true and I approve this comment

To be clear, the river can divide, too. That's speciation, where populations no longer interbreed because their offspring are either unviable or themselves cannot produce offspring.

> It helps to visualize this more as a mesh than a tree, or as a flowing river.

Or: it's a DAG, not a tree.