4 ms·
An intuition about the "appearing trillions of times in a family tree" is to realize that family trees are actually a binary tree extending backwards in time--i
by titzer 6y ago
An intuition about the "appearing trillions of times in a family tree" is to realize that family trees are actually a binary tree extending backwards in time--i.e. each person has exactly two parents, who have exactly two parents, and so on. So N generations back, there are 2^N individuals. Obviously, there weren't 2^100 people on Earth at any point in time, so this backwards tree starts folding back on itself, containing duplications. 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.
It helps to visualize this more as a mesh than a tree, or as a flowing river. The river divides around rocks and obstructions (those are points of temporary divergent evolution) but ultimately all flows together (convergent ancestry). We are truly one body of genetic material, and yet each is a unique combination of backwards traces.
It's really very cool IMHO.
- TeMPOraL 6y agoTo 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".
- titzer 6y agoTo 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.
- rendall 6y agoTo 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.
- throwanem 6y agoThe metaphor appears to have run away with you somewhat.
- rendall 6y agoIt is true. It has run away with us all
- throwanem 6y agoWhat are you actually talking about, though? Veiled and gnomic proclamations can be fun, but sooner or later you have to show your hand.
- rendall 6y agoThe 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.
- coldtea 6y ago>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? :-)
- genghizkhan 6y agoAdam: https://en.wikipedia.org/wiki/Y-chromosomal_Adam https://en.wikipedia.org/wiki/Y-chromosomal_Adam Eve: https://en.wikipedia.org/wiki/Mitochondrial_Eve https://en.wikipedia.org/wiki/Mitochondrial_Eve
- acjohnson55 6y agoI'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.
- bzbarsky 6y agoThis 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.
- sjburt 6y agoAdam 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.
- nextaccountic 6y ago> 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.