2608: Family Reunion

Explain xkcd: It's 'cause you're dumb.
(Redirected from 2608)
Jump to: navigation, search
Family Reunion
Grandma says that because of differences in primate and feline lifespans, the cat is actually my 17,000,000th cousin 14,000,000 times removed.
Title text: Grandma says that because of differences in primate and feline lifespans, the cat is actually my 17,000,000th cousin 14,000,000 times removed.


Because all humans are descended from a common ancestor, every human is, at some point, related to every other human, albeit distantly. Similarly, all life forms on Earth are presumed (with good reasons) to be descended from a single even more distant relative whose ultimate lineage became more relevant than any from its own 'cousins' at the time, and thus all life forms are distantly related. This makes every interaction with another life-form, technically, a family reunion, if not in the traditional sense.

The general English definition of a cousin, which is a person sharing an ancestor who is not a direct parent of either party, can be qualified by two numbers. There is the nth-ness of the relationship (the fewest generations you need to go beyond one's parentage, "a first cousin" implies that a grandparent is the key link) - for example, this Cueball's relation to White Hat is via a great-grandparent, whilst that with Hairbun is through a great-great-great-great-great-great-great-great-grandparent. A "removed" number is any difference in this number between the two individuals, such that a child of a direct cousin invokes a "once removed" relationship between the two (without individually qualifying who is the 'senior' generation, from whom the 'nth' count is determined). You would normally only qualify "first cousin" if this fact is considered important, and "zero times removed" would also be considered implicit.

In this strip, all the humans are shown to be no more distantly related than 35th cousins. This may seem closely related for randomly selected humans, but the number of cousins a given person has increases exponentially with successive degrees of separation. For example, if family lines are separate, each person would have 64 pairs of sixth-generation great-grandparents. How many descendants those grandparents have now depends on how many living children each couple has, and how often breeding lines cross, but [studies in Britain] concluded that the average person in that region has nearly 200,000 sixth cousins. 35th cousins is probably nearly the most distant relationship you're likely to find among people who have ancestors from the same geographical region.

As pointed out in the title text, cat lifespans (or, more importantly, inter-generational breeding cycles) are somewhat different from those of humans. Although they would have still been very similar immediately after the divergence from the appropriate most recent common ancestor (MRCA), the differences will have built up to a generational-count displacement of a similarly extreme nature. i.e. that while the shared ancestor is Cueball's 17-million-or-so-Great Grandparent, the cat is in turn the 31-million-or-so-Great Grandchild. Exactly how accurate, or even precise, Randall considers these numbers is unknown, but it is the kind of fact that we know he likes to research and use expert opinion for.

The Evogeneao Tree of Life diagram indicates that humans and cats diverged around 90 million years ago and humans and plants diverged around 1.8 billion years ago.

If we presume that generations of humans (including proto-humans, pre-humans, etc) since the divergence from cathood (including proto-cats, pre-cats, and the rest, back to the common ancestral form) have averaged around 5 years, then a 17 millionth cousin may be about right. Many of our (and cats') early ancestors will have necessarily been small burrowing mammals — to have been amongst the ones who survived the asteroid around 66 million years ago that killed off most of the dinosaurs — with contemporary equivalents having breeding cycles in terms of a year at the most. But we currently have a large feasible range of generational cycle (15-50 years, very roughly, with or without technical/social help or hinderances), that may have started to drag our long-term average upwards since at least the age of the early hominids, if not the age of our primate forebears or earlier.

To get a 50 billionth cousin from the potted plant, then the generations of (eventually) humans since we were of the same form as that time's ancestral plants (or vice-versa) would need to average two weeks. This is possible, but difficult to be precise about due to the lack of much of the required evidence in the known fossilized remains. Any reasonable estimate, however, should be heavily weighted towards generation spans common for unicellular eukaryotes, rather than the longer generations common for multicellular eukaryotes: the general consensus on the most recent common ancestor for of animals and plants identifies it as a unicellular eukaryote. Given the above analysis of eukaryotes as cousins one wonders why Randall didn't include that every lack of gathering is also a family reunion.


[Megan, White Hat, Cueball, Hairy, Danish, a white cat with black patches on its back, Hairbun, a chair with a half-full wine-glass on the seat, and a potted plant on a cabinet are "standing" in a line. White Hat is holding a glass and Hairy has his hands to the side in a "shrug" position. Megan and Cueball are facing right and everything/one else is facing left (except for the potted plant, which is not facing any direction). There are arrows pointing to each of the living creatures.]
14th cousin [Megan]
2nd cousin [White Hat]
Me [Cueball]
12th cousin [Hairy]
35th cousin [Danish]
17,000,000th cousin [cat]
9th cousin [Hairbun]
50,000,000,000th cousin [potted plant]
[Caption below the panel:]
Really, every gathering is a family reunion.

comment.png add a comment! ⋅ comment.png add a topic (use sparingly)! ⋅ Icons-mini-action refresh blue.gif refresh comments!


How are relatives related by asexual reproduction defined and named?

While False (talk) 21:01, 18 April 2022 (UTC)

Any particular organisms in mind? It seems like you'd still have a generational parent, but there are a number of forms of asexual reproduction, blurring the line of what is a new lifeform and what is part of the old. Many plants, for example, can spread via colonies of their roots, whether severed from each other or not. I'm curious if there are organisms with more than two parents, and how many generations out of sync those parents can be. I think some plants might do this as well. 22:06, 18 April 2022 (UTC)
Look up mitochondrial donation for a case of three parents in humans, with two of them being the regular genetic parents for the 23 chromosome pairs and one extra parent (mother) for the mDNA. In nature you also have cases of horizontal gene transfer (for example via plasmids) where genetic information is passed outside of linear inheritance. 12:59, 19 April 2022 (UTC)
For the above case the definition would be a bit tricky, but for asexual reproduction each time the organism reproduces/goes through mitosis would be considered one generation I suppose. That does solve the _cousin _-removed thing, although perhaps not anything connected to which side of the family (maternal, paternal, etc). Then again those things aren't comprehensive for humans either, so bacteria certainly would be new. Wielder of the Staple Gun (talk) 00:25, 19 April 2022 (UTC)

This suggests humans have a common ancestor no earlier than 3000 years ago or so, due to inbreeding. So if we assume 20-25 years per generation, that works out to a max of 120-150th cousins.


This is an important comic in xkcd lore

If the chair is made of wood, doesn't it mean that it's made of the dead remains of another relative? Isn't that rather gloomy?

Eje211 (talk) 21:30, 18 April 2022 (UTC)

"Good old Umpteen-billionth-cousin oaktree has practically become part of the furniture!" 22:00, 18 April 2022 (UTC)
If it is made from coal or oil-based materials, it might just be a great * (10^7) grandparent (or 1.7 * 10^8 cousin, 10^8 times removed). Cwallenpoole (talk) 13:16, 19 April 2022 (UTC)

Siblings would also technically be zeroth cousins, and the self would be a negative first cousin. No idea what higher order negative cousins would imply. Incidentally, the this structure fixes the lack of a gender neutral term for uncle/aunt/nephew/niece, who could be referred to as zeroth cousins, once removed. This does remove the directionality of the terms, though. 22:09, 18 April 2022 (UTC)

Higher order negative cousins would require that you are both descended from one of your own descendants. This either requires time travel, as in "All You Zombies", or counting step-parents, as in I'm My Own Grandpa.-- 22:22, 18 April 2022 (UTC)
Siblings share a parent, which is already a explicit disqualification from any 'cousin-counting' process. As is the <nephew and/or neice> to <aunt and/or uncle> relationship (via a parent of the latter party, though extend that minimal leg of relationship just one generation further and you get your archetypal first-and-zero cousins).
Yes, it would be nice to have a good term for that, but we already (in English at least) don't have a good word for "aunt/uncle/nephew/niece-by-marriage" unless you start to add "-in-law" to it (which itself is open to certain vagueness.
Though some languages do a lot more than English. 22:35, 18 April 2022 (UTC)

According to a family tree app we have, I'm my own 9th cousin once removed. 02:16, 19 April 2022 (UTC)

What about the drink on the chair? Assuming it might be an alcoholic beverage, would the bacteria and/or yeast organisms which catalysed the fermentation not also have to be considered as (very) remote relatives? Captain Nemo (talk) 09:52, 19 April 2022 (UTC)

Not to mention the fruit (or possibly grains, though the drawing suggests wine). Troublingly, every part of the meal except salt would have to be counted, too. 16:34, 19 April 2022 (UTC)
Water wouldn't be counted as well. 22:01, 19 April 2022 (UTC)
Yeast, yes, and as a member of the fungus "kingdom" that's a closer relative to us than plants. See 1749: Mushrooms. 17:17, 19 April 2022 (UTC)

According to TimeTree, Felis catus and Homo sapiens diverged about 95 Mya. This gives a mean generation time for the 17,000,000th cousin of just over 5 and a half years. Cats can expect their first litter at 15-18 months but probably lived ~5 years through most of their recent history, so say the average generational time is 3 years. Between that and the generation time of humans until recently, 5.5 years for most of those 95 million seems plausible. 18:22, 19 April 2022 (UTC)

OTOH, about 1,500 Mya for the aspidistra/human divergence yields a 3.5 month generation time, which seems low :-) 18:27, 19 April 2022 (UTC)
I actually wrote a new section earlier (like a Trivia section, about 'what we know and what we don't', but I forget now how I worded it) going through the Cueball-to-Other relationships in order of greatest to least proximity-of-relation, and gave the basic relationship and then introduced caveats ((that we don't know there are not unknown "removed"s, other than the states cat one (almost certainly the plant, especially!); the issue of age-disjointedness/re-convergence; that going back to Hairbun's common-ancestor (1024 (g^8)-grandparents) you likely get multiple MRCAs and/or single people in multiple positions in one person's ancestry; etc)).
There is nothing to say that the removedness is on the cat's side (felis/homo common ancestor might have spawned pre-homo generations that cycled quicker than pre-felis ones, though in recent times we do know that the typical age of primigravida is of course far less in cats so they could be heading back towards zero-removed, if so) and evidence is sketchy about even the early-hominin situation, never mind the creatures that came of the initial divergence. But the wide uncertainty certainly comfortably allowed the generational assumptions that the given figures seem to suggest. Yes, there were many, many words. But I made more effort to be economical than I have just now.
I was about to do the same with the plant when I tried a Preview submit which crashed things (or just happened to coincide with an unrelated browser hiccough, prob.) and I'm afraid I didn't have the heart to redo it from scratch. So I never actually got into the details of cross-Kingdom inter-relationships from extreme archeohistoric times, or try to work out which side might have been the shortest chain back to the MCRA (some current plants can live a long time, generating viable seedlings after centuries, but others also repropogate their seeds extremely rapidly!).
I've a feeling that the comic's assumptions would sit well within the huge uncertainty, and probably is based upon the general concensus of the top few expert papers presented upon Randall's idea-inspired trawl through the peer-reviewed literature. Or maybe just the one rather fanciful academic source that spontaneously generated the spark of inspiration that led to this comic... ;) 19:07, 19 April 2022 (UTC)