Family Reunion
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.

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 n th-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.
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.
According to this Quora page , there have been about 13,000 generations of modern humans, so the people at this party would be quite closely related, all things considered.
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 Randal Monroe 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.
