Every Friday except the last Friday of each month, I've been posting a long tweet for Twitter subscribers and Patreon supporters, usually based on that week's book research (the last Friday of the month is a video just for Patreon). I'm making this week's Friday tweet public to give you an idea of what I've been posting for subscribers: One Big Family The size of your family tree is a pretty crazy concept when you think about it. You are most closely related to your siblings because you share two parents. You and your siblings, in turn, are less closely related to your first cousins, with whom you have half of your four grandparents in common. You and your first cousins all share one quarter of your eight great-grandparents with your second cousins, and you and they share two of your 16 great-great-grandparents with your mutual third cousins. Beyond second or third cousins, we tend to stop thinking of more distant relatives as family members, but there’s no clean line where your family ends and strangers begin. In fact, there’s no such thing as a person who’s not your family member. Scientists estimate that your most distant relative on the planet is about your 50th cousin. But when you read about evolution, you’re reminded that the family tree doesn’t stop with humans. If we think of our species as a single individual, we see that the relationship between species is analogous to the relationship between people. If humans are an individual, other homo species, like Neanderthals and Denisovans, are our (no longer living) sibling species. Chimps and bonobos are siblings with each other and the first cousins of humans and our homo siblings. Gorillas are the mutual second cousins to humans, chimps, and bonobos. Orangutans are our third cousins, and gibbons our fourth. We can keep extending the scale. If primates are an individual, rodents are our siblings, while the big group called laurasiatheres—which include dogs, cats, horses, hippos, whales, rhinos, and bats—are our first cousins. Elephants are the second cousins to all the animals mentioned so far, and marsupials are our third cousins. Going up another level, mammals become the individual, with reptiles and birds as our amniote sibling, amphibians the first cousins to the mammal/reptile/bird family, and fish our mutual second cousins. When vertebrates are the individual, echinoderms like starfish and sea urchins are our sibling and mollusks and arthropods our first cousins (and siblings with each other). Jellyfish are our second cousins and sponges our third cousins. When animals are the individual, fungi are our sibling, plants our mutual first cousin, and bacteria our second cousin. (I'm simplifying a bit—there are more layers in many of these cases.) Scaling up levels like this helps clarify the story, but in reality there are no levels, just one giant family tree of individual organisms. All living things are your cousin—the only thing that varies is what number cousin they are from you. Your 50th cousin is a human. A chimp is somewhere around your 250,000th cousin. A rat is around your 15 millionth cousin. And so on. By the time you get to mushrooms, trees, and bacteria, you’re looking at your some-billionth cousins. Life on Earth is one unthinkably large family, all descended from the same common ancestor. It’s a true story crazier than any piece of fiction.