What is a human? A miserable little pile of clades! | Approximate Knowledge

Wait, a pile of what? “Clade” is a scientific borrowing from Greek κλάδος (kládos), meaning “branch”. It means a common evolutionary ancestor and all of its descendants, representing a branch on the tree of life. Since cheap genetic sequencing enabled its wide adoption in the 1990s, cladistics has become the dominant way biologists classify life.

An idle conversation about octopuses and jellyfish earlier this week inspired me to learn about the concentrically nested clades humans are in. One way I like to learn more about things is summarizing the new information in a digestible way. So that’s where you, dear guinea pig reader, come in. Evolutionary biology is not my field, so I want to hear about any mistakes I’ve mad in this summary.

I find cladistics especially inspiring because it’s a recent total paradigm shift in a hard science. If I were writing this article in 1996, it would look completely different. Sure, it would probably be published in a local science fiction zine instead of a personal blog, and it would probably have taken ten times longer to research. More importantly, the methods we used to ascertain the truth would be different. So the resulting information would be different, and the inferences that we would make about life would be different. It’s wild to consider.

Let’s climb the tree of life from most to least specific. We’ll go through what each clade is named for, its distinctive features, and what’s in it that wasn’t in the next smaller clade.

Humans are hominins . Hominins are apes named for their humanity. Hominins walk on two legs and have larger brains. Humans are the only extant (non-extinct) hominins. Extinct hominins include neanderthals, australopithecuses, and paranthropuses.

Humans are apes . Apes are primates that are also called hominoids for their similarity to humans. Apes have long arms, no tails, and more complex brains. Apes that are not hominins, from closest to furthest, include chimpanzees, gorillas, orangutans, and gibbons.

Humans are primates . Primates are mammals named hundreds of years ago for being the best or highest kind. Primates have color vision, more complex brains, five digits on each limb, and opposable thumbs. Primates that are not apes include simians (monkeys) and strepsirrhines (lemurs and lorises).

Humans are euarchontoglires . Euarchontoglires are a recent grouping of mammals named for being the union of the best kind (euarchonts) and rodents (glires). Euarchontoglires that are not primates include all rodents (such as hamsters, squirrels, guinea pigs, and beavers) and lagomorphs (bunnies).

Humans are placental . Placental mammals are named for their extended development within their mother’s uterus. Placental mammals have a corpus callosum connecting their brain hemispheres, and a separate anus and genitals rather than a cloaca. Placental mammals that are not euarchontoglires include nearly everything that we think of as mammals. Anteaters, elephants, manatees, bats, deer, giraffes, horses, dogs, cats, the whole lot.

Humans are mammals . Mammals are vertebrates named for their milk-producing mammary glands. Mammals all have a neocortex, sweat glands, a diaphragm, a middle ear with three bones, and a four-chambered heart. Mammals that are not placental include monotremes (echidnas and platypuses) and marsupials (such as kangaroos, koalas, and opossums).

Humans are cynodonts , therapsids , and synapsids . Cynodonts are named for their canine teeth. Therapsids and synapsids are named for the bony arches in their skull behind each eye. Cynodonts have fur and are warm-blooded. Synapsids have different types of teeth, typically canines, molars, and incisors, and stand upright on four legs. There are no extant cynodonts, therapsids, or synapsids that are not mammals.

Humans are amniotes . Amniotes are vertebrates named for being able to reproduce far from bodies of water. Amniotes have skin made of keratin, breathe using a ribcage that expands and contracts, and have adrenal glands. Amniotes that are not synapsids include sauropsids, which include all reptiles and all birds.

Humans are tetrapods . Tetrapods are vertebrates named for their four limbs. Tetrapods have eyelids, tear ducts, tongues, and a middle ear. Tetrapods that are not amniotes include lissamphibia, which include all amphibians.

Humans are gnathostomes . Gnathostomes are vertebrates named for their jaws. Gnathostomes have teeth, a stomach, a spleen, and a thymus. Gnathostomes that are not tetrapods include nearly all extant fish, including cartilaginous sharks.

Humans are vertebrates . Vertebrates are animals named for their spinal column. Vertebrates have a cranium, eyes, ears, and a nose, a multi-chambered heart, and a digestive system that includes a pancreas, liver, and intestines. Vertebrates that are not gnathostomes include jawless fish (hagfish and lampreys).

Humans are chordates ....