Wait, a pile of what? “Clade” is a scientific borrowing from Greek κλάδος (kládos), meaning “branch”. It typically refers to a common evolutionary ancestor and all of its descendants, figuratively representing a branch of the tree of life. Since cheap genetic sequencing enabled its wide adoption in the 1990s, cladistics has become the dominant way biologists classify organisms.
An idle conversation about octopuses and jellyfish earlier this week inspired me to learn more about the concentrically nested clades that contain humans. 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’d like to know about any mistakes I make 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. But more importantly, the methods we use to ascertain the truth would be different. So the information that results would be different, and the inferences we would make about life would be different. It’s wild to think about.
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, australopithecus, and paranthropus.
- Humans are apes. Apes are primates 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 fingers or toes 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, capybaras, squirrels, and beavers) and lagomorphs (bunnies).
- Humans are placental. Placental mammals are named for their late-term development inside 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 a neck that is distinct from their head 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. Chordates are animals named for their notochord, a body-spanning column of cells that becomes the spinal column in vertebrates. Chordates have tails and thyroid glands. The few chordates that are not vertebrates include tunicata (sea squirts) and cephalochordates (lancelets).
- Humans are deuterostomes. Deuterostomes are animals named for their first embyronic opening becoming an anus, rather than a mouth. Deuterostomes have gill slits that open into their pharynx, a hollow nerve cord, and a segmented body. Deuterostomes that are not chordates include echinoderms (starfish, sea urchins, and sand dollars) and hemichordates (acorn worms).
- Humans are bilateria. Bilateria are animals named for their bilaterally symmetric body plan. Their symmetry gives bilateria a front head and rear tail, a ventral belly and dorsal back, and left and right sides. As they mostly move in the direction of their head, it has sense organs and a nerve center. Bilateria that are not deuterostomes include protostomes (all insects, earthworms, squids, and octopuses), named for their first embryonic opening becoming a mouth, rather than an anus.
- Humans are animals. Animals are multicellular organisms named thousands of years ago for having a soul. Animals can move and must get energy from eating other organisms using an embryonic opening called a blastula. Animals that are not bilateria include cnidarians (anemones, corals, and jellyfish) and porifera (sponges).
- Humans are opisthokonts. Opisthokonts are organisms named for the rear flagellum they use to move. Opisthokonts cannot produce their own food and must consume other organisms to survive. Opisthokonts that are not animals include all fungi.
- Humans are podiates. Podiates are organisms named for having pseudopods for movement. Podiates that are not opisthokonts include amorphea (slime molds and amoebas).
- Humans are eukaryotes. Eukaryotes are organisms named for having a cell nucleus. Eukaryotic cells have organelles, mitochondria, chromosomes, and a cytoskeleton, and reproduce through mitosis. Eukaryotes that are not podiates include diphods (all plants and algae), which have plastids that generate energy from the environment.
- Humans are alive. Living organisms are named for being alive. Living organisms all die, encode information about themselves in DNA, and can reproduce. Life that is not eukaryotic include archaea and prokaryotes (all bacteria). Living things that are not organisms include viruses.
And that’s life.