The closest relatives of humans are apes, monkeys, lemurs, but once you go beyond primates and tree-shrew type animals, we are Euarchontoglires, and our next closest cousins are rodents and rabbits. Not lions, horses, dogs, deer or whales, but rats and mice. I find this fascinating and beautiful, linking us in to life on Earth, showing the weird accidents resulting in our presence here; and I am proud to be related to such a successful animal as a rat.
Euarchontoglires are Boreoeutheria, distinguished by the scrotum in males, though some animals have evolved to no longer need a scrotum- hedgehogs, rhinoceroses, and some trans women. The evidence of this is in our DNA, rather than in fossils: the common ancestor lived between 100 and 80 mya (million years ago), and though its fossils may never be found its genome sequence might be predicted from its descendents to 98% accuracy.
Laurasiatheria are also Boreoeutheria: shrews, moles, hoofed animals, animals that chew cud, whales, bats and pangolins, and all the Carnivora in one Order. Most animals constantly renew their teeth, but mammals stopped doing this, with most only having an infant set and one adult set, because mammals lives were so short there was no need. Some Euarchontoglires started growing teeth again, though not humans- yay Euarchontoglires! Sharks grow new teeth constantly. Tyrannosaurus Rex, whose jaw could exert 3000lbs of pressure on one tooth, broke teeth and renewed them constantly. The whole human mouth can bite at 175lbs.
Eutheria, with a common ancestor in the early Cretaceous or late Jurassic about 161mya, were named “True beasts” by Theodore Gill, and are defined by certain aspects of the skeleton such as the joints between the metatarsals and the cuneiform bones. Every football fan knows what a metatarsal is. I thought a “beast” was an animal, which made me wonder what metatheria, not quite beasts, were. They are marsupials; all living eutheria are placental mammals, though some eutheria were not. Theria- beasts and sort-of beasts, includes marsupials.
Mammals all give milk to their young. Nipples evolved from sweat glands, so there were young of ancient nearlybeasts that licked their mothers’ sweat. All mammals alive now are Trechnotheria; a mammal called Kuehneotherium with different teeth evolved in the late Triassic, and that line all died out just like most animals. There were Mammaliaformes, mammal-like animals, which gave rise to mammals and to Morganucodontidae, “Glamorgan-toothed” animals. This says nothing about Welsh people now. One hypothesis of mammal relationships shows Mammaliaformes, mammalia, theriformes, holotheria, trechnotheria, cladotheria, zatheria, tribosphenida, theria then eutheria. So if you want to confuse someone, which of course I often do, you can say you are a theriforme. Therefore you are not a monotreme, an egg-laying mammal, but closely related.
Before Mammaliaformes there were Mammaliamorpha, a wider clade including more extinct animals. That included the Tritylodontidae, three-knob teeth animals, which also died out and left fossilised teeth. The name shows the evidence: dont appears in the name of lots of extinct life. The Morganucodontidae’s teeth survived, showing a difference from all beasts, a branch on the single tree of life. And teeth are
close to my heart on my mind in my thoughts at the moment- those first two images are horrible- as I might lose one.
Mammals are not descended from reptiles. We are Therapsids, not Therapods (or, mostly, therapists) and Synapsids, evolving separately from Amniotes, animals laying eggs on land. It’s all to do with how there are holes in the skull for the eyes to peer through, though I am not quite sure how they differ and even I can’t be bothered finding out how, now. So reptiles or Sauropsids, with different eyeholes, and in turn dinosaurs and birds all with reptilian eyeholes, evolved separately from the amniotes laying eggs out of water. The word Amniote comes from Greek, meaning “membrane surrounding the foetus”, and earlier “bowl in which the blood of sacrificed animals was caught”. Amniotes evolved in the Carboniferous period, 323-298mya.
So we are related to all reptiles, because they have a membrane round the foetus, and then to all amphibians, even though they don’t, because the adults breathe as land-dwellers. Then we are part of the clade tetrapoda, four-footed creatures, with the same basic body structure- heart, lungs, liver, spleen, legs and toes. Tetrapods evolved from Sarcopterygii, lobe-finned fishes, around 390mya, crawling onto the land.
Tetrapods evolved from Chordates- animals with a nerve cord, a tail behind the anus, and blood circulation. Chordates include vertebrates, with a backbone, so including fish, as well as Tunicata such as sea-squirts. Even a sea-squirt has a heart. Chordates evolved from bilaterians, which are tubes- at first animals took in food the same way they excreted waste, but then evolved into one-way systems. Humans are deuterostomes: in our foetuses, the anus forms before the mouth does. Insects are Protostomes, where the mouth forms first. We are “Bilaterians” because we have a distinct left and right side, not because we have two ends of a tube to feed through. All bilaterians are symmetrical as embryos, but some are not as adults, including echinoderms, such as starfish.
Simpler than that, we go back to the Ediacaran, 600mya. So we are related to all multicellular animals. With more distant common ancestors, we are Eukaryotes, because our cells have a nucleus enclosed within membranes, and mitocondria. Those mitocondria may have been prokaryotic cells, absorbed into others in a symbiotic relationship, though there is also the autogenous hypothesis, where they evolved within a more primitive cell. So we are related to plants and fungi. And beyond that we are related to archaea and bacteria, and all life on Earth.