11. In 1868, Charles Darwin was the first to document a collection of physical and behavioral traits seen in domestic animals, particularly mammals, but not their wild relatives.

12. It wasn’t until 2014, however, that researchers offered a single explanation for the phenomenon of floppy ears, smaller teeth, tameness and other “domestication syndrome” traits: a mild deficit in neural crest cells.

13 In vertebrate embryos, neural crest cells (NCCs) form along the dorsal side, or crest, of the neural tube (the proto-central nervous system).

14. NCCs spread through the embryo as it develops, and directly or indirectly affect a range of other functions, from the adrenal gland, which controls the fear response, to pigmentation, which can differ between domesticated and wild populations.

15. Also different between the two populations: the speed of development. Domesticated animals develop more slowly, so their NCCs spread at a slower pace, having minimal or no effect on the function they typically target. It’s that deficit that results in domestication syndrome.

16. Subsequent genetic studies have supported the NCC deficit hypothesis, most recently an April report in Science on the sequencing of 14 ancient horse genomes up to 4,100 years old; that’s close to the estimated dawn of domesticated horses some 5,500 years ago.

17. Sheep were domesticated twice as long ago as horses, with goats (10,500 years ago), pigs and humpless cattle (both about 10,300 years ago) hot on their heels.

18. Dogs have the distinction of being the first animal humans domesticated — and the only one before the advent of agriculture. A 2016 paleogenetic study found two doggie domestication events perhaps 14,000 years ago: one in Europe and a second in East Asia. The latter population eventually spread west and replaced the former.

19. There’s genetic evidence for a similar double domestication in cats, too. In June, a study in Nature Ecology & Evolution found the African wildcat Felis silvestris lybica was first domesticated regionally in the Near East before 4400 B.C.

20. But a second domestication in ancient Egypt, before the first millennium B.C., proved more successful: This later lineage spread rapidly around the world via land and sea trade routes — and eventually took over the internet. Who’s the boss now?