One doesn't hear much about "the history of ideas" any more, but back when I was a young man it was all the rage. Credit for establishing that busy new discipline is usually given to the American scholar Arthur O. Lovejoy and his 1936 book The Great Chain of Being. In a dense and (for me) almost unreadable book, he limned a scheme for understanding the world that served Western civilization from the time of Plato into the 19th century.
The great chain stretched from the foot of God's throne to the dregs of inanimate matter at the center of the Earth. The chain had no gaps. Every created thing in their almost infinite plenitude had a place in the chain, more worthy than the creature below, less worthy than the creature above. The lion was king of the beasts, the eagle the prince of birds. Mammals were higher on the ladder than birds, birds higher than plants. The oak was the lord of trees. And so on.
Every link in the chain was allowed to excel in some way. A stone lacks sensation and consciousness, but it bests animals in endurance. A gazelle might lack an immortal soul, but it excels humans in fleet of foot.
And humans? We stand on the top rung of material beings, possessing the immaterial, immortal souls that place us just below the angels, a "little world made cunningly of elements and an angelic sprite." A flattering place to be, above all worldly things, only a shade below the heavenly spirits.
There has perhaps been no more shattering disjunction of human thought than the replacement of the great chain by the tree of life, on which our species is a fluttering leaf among a myriad canopy of leaves. The rupture came not from philosophers or theologians, but from biology. If you want to pick a moment for the idea's genesis as a force powerful enough to change history, why not this drawing from Darwin's notebooks.
OneZoom.org, the beginning of a project to create an interactive tree of life containing all known organisms. Without watching the tutorial first, see how long it takes you to find Homo Sapiens among the mammals.
The tree, of course, has long since replaced the chain in the sciences, but not yet in the minds of the majority of people on he planet. The implications of the transformation are enormous, but by and large they have not been faced. Tomorrow, I will ponder why so many of us cling to the chain, and the importance of transferring our collective allegiance to the tree.