You are a teacher. You are looking for an example to illustrate symbiotic evolution. Consider the mulefa, a race of creatures living, in effect, in the Garden of Eden, who have developed an interdependence with a species of tree, the seed-cases of which are the right size and shape to be used as wheels.
The claws of the mulefa have evolved as axles that fit exactly into the hole in the centre of the seed pod which exudes a lubricating oil. It is the use of the adamantine seed-pods as wheels that breaks the pods down and liberates the seeds. The mulefa live in harmony with their environment; even when they kill for food not a scrap of the carcass is wasted, in which they resemble the idealised view of the Plains Indians before the coming of Europeans.
"Creatures" is not, in fact, the right word in this context. The mulefa were not created; this is the whole point. They have no creation myth, but they have a myth of enlightenment, the moment when a snake prompted a female mulefa into discovering how to use the wheel-pod, similar to the moment when Eve ate the apple at the behest of a serpent, which for centuries has been designated as The Fall, source not of enlightenment but of original sin, after which Adam and Eve were cast out of Paradise and their generations doomed to centuries of blackmail.
The message of The Fall runs: Paradise is still there; endure unspeakable and unnecessary miseries while alive and do as you are told and you may get back to heaven when you die. Step out of line and you suffer for eternity. Knowledge is evil, ignorance is a state of grace; a doctrinal system exemplified throughout history by Bible translator William Tyndale, strangled and burnt at the stake for heresy, Galileo arraigned before the Inquisition for his treatise on terrestrial double motion, women in childbirth denied pain relief on the grounds that God condemned Eve and her successors to bring forth children in suffering, homosexuals traduced by edicts that date back to Leviticus. And a species swollen to plague proportions forbidden to practise birth control.
In the first two parts of Philip Pullman's trilogy, His Dark Materials, it was possible to interpret the Church as a metaphor for intellectual and sexual suppression. The Amber Spyglass admits no such evasions. Pullman is addressing the Church as it has been and is; not Christ, or Christian faith, or any other faith come to that, but the organism that has grown out of man's word ascribed to God. No other institution has so elevated ignorance as an ideal, has so vigorously and violently defended it with obfuscation, obscurantism and, frankly, terrorism. We are not talking here of old maids cycling through the morning mist to Holy Communion, but a global malignancy of oppresson that can be defeated only by a new war in heaven, a new Eve, a new serpent - and a new Fall, which is no fall but a great leap forward.
The first book, Northern Lights, introduced us to science and sensuality in the opposing persons of Lord Asriel and Mrs Coulter, each in ruthless pursuit of power by any means, and their daughter Lyra with her daemon Pantalaimon (her separate but inseparable soul). Their world was parallel to ours but with less advanced technology and an ecclesiastical regime closer to the Middle Ages than the 20th century.
At the end of the book Lord Asriel tore an entry from his world to another and Lyra, accompanied by her daemon, passed into it. At the beginning of the second book, The Subtle Knife, Will Parry found a crossing point in the Oxford of our world, encountered Lyra and took possession of the knife with which he cut entrances to yet more worlds. Following his route, a scientist, Dr Mary Malone, set out to establish the truth about dark matter, the particles that she had been researching. In the mulefa's world they are known as sraf; in the world of Lord Asriel and Lyra they are called Dust, a phenomenon that attaches itself to adults but ignores children below the age of puberty.
The antithesis of this phenomenon is the existence of Spectres, entities with identical habits but opposite effects - invisible and harmless to children, disastrous to adults.
As The Amber Spyglass opens, Lord Asriel and Mrs Coulter have discovered the power invested in their once-unwanted daughter. Lyra is in the hands of Mrs Coulter (part mother, part proprietor); Lord Asriel plans to steal her rather than rescue her; Will seeks only to find her because he loves her for herself and Lyra dreams of another bond, with the dead, that must be honoured. Meanwhile, Mary Malone passes into the world of the mulefa as the incarnation of intellectual curiosity, a serpent in Eden. Making their hazardous, uncomprehending way to the same destination are Lyra and Will.
Passing through warfare, cataclysm and the Valley of the Shadow, they encounter old friends and enemies - Mrs Coulter, the cliff-ghasts, the Arctic witch-clans, Iorek Byrnison the armoured bear - and new ones: the diminutive Gallivespian spies, guardian angels, Metatron the usurping angel, God, death and harpies. They are pursued all the while by Father Gomez, sent by the Church to destroy them as a sacred duty.
As before, readers on whom the Dust has not yet fallen can devour the hair-raising adventures, wonder and shudder and grieve at glimpses of things not yet understood, invest with personality Lyra and Will who are, as can be seen now, basic prototypes. The rest of us, the Fallen, can enjoy the mind games and allusions, admire the vision of a formidable thinker, applaud the daring of the enterprise and marvel at its execution.