Chapter four: Twilight Sparkle.
Twilight was young when the explosion happened and unlike the unicorn that were actually born as ponies she struggled with magic like Sweetie Bell because she was actually human and her mutation was settling. She got her cutie mark when she accidently sped up the settling of her mutation as seen in the flashback of her getting her cutie mark.
In the human world she was actually the daughter of a female scientist thus giving her the love for her books that her human mother read to her as a child or she would sometimes get into her mother's books.
Twilight got Spike as an egg that was actually an egg of a lizard mutated to create a dragon.
Though in the human world the scientist witnessed Twilights return and Spike becoming a dog in which he had already done since he thought Twilight should have a dog not a lizard.
Spike can talk because he believes he was hatched after a mutation changed his egg and himself before he hatched.
Twilight and Spike grew up together but Spike grew up much more slowly because his mutation was from when he was an egg and well this affected his aging because of it.
Spike can send letters to Celestia and receive them because Celestia usd her magic to make it so since Sunset took the cellphone book with her when she left.