I like what Hook wrote. If I believed that babies were valuable because they have souls and then was told, "no they don't have souls", I might for a while value them less. But it has been a very long time since I believed in souls and the value I assign to babies is no longer related at all to my belief about souls (if it ever was).

After all, in the space of "things that one can construct out of atoms", humans and goldfish are very, very close.

Sure, they just don't resemble each other in many morally significant ways (the exception, perhaps, being some kind of experience of pain). There is no reason to think the facts that determine our ethical obligations make use of the same kinds of concepts and classifications we use to distinguish different configurations of atoms. Humans and wet ash are both mostly carbon and water, and so have a lot more in common than, say, the Sun. But wet ash and the sun and share more of the traits we're worried about when we're thinking about morality. The same goes for aesthetic value, if we need a non-ethics analogy.