Chicken and egg, Ent and tree...

[Sardy]

According to Treebeard, it was the Elves who woke the trees and taught them to speak.

Yet, Gandalf describes Treebeard as the oldest living thing in Middle-earth. I'd always assumed that while Treebeard was around before the Elves, he was merely "treeish" until the elves awakened consciousness and sentience.

However, the following quote seems to contradict this premise:

""It is not wizardry, but a power far older," said Gandalf: "a power that walked the earth ere elf sang or hammer rang."

Ere iron was found or tree was hewn,
When young was the mountain under the moon;
Ere ring was made, or wrought was woe,
It walked the forests long ago."

A related question: if Ents were indeed around before elves, as well as other less defined but presumably sentient creatures deep in the earth ("Far, far below the deepest delvings of the dwarves, the world is gnawed by nameless things. Even Sauron knows them not. They are older than he.") What are the implications of such creatures and Ents existing before the advent of the elves, in terms of Eru's strict (as demonstrated by the Dwarves) plan for the Elves as the "First-born" on Middle-earth?