Elves were a very special species. They were born with great luck and coincidence. Many elves would call themselves the children of the forest and the children of nature. This was true in a narrow sense, in a broad sense, all creatures were children of nature, but Elves were more special.
Before they became animals... ... they were indeed plants ... And indeed, most of them came from deep in the mountains and forests...
They were usually a very special kind of plant, a type of plant with high psionic power or some fortuitous coincidence that could live for more than a thousand years, which came in the form of a plant...
The evolution of most species in the universe was long. It took many, many years of environmental changes and genetic adaptation to change from one species to another...
But Elves were different. They had very complex and changeable genes, and could completely evolve into a completely different species in a short period of time...