The earliest sign indicating the eruption of a permanent tooth is the resorption of the roots of primary teeth.

Here's why:

- Permanent teeth develop beneath the primary teeth, initially forming as tooth germs within the jawbone.
- Although the formation of the permanent tooth germ and subsequent calcification of the permanent crown are early developmental stages, these events do not directly indicate imminent eruption.
- The key physiological process signaling the upcoming eruption of a permanent tooth is the resorption (breakdown) of the roots of the overlying primary teeth.
- This resorption causes the primary tooth to become loose and eventually shed, clearing the path for the permanent tooth to erupt successfully.
- Thus, the beginning of root resorption in primary teeth serves as an important clinical and radiographic indicator that the permanent tooth is preparing to erupt.

In contrast, the eruption of primary teeth occurs earlier in development and does not indicate permanent tooth eruption.

Reference:
Orban’s Oral Histology, 14th Edition, Chapter on Tooth Development and Eruption