> You are taking 'ancestor' in a strict technical sense, which was clearly not my intention.

Blutly, this was not obvious, and what you suggested would have been equally true if you'd said:

  Learn Lithuanian. You were writing in a descendant language just now. If you learnt the roots, you would understand the descendants better.

Does Lithuanian have a comparable corpus of important literature? Is Lithuanian equally giving when reconstructing the networks of meaning? Is it comparably enlightening in the suggestion of the original common language?