PIE has been pretty well reconstructed up to the time of the breakup
into Hittite, Indo-Iranian, Proto-Germanic, etc. We also have some
tantalyzing clues to its earlier structure.Perhaps before trying to
link PIE with other language families we should come up with a
reconstruction of Pre-PIE.
Some issues to be addressed:
How many laryngeals and what was their manner of articulation?
PPIE was apparently an ergative language. What was the structure of
nominal morphology?
Was there a feminine gender?
How many vowels were there?
What was the origin of e~o alternation?
What, if any, function did thematic vowels have in nouns and verbs?
How, if at all, does nominal morphology relate to pronomial
morphology?