Short answers, pulled from the story.
Proto-Basque is a reconstructed ancient stage of the Basque language, dated roughly to the last centuries BCE, before Roman conquests in the Western Pyrenees. It preceded Common Basque, which is dated to the 5th and 6th centuries CE.
Koldo Mitxelena, a Basque linguist, laid the foundation for Proto-Basque reconstruction. His book Fonética histórica vasca, published in 1961 and based on his 1959 doctoral thesis, launched the field.
The Hand of Irulegi is an inscribed bronze artifact shaped like a right hand, dated to the 1st century BC. It contains a small sample of what is believed to be a form of Proto-Basque, making it one of the few direct physical attestations of the language.
Aquitanian, known only from inscriptions naming places, people, and deities from the first centuries CE, closely matches reconstructed Proto-Basque forms. For example, Aquitanian name elements Seni- and Sembe- correspond to the reconstructed Proto-Basque words for "boy" (seni) and "son" (sembe).
Proto-Basque lacked the consonants /m/ and /p/, the semivowels /w/ and /j/, and the entire palatal consonant series. It also had a fortis-lenis contrast for nasals and laterals that does not survive in modern Basque dialects.
Pre-Proto-Basque, also called Old Proto-Basque, is a hypothetical earlier layer of the language predating even the Celtic invasion of Iberia. Joseba Lakarra proposed that this stage was characterized by extensive reduplication, with later deletion of initial consonants producing the vowel-initial word patterns common in Proto-Basque.