What evidence supports the idea that Proto-Indo-European had laryngeal consonants?

The existence of laryngeal consonants in Proto-Indo-European (PIE) was initially hypothesized based on internal linguistic evidence, specifically their effects on neighboring sounds. Because these consonants largely disappeared or merged with other sounds in most daughter languages, their reconstruction relies on two primary types of evidence:

• Internal Reconstruction: Before the laryngeal theory was widely accepted, linguists reconstructed a "schwa" vowel (schwa indogermanicum) to explain certain vowel correspondences across languages like Greek, Sanskrit, and Old English. It was later determined that these patterns resulted from the loss of laryngeals between consonants. Additionally, laryngeals are credited with influencing the quality and length of adjacent vowels—for instance, by lengthening a preceding vowel or changing an e to an a or o.12
• Anatolian Evidence: The most direct confirmation came after 1927, when Jerzy Kuryłowicz demonstrated that Hittite and other Anatolian languages preserved these sounds as a consonant ḫ in positions where laryngeals had been theoretically posited.132