First permanent molars serve a critical role in maintaining proper occlusion and arch integrity. Extraction should be avoided if third molars are absent because there will be no natural posterior replacement to prevent mesial drift of second molars. This can lead to loss of arch length, malocclusion, and difficulties in future prosthetic or orthodontic treatment. When third molars are present, they can potentially erupt into the space of the extracted first molar, preserving function. Therefore, the presence or absence of third molars is a key surgical consideration before deciding to extract first permanent molars.

Reference: Shafer's Textbook of Oral Pathology, 8th Edition.