This study investigates student conceptions of the facial angle as a way to attain understanding elements of the theory of human evolution. The chosen laboratory work involved determining the species of a human cranium, and students had to design and write down their own experimental procedure. Three versions of the laboratory work were carried out leading to different student productions. Three aspects will be presented in the present paper, which are related to the three conceptual difficulties that appeared in our a priori analysis: the importance of students' everyday knowledge of angles and of anatomy of human crania, the problem of knowing how many points make up an angle, and finally, the way in which students determined a reference system to construct an angle.