Mafatu finds a spearhead in the Sacred Place.

When Mafatu discovers the Sacred Place, he sees

"a series of wide stone terraces rising in a pyramid many feet high; on top of this pyramid a grotesque idol, hideously ugly, rear(s) in the brilliant sunshine.  It (is) and ancient idol, its contours softened with fungus and lichens...around the base of the idol, he (sees) piles of bones, charred, but not old.  The platform (is) strewn with them.  Bones too large for dogs, too large for pigs...Here the eaters-of-men (make) their terrible sacrifices to the Varua Ino".

Examining the monument from a short distance, Mafatu sees a spearhead on the sacred platform.  "Finely ground, sharp-edged", it would make a perfect "spear for food, a weapon against attack".  He wonders if he should take it, knowing that to do so might mean death for him.

Although the term is not used in the book as far as I can see, another name for the "eaters-of-men" is cannibals.  In the book, they are referred to merely as savages.

Mafatu first realizes that people have been on the island when he notices that fruit on a banana tree has clearly been severed by a knife.  That tells him that beings that are capable of making and using tools (the knife) have been there.  He then discovers the Sacred Place, which proves unequivocably that people have been there, recently.  The fact that the piles of ashes resting on the platform have been "undisturbed by the wind and storm" tells Mafatu that the savages have been on the island not too long ago ("The Island").