In Call It Courage, what does Mafatu realize once he sees the island has a sacred place?

In Call It Courage, when Mafatu finds the marae, the Sacred Place, on the island, he knows that he has found a forbidden island, or motu tabu.

This realization can be found on p. 46 of the book. At this point, it is the first day that Mafatu has spent on the island and he is exploring. While exploring, he finds a long lava slide and decides to sled down it on a sled of leaves. At the bottom of the slide, he finds a trail that is too fine to have been “made by the feet of wild pigs (p. 46).” He walks along the trail and comes to the Sacred Place. Mafatu looks at the idol and then looks around its base. He sees burnt bones all over the platform. They are too large to be dog or pig bones so he realizes that they must be human bones. At this point (still on p. 46), we are told that

And then he understood. His heart congealed.  This was a motu tabu, a Forbidden Island.