What does Mafatu discover when he looks at the banana tree?

In Call It Courage by Armstrong Sperry, Mafatu, the main character, lands on a deserted island after leaving his own home in a dugout canoe. When he explores the opposite side of the island from where he landed, he views a sheltered valley below him where a stream flows. He wonders whether he will find people there.
He is at the base of a volcanic crater, and natural "lava slides" flow down into the valley. He remembers a tale of boys who slid down such a slope on banana leaves. He approaches a nearby lava tree and tears off some leaves, with which he can make a toboggan. To his surprise, as he examines the tree more closely, he realizes that the spots where the fruit had grown on the tree show evidence that the fruit was cut off with a sharp knife. This captures his attention, for it is undeniable evidence that other people have been on the island and perhaps still are. At this thought, "his heart gave a great thump and hung fire."
As he examines more trees, he sees that many of them show evidence of their fruit being picked using knives. He determines that the knife marks are no more than a week old. He resolves to find out who picked the fruit from these trees and sets off down the lava slide on his banana-leaf sled.