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The Cannibals
Answer
First, two cannibals go across to the other side of the river, then the rower gets called back. Next, the rowing cannibal takes the second across and then gets called back, so now there are two cannibals on the far side.
Two anthropologists go over, then one anthropologist accompanies one cannibal back, so now there is one anthropologist and one cannibal on the far side.
The last two anthropologists go over to the far side, so now all the anthropologists are across the other side, along with the boat and one cannibal.
In two trips, the cannibal on the far side takes the boat and ferries the other two cannibals across the river.