Evolutionary progress at times rewards less intelligence in a species.
“High intelligence is just one evolutionary strategy,” says Georg Striedter, a neurobiologist at the University of California, Irvine and author of Principles of Brain Evolution. “Some species do well in evolutionary terms precisely because they invest in other features, not intelligence.”
The Virtue of Being Simple, by Brandon Keim.
It is survival of the fittest, not the smartest.