(A certain amount of) Danger was good for you

In a benign environment an optimal survival machine might invest relatively few of its material and energy resources in thought and mind. In an extremely challenging environment there may be no opportunity to evolve a mind. The process may be just too costly to undertake. Only in an environment offering just the right mix of danger and resources will brain and mind evolve. The environment itself may then change of course. When a sea squirt adapts to its new living environment it absorbs its "brain" and gives up the (then) unnecessary act of "thinking." Intelligence is an adaptation to (an emergent specialization in) a particular range of environments, it is not always ideal/warranted.