The savvy of life forms is a life-history variable. The manipularity–intelligence hypothesis correlates innate shrewdness with manipularity: the ease by which an organism can manipulate its environment. Basically, organisms which have lesser ability to change their environment must have more “on the ball” to survive.
Microbes individually have little ability to alter their world. As such, they must rely upon their wiles: discernment at the molecular level, including being able to assess the utility of information resident in DNA. Microbes use sophisticated quorum sensing to assess the power they may have as a crowd. Pluricellularity has a power to craft fate which individual cells may only dream of.
Sessile plants survive by their wits. The head start of autotrophy – not needing to find food – belies the difficulties of managing the myriad of risk-based potentialities in a largely uncontrolled environment. A universe of decisions must constantly be made related to resource allocations and the probabilities of chance encounters, especially interspecifically (e.g., herbivory by savage animals). The unsurpassed intelligence of plants is amply illustrated by how they conduct their sociality with bacteria, fungi, other plants, and animals.
Problem-solving as a proxy for intelligence is, obviously, a partial indicator. Intelligence originates with perspicacity which gives rise to comprehension. At the base of intelligence is the mental ability to manage symbols (e.g., concepts) to survive.
Intellectual capacity in animals is exemplified by how frequently they resort to trial and error in problem-solving. With the limited manipulatability of beaks for hands, birds commonly think through a problem to its solution before beginning implementation. Corvids, for example, are known to act only after they have mentally figured the steps for solving a complex task. Methodological adjustment only happens when new relevant information is revealed during implementation.
Like birds, dolphins cannot easily manipulate their environment. They are compensated with generous aptitude for problem-solving, abetted by a convivial sociality which engenders cooperative effort.
In contrast, humans, who have unrivaled ability to control their environment, constantly resort to trial and error to solve problems. Human memory and ability to mentally map out solutions is feeble compared to rodents, who have much less facility for physical manipulation.
The relative stupidity of people is demonstrated by their invariable environmental destruction. Throughout their existence, humans have degraded their habitats. (Habitat destruction by species is rare, as it is an excellent formula for self-extinction, as we see with humans.)
Rare indeed have been the instances where humans have been able to sustain their populations without resort to technological advances, which only accelerate the sapping of natural resources. That humans have been unable to devise a social system to sustain themselves without essentially enslaving sizable percentages of their societies further evidences the relative mental weaknesses of this species.
Human overestimation of their own intelligence is laughable. Dimwits smugly think they know what is going on. In this, so-called geniuses are no different. Despite copious evidence to the contrary, the resolute conclusion of modern science is naïve realism and a simple-minded empirical matterism: that the experienced actuality of materiality is objective reality, and that what comes to mind has validity (notwithstanding having experienced countless instances of false thoughts and self-deception).
People think they are smarter than other animals merely because they cherish their beliefs, and the artifacts which abstractions have brought: technology. But intelligence is not the capacity to have woolly thoughts or to produce mass consumption and destruction. Intelligence is instead the ability to behave aptly. On this count humans have failed miserably.