Interesting book on this topic:

>In The Gap, psychologist Thomas Suddendorf provides a definitive account of the mental qualities that separate humans from other animals, as well as how these differences arose. Drawing on two decades of research on apes, children, and human evolution, he surveys the abilities most often cited as uniquely human -- language, intelligence, morality, culture, theory of mind, and mental time travel -- and finds that two traits account for most of the ways in which our minds appear so distinct: Namely, our open-ended ability to imagine and reflect on scenarios, and our insatiable drive to link our minds together. These two traits explain how our species was able to amplify qualities that we inherited in parallel with our animal counterparts; transforming animal communication into language, memory into mental time travel, sociality into mind reading, problem solving into abstract reasoning, traditions into culture, and empathy into morality.

>Suddendorf concludes with the provocative suggestion that our unrivalled status may be our own creation -- and that the gap is growing wider not so much because we are becoming smarter but because we are killing off our closest intelligent animal relatives.

https://www.amazon.com/Gap-Science-Separates-Other-Animals/d...

There had been lot of theories but to every theory there had strong counterpoint found. For example, speech, language and math was considered unique to humans but several animals have been found to speak, have elaborate language and even do counting and addition. Then there was belief that animals can’t do counterfactuals, for example, making decisions based on simulating what-if scenarios but that also has been proven false. My take is that there is no fundamental lack of ability in animals but the fact that those abilities are a magnitude or two lower than humans and animal species tend to specialize on very specific abilities.

There is book written on this subject that argues that most fundamental human abilities not yet seen in animals is building nested scenario such as telling a fictional story and desire to share knowledge among each other:

https://www.amazon.com/Gap-Science-Separates-Other-Animals/d...