Psychologist Thomas Suddendorf has written an interesting book ("The gap" ) about what sets humans apart from other animals.
He distinguishes six domains, the first term is related to features we can find in animals, the second term belongs to Homo sapiens (the sign "/" representing the "gap" ):

• Communication / language
• Memory / mental time travel
• Social reasoning / mind-reading (theory of mind)
• Physical reasoning (problem solving) / scientific theories (abstract reasoning)
• Empathy / morality
• Tradition / cumulative culture

In all six domains we repeatedly find two major features that set us apart:
a.) our open-ended ability to imagine and reflect on different situations
b.) our deep-seated urge to link our scenario-building minds together (joint attention and collective intentionality, according to Michael Tomasello).

It seems to be primarily these two attributes that carried our ancestors across the gap, turning animal communication into open-ended human language, memories into mental time-travel... and so on...