"To be human is to yearn . . . "

"To be human is to yearn . . . " The Crane Wife, Patrick Ness.
There are more theories out there than I can remember, of what made us human, what small change lead us down the evolutionary path toward being so very different from other animals. No, we are not entitely different, but in so many ways, we are different by degrees, huge degrees.  Animals seem to use tools, but only one or two at a time, not whole arsenals and kits and versions and variations like we do, from trivia like dental floss to massive machines that we set loose underground to dig a tunnel for us and process and eject the debris.  
Animals play but their play seems more like practicing for life, such as pretend stalking and pretend fighting, and seems not to have the imaginative pretending that includes building fantasy worlds to the extent that we humans play.  Animals seem to mourn but never so long as we and never to the extend of creating and visiting memorial sites.  Animals seem to not look much to the future, beyond storing food for an immediate next season.  They certainly do not seem to plan ahead to stock pile building materials for future use or build and store tools they won't need until later.  Animals communicate about territory and mating and immediate needs, but certainly don't seem to possess any story telling capacity.  A few animals seem to ornament their environment, but these are en extension of mating attraction or nest building, and never come close to the human making of art as story telling and expression of concepts.  The human brain is more complex and therefore permits and accomplishes far more complex behaviors than any other creature on our planet.
There was a theory long dismissed that we evolved to stand up in water, and once our hands were free, we could make tools.  But other primates have hands free and have not embraced the tool making to the point we have.  There was one about us evolving in grassland and standing to see above the grass.  There was one about how once we began to cook food, we could get more nutrition from it and did not need to eat and hunt and gather as much.  Other theories say that it was language, moving from simple grunts and gestures to more complex sounds that become increasingly complex symbols for objects and actions and emotions.  Did language and food cooking and tool use and standing upright cause us to become human, or were they merely products of some other change that made us human first then lead to us evolving those traits?  
Maybe it was as simple as wanting more.  Maybe instead of being content with enough food, enough shelter, we began to yearn.  To want to try different foods.  To want to see what was just beyond that hill, that forest, that plain.  To make a tool and make another, and then more after that, every improving.  To not be content with basic words but to want to add detail and intricacy.  To want the shelter to be stronger, higher, more portable, more lasting. To see just one day or one season or one cycle of the sun beyond and want that one to be better and more than this one. To see colors and shapes in nature and want to decorate our things with them to have the colors and shapes with us even out of the season of the flowers and the birds and the insects. To not be content with now, but to wonder about before and wonder about after.  To  not be content to let happen but to plan and cause to happen.  To not just be but to cause and make.  
What if it was as simple as just not being happy with what was and wanting more?
Is that what sets us apart? We feed the dog and he lies down for a nap, content.  We ponder how the meal could be a little better next time, what to have for dinner.  We give the dog a nice cushion and he scrunches it up a bit and turns and lies down, content to be comfortable.  We get a new home and paint and fix and tile and sit down to rest and ponder if maybe different window blinds are needed or maybe a bookshelf over there, never done, never happy to let things be.  The dog is content with the same walk every evening, while we tinker with it, one more block, east instead of north, a loop down past the park, what about over there?  We have myriad forms of transportation to take us farther and faster.  
We get a job and we complain it isn't challenging enough, it isn't interesting enough.  We get a car and we add new floor mats and special wiper blades.  We tinker and tweak and adjust and change and add and grow and accumulate

and still, we aren't happy.  We travel on vacation and are pondering while there what we should do next time, instead of being in the moment now, we are far out ahead making plans, setting goals, leaning forward, yearning.  We analyze the past and constantly ponder the future.  It is our curse to do these things to the exclusion of experiencing the details and nuances of the present moment, We yearn.  Is that what got us here?  And where will it take us?