From Sarah Blaffer Hrdy, quoting Michael Tomasello in Mothers and Others:
We propose that the crucial difference between human cognition and that of other species is the ability to participate with others in collaborative activities with shared goals and intentions.
From Luci Tapahonso in "A Radiant Curve,"
One fall afternoon in 1998, my daughter Misty called to tell us that her infant son Isaiah had just laughed aloud. It was anticipated, because when a Diné, or Navajo, baby laughs aloud for the first time, a First Laugh ceremony is usually held. Shisóí Isaiah (Shisóí means "my daughter's child") had laughed in his sleep, and as a baby he still lived in the world of the Diyin Dine'é, or the Holy People. But this time he had been awake when his father, Lloyd James, tickled him.