The special aims to answer such questions as: How did we get here? How did mutations create male and female sexes? And were we actually fish at one point during the evolutionary chain?
"This is the improbable story of human existence -- where we’ve risen from raw materials of Earth to the big-brained, two-legged mammals we are today," the episode description reads. "One change or predator along the way and this story would have never been told."Source: