Long after Earth has past its liveablility and humanity is a myth, Aliens have ben able to reconstruct a documentary on some of the past lifeforms that existed.
Read in a Morgan Freeman voice for added effect.
“Cranes. These majestic tall metal structures once lived in the most densly populates areas of humankind. They had a short lifespan of about a year. They raise stone-like cocoons around themselves for their offspring, a sub-species of humans called sitee-dwellers, to live in. In return, their offspring would cannibalize their parents, go out and reproduce their parents who then build more coccoons and perpetuate their existence. Truely proof of the circle of life.”