Given scientific fact as input, generate the question from this fact such that it can be answered from the input.
--------
Question: Prokaryotes group of simple organisms recycles nutrients and has been on earth since long before multicellular life appeared.

Answer: What group of simple organisms recycles nutrients and has been on earth since long before multicellular life appeared?


Question: A skeleton that consists of hard, mineralized structures located within the soft tissue of organisms is called endoskeleton.

Answer: A skeleton that consists of hard, mineralized structures located within the soft tissue of organisms is called what?


Question: Calcium ion is an example of a second messenger.

Answer:
Calcium ion is an example of what kind of messenger?