instruction:
Given scientific fact as input, generate the question from this fact such that it can be answered from the input.
question:
600 million many years ago may the earliest fungi have evolved.
answer:
How many years ago may the earliest fungi have evolved?


question:
Deep currents are caused by differences in density at the top and bottom of the ocean.
answer:
What is caused by differences in density at the top and bottom of the ocean?


question:
Birds have four limbs.
answer:

How many limbs to birds have?