Meaning of "took a few feet" in the last line

Solution 1:

It's a reference to the expression give someone an inch and they'll take a mile

Defined by the Cambridge dictionary as being

said about someone who has been given a small amount of power or freedom to do something, and then has tried to get a lot more

and by dictionary.com as

Make a small concession and they'll take advantage of you. For example, I told her she could borrow the car for one day and she's been gone a week—give an inch!

The latter website also says that the expression has its origin in or before the 16C with the reference

This expression, in slightly different form, was already a proverb in John Heywood's 1546 collection, “Give him an inch and he'll take an ell,” and is so well known it is often shortened (as in the example). The use of mile dates from about 1900.

The writer of the piece you quote is using a modified version of the "give them an inch" expression to say that early terrestrial vertebrates took the very slight advantage of being able to breathe air inefficiently and made it a slightly greater advantage which, in time, led to the development of land animals proper.