What is the difference between "something else" and "something extra"?

What is the difference between "something else" and "something extra"? The answer for the blank in this question is "else", but is "extra" an alternative answer? If "EXTRA" IS WRONG, WHY IS IT WRONG?

"A land free from destruction, plus wealth, natural resources, and labor supply – all these were important factors in helping England to become the center for the Industrial Revolution. But they were not enough. Something __ was needed to start the industrial process. That “something special” was men – creative individuals who..."

To be picky: the nuance of "something else" implies an alternative; "something extra" implies an addition.

Examples: "I am allergic to peanut butter, so I need to use something else in my sandwich."

"I love to put something extra in my peanut butter sandwiches, like bananas."

So in the sentence in the question, I would use "extra" if I was being very picky; however, I think the colloquial understanding of "something else" would give the reader the same idea. In other words, I don't think the reader would see "something else" as implying that men were a different impetus as opposed to an additional impetus.

"Don't give me x, give me something else something extra"

"Let me keep x, give me something extra" (but "something else" would be O.K. for most people)

EDIT: In your test, neither something else nor something extra is the right answer.

Something __ was needed to start the industrial process. That “something special” was men – creative individuals who..."

This only works if you write something special in the first sentence. "That 'something special'..." when the antecedent it refers to was "something else/extra" is poor writing.

Your teacher may have marked down something extra because it is more appropriate for conversation than for formal writing.

Even though, semantically speaking, perhaps some ingredient is needed which is in addition to the existing precursors for the industrial process, the word you need there is else.

The word else is not strictly for alternatives. It is possible to "(also) want something else." which sometimes means in addition to other things that were previously stated. If also is present, it certainly means that.

A good example is: "There is something else I wanted to ask you." which does not mean that I wasted time asking you the wrong question and that actually I just want the answer to this new one!

Extra is something beyond the norm: "Extra large order of fries with the burger." Note that in the sentence we are examining, "needed" follows the blank space. Something which is extra usually cannot at the same time be needed; this is oxymoronic.

There are exceptions. For instance, "This morning Bob needed an extra cup of coffee to get rolling". In this case we are comparing with other mornings, when that cup is not required, expressing the idea that today's requirement is beyond the usual norm.

In the given question, there is no implied comparison to other times when other industrial processes got started. So there is no basis against against which extra can be considered.