Question

Without DNA, humans wouldn’t exist. Is it wouldn't or won't?

Answer 1

The word required here is wouldn't.

Without DNA, humans wouldn’t exist.

Explanation:

Wouldn't is the conditional form of the verb will which, when used in this sentence describes the outcome of a possible event that may or may not occur. We know that DNA does exist and so do humans, in spite of the speculation.

Won't is the abbreviation of the verb phrase will not. Will is the positive form of the verb, but it is not conditional. As a result, it cannot be used in a sentence such as this where there is an extenuating circumstance.

Answer 2

Without DNA, humans wouldn’t exist.

Explanation:

Without DNA, humans wouldn’t exist is correct because wouldn't is the abbreviation for would not and won't is the abbreviation for will not. So, lets see the sentences with the full form of the words:
1) Without DNA, humans would not exist.
2) Without DNA, humans will not exist.

Hmm, look at it yourself, 1) seems right to me. Well, it is!

I hope this helps!
-Moksha