Double negative in "...the most dangerous threat was not not getting..."
You are looking for a rephrasing of the sentence in which the double negative is removed—but which results in a different meaning than simply turning it into a positive.
For your ancestors,the most dangerous threat to survival was not not getting the social vibe of your group.
Compare the following two sentences. The first is the interpretation you suggest. The second is an alternative, where the second not phrase is replaced by something with the same meaning—and to which I've added something else for additional emphasis:
For your ancestors,the most dangerous threat to survival was getting the social vibe of your group.
For your ancestors, the most dangerous threat was not failing to get the social vibe of your group [but failing to observe the presence of predators].
If you simply replace the two negatives with a positive, the meaning of the sentence is completely different than if you replace the second negative of not getting with the synonymous negative of failing to get.
This results because of the difference in parsing. The first parsing is concerned with (not not) getting, while the second is concerned with not (not getting).
The cancellation of negatives occurs when they are applied successively onto the same target ¬¬p = p. In symbolic logic the restrictions of the formal language and inference laws help you inequitably to determine whether a negative act on top of another. For example in ¬(p and ¬ q) the parentheses (and DeMorgan's law) allow you to determine that this is equivalent to (¬p or ¬¬q) in which the negations acting on q cancel to (¬p or q).
The grammar in natural language is more flexible and allows both syntactic and semantic ambiguities. In your sentence the two words was and getting could be two independent verbs or it could be the auxiliary+verb was getting. If we ignore the nature/meaning of the nominal elements in that sentence then there is no way to tell which of the two options is intended.
X was getting Y
could be that X was in the process of getting Y, or it could be saying that getting Y was a property of Y.
Now, adding the two negations remains ambiguous.
X was not not getting Y or X wasn't not getting Y
could still mean either that X was not in the process of not getting Y or it could mean that not getting Y was not a property of X. This is because in one option one assumes that the two negations act on the verb was getting.
Finally, in your original sentence there help deduce that we are in the second option.
1) It is in general awkward to want to express an affirmation by doubly negating to. Specially in this case which would require doubly negating a verb that is already relatively complex, which uses two words.
2) The particular nature/meaning of the subject the most dangerous threat to survival is unlikely to be performing the action was getting if the predicate will be the social vibe of your group. A threat was getting a social vibe? That's an unusual claim. On the other hand a threat was (or wasn't) some action fits.
These two reasons make the reader confirm that the two words was and getting are two separate verbs belonging to two separate clauses, and therefore, that the two negations act on each of them as (was not) (not getting), where there is the extra complication that in English a negation can be applied on a verb post-fixed (was not) and pre-fixed (not getting). In this case both encodings appearing simultaneously.
As I told you before -interesting sentence!