Ancient hunter-gatherers developed instincts that stigmatized selfishness, and voluntary cooperation was encouraged as well, whether it be within the group or also with outsiders.

 A

selfishness, and voluntary cooperation was encouraged as well, whether it be within the group or

 B

selfishness, encouraging voluntary cooperation as well, both within the group but

 C

selfishness and also encouraging voluntary cooperation, within the group and

 D

selfishness and they encouraged voluntary cooperation, whether it be within the group or

 E

selfishness and encouraged voluntary cooperation, not only within the group but

【申友解析】

正确答案：E

THAT做instinct的定语从句
A.it没有可指代的对象；
B.encouraging voluntary做状语修饰前面整个句子，逻辑错误，both but结构错误
C.and also语义重复，encouraging没有和stigmatized平行；
D.主句和从句不能并列，they指代不清，it指代不清