句子主干:A study ..yeilded results....
修饰成分(按顺序):
published in the journal Nature 为过去分词短语作后置定语,修饰前面的名词study;
which compared the problem-solving abilities of young chimpanzees and human children,为which引导非限制性定语从句,同样修饰前面的名词study,对其进行补充说明;
that surprised the researchers themselves为that引导的限制性定语从句,说明是什么样的结果-“令研究者自己都感觉到惊讶的结果”。