英语达人进！

这句话没有从句，你怀疑是从句的部分---resulting from improved health and living conditions，实际上是the rapid extension of the lifespan的后置定语。但这个后置定语，的确可以改成一个定语从句：
The rapid extension of the lifespan, which resulted from improved health and living conditions, is a feature of all developed countries in the 21st century.
其中，result from是固定短语，意思是“起因于”，这句话的意思是：
由于健康水平和生活条件提高而出现的人们寿命快速延长的现象是21世纪所有发达国家的特点。
再举几个例子体会一下：
1. The boy sitting on the chair is my brother.=The boy who sat on the chair is my brother.
2. The disease resulting from poor sanitation is difficult to be cured.=The disease, which resulted from poor sanitation, is difficult to be cured. （因卫生条件差而引发的这种疾病很难治愈。）
希望有所帮助。

resulting引导的是原因状语从句，用来修饰前面的The rapid extension of the lifespan
整个句子里面就只有这一个从句

resulting表示lifespan的动作..其实没有从句..两个逗号之间只是一个伴随状语

结果，导致！