这里whose修饰life,作life的定语.引导定语从句的关系代词中只有wose才可以作定语.
题中whose life合起来作这个定语从句的主语.
whose可以转换成of which.这句话可以说成:
He belives that earth is unlikely to be the only planetthe life of which has developed gradually.
或:
He belives that earth is unlikely to be the only planetof which the life has developed gradually.
或:
He belives that earth is unlikely to be the only planetwhich the life of has developed gradually.
人教版高中英语必修(二)附录中好像有相似的例子.