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.