Some historians may write only to promote a particular cause what is the writing considered to be and how is it typically presented in which way

When you write something that promotes a particular cause, it is called PROPAGANDA. 

Propaganda is defined as giving out information, especially of a biased or misleading nature, and using it to promote a political cause or point of view. Most propaganda list down all the positive aspects of the cause and diminishes its negative aspects. There are those who write propaganda by citing all the negative aspects of the alternative, making sure that their specific cause is more attractive to their target audience.

Because the discoveries were some of the oldest human remains.