Strictly from a biological perspective, what is the functional expectation of a human being
I will start with a quote from François Jacob, a French geneticist:


  The sole ambition of a bacterium is to make two bacteria.


The same reasoning can be extended to all life on Earth, including humans.

There is no purpose to human existence, in the same way that there is no purpose for life at all. Life is there because it can. It works, chemically, physically and thermodynamically, given the physical conditions present on Earth.

Biology does not give any moral reason for our existence. It only describes what is there. Using experimental techniques we can infer the purpose of the human feet. All we need to do is to remove the feet of a human subject and see what happens. The subject will be unable to walk, and therefore we will be able to infer that the feet's purpose is to enable locomotion. In biological research, we usually remove or silence genes or proteins from an organism to infer their purpose. We then observe the phenotype. Most of the time however, such an experiment does not give any information on the purpose of the gene because its function is not obvious enough to be observed. 

In the same vein, one could remove humans from the Earth and see what happens. If something changes for the worse on Earth, then we would be able to infer that our purpose was to prevent such event from happening. Obviously, such an experiment is impossible to perform, therefore we will never know what is our purpose, neither will we know whether we actually have a purpose.