Thanks for your answers, which help me to formulate things more accurately.
I totally agree that it does not exist a pure scientific definition of “human”.
My view is as follows:
The “main piece of evidence” entitling me to claim for my rights, and especially not to be harmed, is my specific body, that is a body that can be distinguished cut-off as a human body.
So if I wish to be respected, I have coherently to respect all creatures exhibiting a human body.
Accordingly Humanity means primarily the “life form” whose individuals share in the sense of law and are called to live according the “Golden rule”.
The amazing thing is that this is possible, because Evolution worked in such a way that today there is a clear distinction between the human body and the body of all other “life forms”, even the genetically nearest ones (chimps, bonobos). The “life form” Humanity can be today distinguished from other living forms by observable biological means.
Accordingly, although Humanity is mainly defined as a community of creatures called to live according to the “Golden Rule”, at the moment of implementing this rule and assign rights the “Golden Principle” is the specific human body. Thus the foundation of law can be formulated as follows:
“The human person and her rights are defined by her belonging to the human species. A human individual shares the status of a person, and personhood is inseparably united to humankind. This principle means that the fundamental rights of a person cannot be established by belonging to a subgroup of humankind, be it by race, religion, nation, or political class. Neither can one reduce the rights of humankind to the rights of the present-day generation.” [M&M, 16-1 (2013) 85]
In this sense not only Humanity but also the concept of Species itself cannot be defined by “pure scientific means”: It is motivated by the will of assigning rights coherently and refers firstly to Humanity; only thereafter it is extended to other “life forms”. In the maelstrom of Evolution the concept of Humanity has become totally “biologized” and now is often confused with the evolving species Homo sapiens.
So one has to be careful and not be caught unaware by two pitfalls:
Trying to define the beginning of Humanity through a biologically sharp beginning of Homo sapiens from a genetic common ancestor (@agauger position, as I understand it).
Assert that there is no common ancestor of Homo sapiens without clearly distinguishing between the evolving species Homo sapiens and Humanity, and thereby suggest that Humanity cannot have a sharp beginning in time by God’s intervention either (@DennisVenema position, as I understand it).
If one distinguishes between the beginning of Humanity and the (non-existing) beginning of evolving Homo sapiens, then (whichever way one looks at it) one acknowledges that Humanity begins at a time when Homo sapiens already has a large population size. It seems to me this is what you assume in your genealogical model, and it is what I assume in my model with a primeval little community of free accountable human beings (in my view both models fit to the BioLogos principles, and may be equivalent in the end, as I will discuss in a coming post).
Before continuing I would be thankful to know whether you agree to my claims in this post or could eventually suggest how to formulate things better.