Why do you want it? What additional understanding of the issues do you think a definition of human would help you gain?
Let's put it another way. What part of my thought experiment of lining up all our ancestors and being unable to point at a dividing line between human/non-human did you not understand. Why do you think (supposing for a moment that you accept evolution) that you should be able to draw such a line? I'm not trying to sneer or talk down; I'm trying to help you understand how we see it. I hope my explanations are helping.
I cannot believe there is no scientific standard for human. So with your explanation we might be human or we might not. I'm pretty sure I'm human, though I have questions about my offspring.