The relation with this problem is that you can offer the following attempt at definition: "it's human if the direct ancestors are human". Of course, it fails ;)
Another attempt is via attributes (fe, "if it has a head, arms, legs, and an IQ of at least X and DNA that has ..., it's human"). This also quickly fails.
I think the current consensus amongst philosophers is that a binary categorization is impossible in these cases. ie, whatever the definition, you will either exclude things you want to be included and/or include things you don't want in there.
There is no firm answer, and the choice depends on one's philosophy. The US Supreme Court - and in this case I am suggesting that the courts are a sort of applied philosophy - took input from biology in deciding Roe v. Wade.
Medical ethics, again a form of philosophy, says that human embryo experimentation must not continue past two weeks, based on input from biology. That's when the central nervous system starts to form, and marks the end of when the embryo can split into twins. It was also not believed possible to keep an embryo alive outside the womb that long, but now it's possible. http://www.npr.org/sections/health-shots/2017/03/02/51628089...
If the definition of human is related to DNA, then is cancer human? What about immortal cell lines? Parasitic cancers like Canine transmissible venereal tumor?
Were Neanderthals human? Denisovans?
so which of the 26 taxonomy models are you referring to ?
Clearly, it's an utter mess.