[mlc9852]

Are you saying science cannot define "human"?

[Thatcherite]

We can define what constitutes a member of the human species now, but if we were miraculously able to summon up all of our ancestors back in time to something that everyone would agree is an ape, and stand them in a line, no-one would be able to point at one particular individual in that line and say "everyone before this is ape, and everyone after this is human."

Well, someone might claim they could do that by a variety of different metrics, but getting everyone to agree on what those metrics should be would be impossible. The entire definition of what consitutes a species is fuzzy, which is a prediction of the theory of evolution, because species are constantly evolving via genetic drift, sexual selection, and natural selection amongst other mechanisms.