So there is no scientific definition of human? That's can't be right.
That is not quite what he said, but if you think you can make hay with it, knock yourself out.
Remember, science deals with facts and theories. If you want certainty, try faith.
Why can't that be right? Because you want it to be right? Because you think it ought to be right? Because you think you've scored some kind of point? Anyway, that isn't quite what I said. But feel free to twist my answer to score an imaginary point.
Species change over time. At any fixed point in time a species is defined, but over geological time the definition of a species changes; every child is the same species as its parents, yet over thousands of generations the great, great..... great, great grandchildren become a different species. Over time the definition of species is fuzzy.