Some apologists assert that a growing entity which has human “DNA” is a human being. Which would mean that a few human stem-cells in a Petri dish already counts as a human being. Even if they are grown to be a replacement for a failing organ. Or even if they are grown to be a twin of an already existing child.
Apart from the obvious problems, there is a fundamental question: “What is a human DNA”? A certain sequence of molecules? But there is no precise sequence of molecules which could be classified as a “human” DNA. There can be mutations, some minor, some major. At what point will the mutated DNA become a non-human DNA?
Biology comes to the “rescue”. In biology the definition of a species is that members of the species are able to interbreed with each other, and produce something that is also able to procreate. That is why the “mule” - the result of the cross-breeding of a horse and a donkey is neither a horse, nor a donkey.
So, if a mutant is biologically unable to procreate with another human, then it is not a new member of the human species. However, it can happen that two new members are compatible with each other, but not with some randomly selected “human”. Therefore we have some members of a brand new species - some entities that are NOT humans. How should we treat them? As “honorary” human beings? Sounds like a sensible idea… but then we treat others as if they were human beings, and yet, they are not.
So the biological definition of “human” is not sufficient. We need the philosophical approach, which is also endorsed by the church: “humans are rational animals”. But this is also problematic. A few cells in a Petri dish are definitely not rational animals. They are a collection of some cells - which MAY grow into someone we call a human being. Let’s place then into the uterus of a woman. Do they become a human being just because they are placed into a uterus? So a few growing cells (blastocyst) is not a human being - it could be called a “potential” human being - but ONLY as long as the DNA enables the result to procreate with an actual human being. Otherwise it could be a member of a brand new species.
And why should the “animal” part be considered essential? If there is a being, which is not composed of animal cells, but is able to think and act rationally, would that being become an “honorary” human?