En embryo might eventually become a human but is not one yet! You need a decently developer brain at least… being a small agglomeration of cells with human dna doesn’t make you a human! A tumor is an agglomeration of cells with human dna but is not treated as a person! On general is a gradual process… but like with every fuzzy system it is easy in some case to say if something belongs ta a category or not but when you reach the overlap of categories to becomes harder. Just set the threshold when it is still clear