The official Catholic answer is that there must be a clear separation between man and animals. Souls are not generated by biology but implanted by God. Therefore at some point undetermined and probably never to be determined a Homo Erectus couple who were apish animals gave birth to a child who had a soul and was the first real human. The story of Adam & Eve with all its explicit details is considered symbolic. Y'know animals are the mere "dust of the Earth".
It was accepted that an immortal soui could not exist in a rock nor a plant nor anything without enough brain to receive it. In the Middle Ages the Catholic Church said the soul was not implanted in a fetus until obvious differentiation of the fetus into male or female. Any priest who carried out any religious service for a miscarried fetus like that would be excommunicated. In effect an early term abortion was OK. Visible differentiation occurs at about 7 weeks
In the middle of the 19th century microbiology reached the point where fertilization of sperm and ovum were discovered. The Catholic Church changed its doctrines and had the soul implanted at fertilization.
In the late 19th century before the discovery of the chromosomes the theory of animals growth was called the Mosaic Theory. Clearly as the cells grew and specialized they would fragment the germ plasm (as it was called) into different somato plasms. So to put it in modern terms the DNA in your right hand would only be DNA for a right hand. Thus the fertilized ovum was unique and special.
So the Catholic Church had updated its doctrines and as it always does, denied it ever had any other doctrine. Whatever its doctrines are now are what it has had eternally. For instance, it has never, ever said you can't make loans at reasonable interest rates even tho you can find its pronouncements all the way up thru the Middle Ages that even declared that anyone who opened a discussion on the matter would be excommunicated.
But we live in an era of not only cheap printing but universal literacy. We now know that when I cut myself I kill lots and lots of divine images of myself. All my cells have divine images = the genome. According to the National Institute of Health
this was also discovered:
The NIH reports, "It is estimated that up to half of all fertilized eggs die and are lost (aborted) spontaneously, usually before the woman knows she is pregnant. Among those women who know they are pregnant, the miscarriage rate is about 15-20%. Most miscarriages occur during the first 7 weeks of pregnancy."
So nature or biology or whatever you want to call it has no great regard for these unique human genomes and discards about half of them.