While I was an undergraduate at Rice, sometime between 1958 and 1961, a wonderful psych professor, Trenton Wann, who influenced me more than any other single person, had us read a novel You shall Know Them by Vercors in translation from the French original. Some years later I found a copy but now it is lost. But it raises the question of what are the criteria by means of which we recognize an entity as a human, an issue that is raised by the abortion issue.
The basic plot is that evidence of a potential "missing link" is found in Africa and the Brits send an expedition consisting of a variety of different sorts of people including a theologian, I believe, and a reporter who serves as the novel's protagonist. Their mission is to find extant members of the group and study them. They are found and are observed. In the meantime the reporter discovers that a corporation, possibly French, plans to exploit these entities as slave labor so he impregnates a female. My memory is a bit hazy but I believe he brings the pregnant entity back with him and when the child is born he kills it and calls the police claiming he had committed a murder. He thereby creates the dilemma for himself that if he gets what he wants -- a determination that the child is a human and that therefore he has, in fact, committed murder -- then he will have to go to prison but the newly discovered people cannot be exploited as slave labor since they would have to be recognized as sufficiently human to make it morally repugnant. If he doesn't get what he wants, he stays out of prison but the corporation would be within its rights to exploit these nonhuman entities as it wishes (pace PETA).
The British parliament takes up the issue as to whether or not these entities are human and they are given access to all of the expedition's information. They decide that they are sufficiently human to warrant being called human (just as Neanderthals are called "human") and their reason was that in smoking their meat briefly before eating it -- too briefly either to cook it or give it a smoke flavor (I believe) -- they were exhibiting ceremonial behavior and that is a characteristic shared by all humans but no animal species. I open to you what you would look for in a newly discovered "missing link" set of beings as evidence that they are or are not human.
Notice how different this question is from the question whether the human foetus is a human. There is no question that a born child would meet our conditions for something being a human being though given how limited newborns are they wouldn't present much evidence. They don't talk, can't sit up to say nothing of being able to stand up or walk. The human foetus is clearly human, that is it has the quality of being human (adjective use). But it doesn't follow from that that the foetus is A human (noun use), a fully human entity, at least early on in the gestation period -- certainly not when it is a zygote or two-celled entity or four-celled entity, etc. You can take a look at a one month old foetus to determine for yourself whether you think it looks like a human.
Religious people, especially Christians, seem most firmly to believe that abortion is immoral and that their basis for believing his would be the Bible. I found a very silly site that gives ten Bible reasons for believing this. After reading the first two "bible reasons", which came down to the fact that the Bible uses "babe" and "infant" not "foetus" in reference to the foetus, I gave up reading this nutty site. I have better things to do. Obviously the Bible would not use "foetus" because the Bible was written in Hebrew, Aramaic, as well as in Greek. And this dude was reading an English language bible.
In any event, this blog site will not accept any religious text, including the Hebrew or Christian Bibles, the Koran, or whatever as an authority on any issue. The problem with arguments that appeal to such texts is that we don't all believe in the same texts so there will be no common ground upon which all of our arguments can stand. In any event, any moral principle that cannot be defended on nonreligious grounds isn't worth the pixels it takes to print it on a computer screen.