A new article by Gonzalo Herranz, “The Timing of Monozygotic Twinning: A Criticism of the Common Model,” appears to deflate one of the most common science-based challenges to pro-life personhood philosophy. According to the theory that has been accepted for more than half a century, early human embryos have the ability to split, forming monozygotic (“identical”) twins, up to 14 days after fertilization. This is often used to challenge the pro-life contention that every human being is an individual, unique person from the moment of fertilization. It is argued that because of the possibility of twinning, there is no reason to suppose that an embryo is individual and unique, therefore the destruction of embryos younger than 14 days is morally permissible for elective and research reasons.
(Note: there are many flaws in the argument favoring the moral permissibility of destroying embryos that have been addressed by pro-life apologists.)
But according to Herranz, we shouldn't be so quick to accept the theory that human embryos can “twin” up to 14 days after fertilization. First and foremost amongst Herranz's points is that the scientific community has largely left this theory unchallenged since it first appeared in its complete form in 1955, never a good environment for scientific advancement. Herranz then lays out four challenges that the prevailing theory does not answer and a new hypothesis for when twinning occurs.
Herranz argues that the prevailing model does not satisfactorily explain the complex mechanisms involved in twinning and that the model is not based on facts, but on “apparently reasonable conjectures.” Further, there has been no observed confirmation of the MZ twinning model during IVF procedures and other assisted reproductive technologies. Herranz concludes that the model is “fragile and untenable” and therefore cannot be used to support biological or bioethical claims on issues related to the early embryo. (Please see the article for specific details of the weaknesses in the current model, according to Herranz.)
Herranz then offers a new theory on the timing and mechanism of monozygotic twinning for “discussion and critical evaluation.” The aspect of the theory relevant for pro-life advocates suggests that MZ twinning actually occurs during the first cell division, within 24 hours of fertilization. When the zygote splits, instead of forming two blastomeres, two distinct zygotes are formed, each then developing independently. This would mean that twinning is part of fertilization process, not an event that occurs post-fertilization.
This new hypothesis, while a long way from being verified, would only reaffirm what all pro-lifers know to be true, namely that from the moment of fertilization, there exists a new, individual, unique human being. It would scientifically undercut any argument that a human embryo is not individual or unique, wiping away many of the arguments for the moral permissibility of destroying human embryos.
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