The twins objection argues that because an early embryo can split into twins, it must not have been a single individual to begin with. The response is that this conclusion doesn’t follow from the biology.
The possibility of later division does not negate earlier individuality.
In nature, a single organism can sometimes give rise to another organism without losing the fact that it was already an individual beforehand. Flatworms, for example, can divide and form a second flatworm, yet no one claims the original flatworm was not an organism before the split. Likewise, cloning an adult animal does not imply the pre-cloned animal was anything other than a biologically distinct individual.
The same logic applies to human embryos. When an early embryo divides and twins arise, that division is a change that happens to an already existing organism. It does not retroactively erase the fact that there was a single, living, biologically unified human organism prior to the split. Biology is still capable of identifying when an individual human organism begins: when fertilization is complete and a single living member of the species Homo sapiens exists. Questions about personhood or legal rights may require additional arguments, but the twins objection does not undermine the biological claim about when an individual human organism begins.
Key Takeaways
The ability of an embryo to split does not mean it was never an individual; division presupposes something already exists to divide.
Other organisms can reproduce by splitting or cloning without losing their prior individuality, showing the twins objection is not unique or decisive.
Fertilization still marks the beginning of a single, unified human organism, even if later biological changes occur.
The twins objection fails to refute the claim that embryos are individual biological humans and therefore does not weaken the core pro-life biological argument.