In What Evolution Is, Ernst Mayr floats the idea that humans are born 17 months premature. I don’t think he intends to critique humans or human development; he is simply describing a difference between humans and primates like chimpanzees. Our neurological development is not very far along at birth; baby chimps are much more capable. Maybe you’re not familiar with baby chimps, but perhaps you’ve seen (video of) a baby deer or horse or giraffe tentatively taking its first steps within hours of birth; that’s a skill humans need a year to be ready for. That’s the price we’ve paid for greater capabilities mediated by a proportionately larger brain; we have to wait longer for the full functionality and spend more time taking care of our infants.
Intentionally or not, Mayr is also reminding us that human development is an ongoing continuum with birth as one significant milestone but not any kind of meaningful end. Fundamental motor skills continue to develop in those 17 months after birth, additional cognitive milestones extend into our twenties, and the learning our large brains enable can continue throughout our lives. To borrow a theological phrase, the human experience is one of “already but not yet.” We are already human, but not yet the human we are headed towards. The human experience is intrinsically one of becoming, not merely being.
I mention all of this because the question of how we define humans and what science has to say about it has once again captured the headlines. This time around the focus is on what constitutes a heartbeat. Heart imagery and heart metaphors are deeply rooted in our language and our sense of identity. We hold the people most significant to us in our hearts. “Have a heart” is an appeal to compassion, to empathize with a fellow human. To be heartless is to be mechanical, robotic–inhuman. So understandably, we attach great significance to the emergence of the heart as a milestone of human development.
Notably, hearts can be heard before they can be seen. That is to say, the earliest sounds described by some as a heartbeat is generated by cardiac cells that are precursors of the organ we are more familiar with. You can read more about the place of those cardiac cells in human development and about the role of their sound in obstetrics practice in this Wired article or any number of other recent news items. That piece stood out to me however for how directly it addressed the root issue. Author Adam Rogers asks “whether you think a 3- to 4-millimeter-long, partially organized blob of cells is a human individual or not” and “whether you think the government or the person in whom those cells reside gets to make that determination.” It’s pretty clear how Rogers answers those questions. I think it should also be made clear that those are not purely scientific answers.
Among the remarkable feats our proportionately larger brains make possible is abstraction through language. We create categories by naming them. We choose which differences to emphasize as defining distinctions and which to smooth over as natural variation. We also choose the level of description. “Partially organized blob of cells” is one level and it is accurate as far as it goes. I am also a blob of cells, perhaps more organized but probably not fully. “Human” is another level of description, not mutually exclusive to any discussion of cells.
These abstractions can be informed by science. We can make observations and measurements related to those category-defining distinctions using the tools of science. We can track milestones in gestation and quantify how they correlate with future viability. We can assess how close or far each stage is from what we recognize visually at birth. We can even detect our own cognitive bias towards emotional attachment to anything resembling infants at birth (suggesting perhaps our relatively lack of attachment to less similar stages may not be purely objective). Those are all issues well suited to the repeatable, observational techniques of science. But all of that information does not fully constrain our options for categorizing and naming.
So I agree that we do have some freedom to decide when to apply the label of “human individual.” As a matter of history more than science I think it is best when we do not let each person make their own decisions about who is human and who isn’t. Thus we need a collective answer, no mean feat given the plurality of other potential inputs from philosophy, theology, personal values, and whatever other -ologies and -isms people bring to the table. It would be nice if science could provide an objective answer. I’ve said why I think it cannot; still, I do think there is an idea we can borrow from science, a heuristic rather than a definitive, quantifiable solution.
If developmental biology cannot fully answer the question of who is human, what about evolutionary biology? Some fear that evolution undermines our humanity, but I see it differently. Let’s return to Mayr, who reminds us that evolution takes us away from essentialism and towards population thinking. Essentialism emphasizes a set of defining characteristics that all members must possess. Framing the question “who is human?” in terms of characteristics like a heartbeat or the ability to feel pain or self-awareness is an appeal to essentialism. If we adopt population thinking, we emphasize community and relationship rather than individual characteristics. An individual is a human because other humans treat them as a member of the human group. With population thinking, we have more freedom, not less, to decide who we want to consider part of our community.