The split-body problem


From Monsters and Marvels by Ambroise Paré. IMAGE/the Wellcome Collection

If you split yourself down the middle to become two people, would you survive the process? And, if you did, would your other half be your child, your clone or your sibling? Would this create two instances of the same you, existing simultaneously in two places at the same time; or would it create two entirely new people, causing you to suddenly cease to exist? While such thought experiments raise baffling questions about personal identity, there is a more fundamental problem I want to consider: would splitting in two be an instance of reproduction or an entirely different kind of process?

When we think about how organisms reproduce, we don’t tend to think of splitting bodies. We think of sex. We tend to think of animals such as panda bears, leopards, ravens or any other large multicellular organisms having sex, becoming pregnant (or laying fertilised eggs), and giving birth. It isn’t surprising that this is how we think new organisms come into existence. Sexual reproduction is, after all, the form of reproduction that nature has selected for creatures like us. But sex is not the way most reproduction takes places.

Most forms of life on this planet create other living beings through asexual processes – and there are many ways this can happen (as we’ll see). Some of the most common forms are similar to the thought experiment above: a body splits in two. Nearly all prokaryotic microbes, such as bacteria, reproduce through various forms of this process, such as binary fission (when a body separates into two new bodies). However, it’s not always clear what kinds of relation result from fission, as in the thought experiment above.

As a philosopher of biology, some of my research considers what counts as a biological individual, and how biological individuals are connected to each other. It seems to me that our concept of reproduction often distorts how we think about organisms coming into existence. This isn’t surprising: it’s a concept developed by 18th-century naturalists who knew very little about the fission processes we can observe today. What needs to be done to align our concept of biological reproduction with the fission processes seen throughout so much of life? Where do we start?

To think about this, we first need to figure out how we understand reproduction itself. This may seem like a theoretical exercise, but a lot is at stake in how we define the concept. Our ability to identify species, aliens and even the branches on the tree of life hinges on the concept.

Composed of reproductive links between organisms, much of the tree of life rests on our understanding of reproduction. How we think about vertical lines of descent (between parents and offspring across generations) can have massive implications for our evolutionary models. Say an organism survives reproductive fission and has 40 offspring. All those offspring – 40 bodies that split off over time – are one generation away from their parent. However, if you think an organism doesn’t survive fission, then the organisms born after 40 splits are 40 generations removed from the first organism to break into two. Thus, whether two organisms are understood as being just a few generations apart or (potentially) millions of generations apart hinges on how you understand reproductive fission.

Astrobiologists are also heavily invested in how reproduction is defined. If we’re ever going to find life outside Earth, they will need to know how to tell the living from the non-living. One of the more prominent theories about how to do this is by determining whether an extraterrestrial candidate can reproduce or pass on its genes. According to this theory, a capacity to reproduce could be one of the defining characteristics of life. Reproduction is also central to the way we define species. The German-born evolutionary biologist Ernst Mayr ’s biological species concept – the most common concept used to define a species – treats them as interbreeding (or potentially interbreeding) populations. Updating how we think about reproduction, then, creates a series of difficult identity problems that challenge the foundations of many of our ideas about life.

It’s perhaps surprising then that the concept of biological reproduction, which underpins our ability to define species, the search for alien life and the concept of evolution, emerged only around the mid-18th century. The work of the French naturalist Georges-Louis Leclerc, Comte de Buffon and the French philosopher Pierre-Louis Maupertuis marks the beginning of the idea that we think of as ‘reproduction’ today. Prior to their work, organisms were believed to form through generation, not reproduction. The French biologist François Jacob’s The Logic of Life (1970) nicely illustrates the importance of the historical shift between these concepts.

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