How to count organisms, and why it matters – a blog post by William H. Morgan

In this post, our centre member William H. Morgan, who has recently finished his PhD entitled “Individuals in Metaphysics and the Philosophy of Biology”, talks about organisms, what they are, how to count them, and why it matters. You can read more on this topic in his recent paper Are Organisms Substances or Processes, published in the Australasian Journal of Philosophy.

Counting and distinguishing organisms often seems easy. Most of us would have no trouble counting the number of animals in the zoo, or the number of people on the bus. Toddlers can do these things too. When teaching toddlers how to count, we even take organisms to be paradigmatic examples of countable things (you probably had picture books as a child asking you to count the number of farm animals on a page). You would be forgiven then for thinking that the question of how to count organisms is pointless for educated adults to engage with.

But what about slime mold? These single-celled creatures tend to live most of their lives independently. When food becomes scarce however, they merge with each other to form a slug shape which can coordinate its movements and even complete a maze. Should we count these genetically diverse worms as organisms? Or is the slug really just a colony of distinct but interacting organisms?

What about a grove of aspen trees? When considered from above ground, each tree in the forest looks like an organism in its own right. Underground however, the trees are interconnected by an intricate root system. The trees are also genetically the same. Should we count the trees as distinct organisms in their own right, or are they actually just parts of one giant organism, like a head of broccoli?. 

Perhaps human beings are actually more like a colony of organisms too. Whilst we don’t usually think of ourselves as being made of lots of tiny organisms, we are each hosts to a vast number of microorganisms such as bacteria, viruses, and fungi. These drastically outnumber our own ‘human’ cells, with it being estimated that as many as ninety percent of the cells in and on us are microbial. Many of these microbes don’t just live in us, but, like our other bodily cells, contribute to our vital biological activities such as digestion, the immune system, the nervous system, and the metabolism. Does this mean that we actually have millions of microbes as parts? Or does it mean that what we refer to as ‘we’ is really just millions of cells rather than to a single organism?

 Furthermore, a fetus is tightly integrated with its mother’s body. Pregnancy requires drastic physiological changes to the mother such as the remodelling of blood vessels, and it is far from clear where the mother’s body ends and the foetus’ starts. The placenta is made up of both fetal and maternal parts with no clear boundary between them, and allows oxygen, nutrients and immune cells to flow from mother to foetus and foetal waste products to be expelled by the mother. This dependence continues sometime after pregnancy, often through breast feeding which provides babies with nutrients, enhances their immune system whilst their own is still immature, and develops their healthy gut bacteria. Given this, should we count a foetus, and even the later baby, as parts of their mother?*

How to count and distinguish organisms matters because it raises a range of other questions. For example: if our microbes are parts of us, are human beings much more genetically diverse than we ordinary take them to be? Does it mean that most of our DNA was not inherited by our parents but from millions of microbes? Does that mean that we each have millions of parents, not two? Should we conclude that we are more microbial and human, and perhaps not even human beings at all? If a foetus is a part of its mother does this mean that many human beings (pregnant ones) have four hands and legs and two heads? Was there a time when we were related to our mothers like their kidneys relate to them? If heart surgery is human being surgery, then is foetal surgery actually maternal surgery too? And does it involve an intrapersonal trade off, rather than the interests of two distinct beings? What does this mean for ethical issues such as abortion?

So how should we go about counting organisms? There are two main approaches favoured by philosophers of biology. According to one approach, we should count organisms by appealing to functional unity. This approach says that an organism is something whose parts work together as a single cooperative system, and that there are two organisms when there are two distinct functional units. How we understand ‘work together’ is up for debate, but some philosophers understand it in terms of metabolic interaction – an organism is something whose parts are involved in the constant exchange of matter and energy – or we may understand it in terms of the immune system – an organism is something whose parts are accepted by the same immune system. This approach seems to tell us that a foetus is a part of its mother, given that it undergoes metabolic interactions with her (such as the use of her nutrients), and because a foetus (in most cases) is tolerated by its mother’s immune system. For similar reasons it tells us that a human organism, along with many of their bacteria, count as a single biological unit. 

According to the second approach, we should count organisms by appealing to evolution by natural selection. This approach says that an organism is something which has the capacity to respond to evolution by natural selection. Using the jargon, we might say that according to this approach, an organism is a unit of selection. The evolutionary approach says that many of our symbiotic bacteria do not count as parts of us. This is because my bacteria and I have different evolutionary fates. There is no guarantee that a human being will pass on their bacteria to their offspring, and my evolutionary lineage could entirely cease to exist whilst the lineage of the bacteria inside of me could continue to thrive inside another host. From an evolutionary perspective then, our bacteria may well be useful to us, but they are not parts of us. It’s not entirely clear what the evolutionary approach says about the mother-foetus relationship, but it may seem that a foetus and its mother are distinct evolutionary units. A foetus, for example, has its own mechanisms for participating in evolution by natural selection such as having its own sex organs and it is genetically distinct from its mother (by half). 

Which account should we adopt? The functional approach does a good job at capturing how we ordinarily think about organisms: as things whose parts work together to preserve life. I imagine, though, that many people would find it surprising to learn that they are partly made up of millions of bacteria, or that their baby is a part of them. The functional approach may also have some troubling consequences:  what does it say about people with Severe Combined Immunodeficiency, who pretty much lack any sort of immune system? Despite looking like human beings, does the functional approach say that they don’t count as organisms? Surely we don’t want to say that! The evolutionary approach is appealing because evolution by natural selection provides the best scientific theory of how multicellular organisms came to exist. Again though, it has some worries. What, for example, does it say about sterile living things like mules, or even some human beings? Since they cannot reproduce, we might question whether they have the capacity to respond to evolution by natural selection. Do we really, though, want to say that they are not organisms?

The question of how to count organisms, therefore, is not just a question that should appear in toddler books. On the contrary, it is a question that has puzzled professional biologists and philosophers, and raises ethical and personal questions too.

* A lot of work on this question has been done by Elselijn Kingma, see for instance “Were You a Part of Your Mother?“, Mind, Vol. 128, Iss. 511, 2019, pp. 609–646.