Competition and cooperation are both ubiquitous in biology

the body as a battleground

Cells from Outer Space Aliens Invading the Planet Brain by Valérie Watters. Source

Competition and cooperation are both ubiquitous in biology. I don’t just mean at the level of organisms – we’re all familiar with the ways that organisms compete with each other (the “Darwinian struggle for survival” where only the fittest survive), and also with the ways they cooperate (ants form colonies, birds form flocks, fish form schools). What was surprising to me is that competition and cooperation occur within individual organisms as well.

Cooperation within the organism is pretty easy to see. The whole body can be viewed as a chaotic jumble of overlapping, cooperative processes. The cells of your nervous system are constantly in communication, computing transformations of sensory input and inner thought that are still far beyond our understanding. The cells of a developing embryo morph from generalized stem cells into specialized tissues depending on where exactly in the body they are. And your immune system coordinates to identify the shape of foreign pathogens and then unleash an army of fighters seeking out that pathogen, to protect your body against infection.

And yet, your body is also a battleground for internal competition. In most regions of the brain, half or more of the neurons that are born die out, suffocated by the competition for limited resources. In the embryo, between days 6 and 7 of development, a full 35% of cells are quickly eliminated, leaving only the most vigorous cells to form the body. And in the thymus—one of the core organs of the immune system—younger stem cells are constantly outcompeting and destroying older stem cells, in a process that’s crucial for the prevention of cancer.

Since the publication of Darwin’s Origin of Species in 1859, which demonstrated that the driving force of evolution is competition for resources among organisms with varying genes, biologists have wondered whether a similar kind of competition takes place at the level of cells, tissues, and organs. In the past few decades, a wealth of evidence has uncovered that it does: we’ve identified competitive processes in tissues across the body, and in organisms across the animal kingdom.¹

Just as humans can determine each other’s “fitness” by evaluating their physical characteristics (to make decisions, say, about who to mate and who to fight), cells use signaling mechanisms to determine the fitness of their neighbors. One of these mechanisms is a protein called Flower, which cells display on their surface, and different variants of the protein (the “losing” variant and the “winning” variant) communicate the cell’s internal state to other cells. When a “winning” cell is next to a “losing” cell, the winner actively triggers the self-destruction of the losing cell, through apoptosis, or programmed cell death. This process underlies a number of crucial developmental and homeostatic functions in the body—like choosing the most vigorous cells for the development of skin tissue, culling stem cells that don’t differentiate correctly, and eliminating cells that have incorrect numbers of chromosomes. Crucially, the signaling proteins are necessary for this elimination to take place: if you remove the signaling protein, the “less fit” cells are not eliminated—they persist, despite being less functional than the fit cells. In other words, it’s not just a blind competition for resources: there are precise mechanisms that determine the winners and losers and that target the losers for destruction. This signaling process seems to be conserved throughout the animal kingdom, whether you’re looking at flies, mice, or humans.

We might ask whether these processes are really “competition” or just more examples of “cooperation,” and it depends partly on the perspective you take. On one level, cells really are competing with each other, because as individuals they have objectives that are in direct opposition.² To see this, consider two neurons both attempting to form a connection with a muscle fiber. Usually what happens is that one neuron “wins” and covers the muscle fiber while the other neuron “retreats.” But how do we know the neurons are “competing”, versus collaborating as a pair to “choose” one of them to take over? Well, it turns out that if you injure the neuron that has won, the losing neuron—which had previously retreated—actually comes back and takes its place. The losing neuron was not “retreating on its own accord”—it was genuinely “competing” with the other neuron and capable of taking its place as soon as it had the chance to.

One cell (yellow) was taking over a neuromuscular junction, but it was then injured with laser microsurgery. Then within 24hrs, the other cell (green) reclaimed the same junction. Source

At the same time, individual neurons, and all other cells in the body, can be viewed as smaller agents within a larger system, and that system is itself an agent (you). From the perspective of that agent, all of this competition is profoundly helpful, it is keeping the system healthy and alive. The competition follows specific rules and has specific purposes, making it a kind of cooperation. Despite the fact that there are winners and losers (cells that survive, and cells that die), the system as a whole is better off for it.

Of course, competition is not always productive for the collective. These same competitive dynamics that are crucial for healthy functioning can be hijacked by cancer cells. Cancer cells have been shown to, among other things, disrupt the ordinary signaling that determines winners and losers in cell competition. Understanding the dynamics of these competitive processes could help lead to new approaches in cancer therapy. We’ve found, for example, that competition within an individual tumor (between different classes of cells) can stop the tumor’s growth; and that often, the growth of a tumor is impeded by other hyper-competitive (but not cancerous) cells that surround and smother it. More research still needs to be done, but among the potential therapeutic approaches are ideas to boost the fitness of healthy cells around developing tumors, or to introduce “more fit” cells through transplantation.

Zooming out beyond the dynamics of individual cells, some biologists speculate that competition between entire organs and limbs also plays a functional role, specifically in the development of high-level structure. Experiments on butterflies have shown that if you remove the butterfly’s hindwings in early development, its forewings grow to be substantially larger, suggesting that the particular size of the wings is determined by competition for limited resources during growth. Similar results have been found in beetles and fruit flies, and there is (as yet contentious) speculation that competition regulates the size of mammalian embryos. And a recent computational study suggests that evolution actually opts to employ a finite resource for cell processes—even when it has the option of employing an infinite resource for the same processes—because the finiteness of the resource offers a coordination mechanism for the whole body. The fact that a resource is finite enables it to form a “global scratchpad” for the body: relative levels of the molecule give you information that you wouldn’t have if the molecule was abundant everywhere.

There is still much to discover about both the detailed mechanics of competition between cells, and the higher-level functions that this competition might serve. But what is abundantly clear is that competition, even within the body, is not always a bad thing: it can just as often be constructive as it is destructive. This is the case in every multi-agent system, be it an organism or a society—competition is sometimes beneficial for the collective (e.g. sports competitions pushing the boundaries of athletic achievement) and sometimes detrimental (e.g. wars leading to irrevocable death and destruction). It’s the finely-tuned interplay of competition and cooperation, across scales from cells to bodies, that makes life work.

---

Thanks to Susie and James for feedback on earlier drafts.

1. “Cell competition” is a technical term referring to cells’ tendency to eliminate other cells that have lower “fitness.” It was originally discovered in fruit flies in 1975, and in later decades it was found in mice, and most recently it has been shown to take place in human cells too.

2. This whole post takes for granted the view that cells can be conceptualized as “agents.” There is an ongoing debate about this in biology (see e.g. the introduction to Evolution on Purpose), but I think this view deserves at least some serious consideration. When I use terms like “attempting”, “competing”, “purpose”, or “objective” as applied to cells, it should be obvious that they don’t mean exactly what they mean for humans. (I’m not, for example, ascribing any conscious feelings to the cells.) I use these words because I’m convinced that viewing cells as agents with objectives (even if very primitive ones) often leads to a better understanding of them and more interesting lines of research, as argued e.g. in Levin and Dennett. We should always be careful of taking this view too far, of course, and I plan to do a more thorough and critical take on the “cells as agents” paradigm in a future post.