Contingency and Convergence: Toward a Cosmic Biology of Body and Mind (2020), by Russell Powell

Behind the abstract title and the stuffy academic cover of this book lies a world of deep thought and wonder. Written by one person, Rachell Powell (formerly Russell as it still says on the cover), a book like this is never actually one person’s accomplishment as if popping out of their forehead whole. Scientists with years of training behind them do their part in pushing all this knowledge forward, and make use of the combined knowledge of thousands of colleagues and decades of questions and research, all gathered together on academic search engines, waiting for the moment when a scientist decides to put in the time to write a grand review on a difficult issue. Academic books like this one are part of that work. It is part of those moments when researchers step out of the narrow holes in which they were working and look up at what the rest of them are doing, and try to gather together some of the latest insights to form new narratives. 

The title alone, very abstract, not very inviting to the casual reader, is a sign that we are dealing with some very fundamental questions. In fact, one of the most fundamental questions in evolutionary biology that has never been solved, and will likely only be solved once we discover complex life on other planets.

What is the fundamental question of this book? 

Powell put it like this: the palaeontologist Stephen Jay Gould posed that if you were to rewind the tape of life and let it play out again, that the tree of life would develop radically differently, giving shape to totally new forms of life. That would mean that the evolution of reptiles, mammals and ultimately us is incidental, the result of a throw of the dice. A “contingency.” On the other hand, we observe so-called convergent evolution, in which the same shapes and mechanisms appear in different branches of life independently, like wings and eyes and flippers. In nature, form and function are tightly connected, and if the same forms develop with similar functions in some natural experimental replication, what does that mean for the chance of complex life emerging in similar ways on other planets? What are the mechanisms behind these convergences of form and function?

The first problems to come up are that (1) the contingency thesis of “happy accidents” has not been clearly described and (2) cannot account well for episodes of convergent evolution. On the other side of the fence, critiques of the contingency thesis have not been robust. Specific compelling episodes of convergent evolution do not discard on their own the core ideas of the happy accidents thesis. If you are a scientist, this is usually the time to dive into the nitty gritty of yelling and hair-pulling (only joking) about minute details in imperfect studies. Or you could wait for the screaming to stop and knock again to ask for a preliminary conclusion.

Powell presents the arguments in a very clear, well-argued way, but ideally you need a background in biology and wikipedia at hand. She’s a philosopher, foremost. The topics that are touched upon along the way are varied and fascinating. What makes this book interesting to me is that it deliberately wants to focus on what can be expected from other habitable worlds, and what the two theses on evolution might mean for the development of complex minds. 

I’ll go over some of the arguments.

The first stumbling block is that we don’t know how hard or rare it is to form unicellular life in the universe. Bacteria are already enormously complex, and the origins of DNA are still murky, but the only way to make complex life as far as we know it is through cellular life and have cells stick together as multicellular bodies. That sounds difficult – a lot of steps to get there. Let’s first observe that when we look at the timing when life first formed on Earth, then it formed as soon as it could. When meteors still rained down with a vengeance in the early days of the planet, cellular life popped up immediately when minimal conditions were met. Quite shocking actually. That might make cellular life highly probable in other places. What’s less probable is complex or intelligent life. After all, it took maybe 4 billion years to go from simple cells to intelligent life on our planet. That’s nearly the entire span in which the Sun gives off the right amount of energy to create good conditions.

Of course, we don’t know if life follows the same laws on other planets. If genetics, evolution and natural selection work the same way. And even if complex life arises, then intelligent, technology-using life may never do so. Thus we cycle back to the contingency thesis of the course of evolution as fundamentally unpredictable about what kind of life forms will emerge in the long run.

The “happy accidents” contingency thesis

The first argument for the happy accidents thesis is that the selection of organisms we find on planet Earth is altered radically every time a mass extinction happens. Our biosphere can accommodate all sorts of variations of dominant lifeforms, and after a mass extinction, life does not return to the status quo. Why did birds or mammals not re-evolve into dinosaurs to fill up all that empty space after the meteor hit? The answer is also the second argument for the thesis: that fundamental features in an animal’s body plan, such as those that are laid down early in an embryo’s development, don’t change easily, so that body shapes don’t go out of whack all the time. This gives an inertia to evolution and a limited space in how much a lineage of organisms can change over time.

In other words, looking at the consequences of mass extinctions, then chance plays a large role in selecting which animals with which body plans will be dominant on the planet, and that selection may not be based on merit or adaptability, and once the selection has been made, basic features of those body plans stick around.

By talking about “happy accidents” and “rewinding the tape of life” we are in danger from arguments from determinism. If the universe is deterministic, every play of the tape of life will give the same outcome. The contingency thesis is about something else. It wants to say something about the mode of evolutionary history, about how sensitive its history is to chance effects. For that, we ideally need to compare different planets with different trees of life to see if this sensitivity is the same everywhere. Determinism is another matter. The thesis also does not mean that there cannot be “laws of nature”. Even if you scramble up all the initial conditions that give rise to lineages of organisms with certain body plans, then within those lineages and within those ecosystems there can be “laws” about forms and functions and species radiation and so on. But we can’t make these laws applicable to all of biology and every possible history of life. 

Strong convergent evolution

This is the counter argument. Some functional forms, like eyes, like flippers, like wings, have arisen multiple times in natural history in lineages whose common ancestors are situated so far back in evolutionary history that these forms cannot be inherited, but evolved independently from one another. We may have one planet and one tree of life, but these examples can be considered as “replays” of the tape, giving rise to similar shapes. But it is not easy to marshal this as strong evidence against the contingency thesis. It is in fact very difficult. Philosophically, we can’t even narrow down what are traits and what kind of natural laws you could pull from this.

In fact, if you take the fact of developmental constraints in an animal’s body plan, which is one of the core arguments of the contingency thesis, then you would expect a lot of convergence among animals, because it restricts the morphological choices that are available for adaptation. Animals that are closely related arrive at the same solutions again and again, all within a restricted frame of possibilities. So even in the happy accidents thesis there is room for “weak” convergent evolution. The case for “strong” convergent evolution is at its strongest if you have very complex traits that converge to the point that they are very similar, but from very different starting points, meaning from ancestors that are very far removed from each other in the tree of life.

Powell gives an example. There are a couple of families of cat species in history that developed saber-teeth. But there is also a marsupial (ancestor to many Australian animals) that was saber-toothed. Clearly, to have saber-teeth is some kind of feeding adaptation that is helpful in some way, and not exclusively reserved for cats. Is the fact that a marsupial developed that same trait an example of strong convergent evolution? Hard to say. Powell’s greatest contribution to the debate is that she points out that cases of strong and weak convergent evolution are all lumped together because researchers only look at taxonomic information and not at the developmental constraints within lineages, and therefore their argument does not refute the contingency thesis.

What Powell does next is to draw up a framework for how we could identify strong and promising cases of convergent evolution, what conditions they have to meet to figure as evidence, to say anything about cosmic laws of form and function. But this remains a first attempt. 

The Rest of the Book

What the two theses mean for the shape of alien life is basically that the contingency thesis agrees that there could be similarities in shape amongst alien life in very general terms. Such as, creatures that swim in water are likely to have spindle-like shapes. But the convergence thesis hold that there could be very specific similarities, and those would be much more interesting, scientifically. 

Here is a most exciting part: Russell argues, quite thoroughly, that the development of “seeing” is a true example of convergent evolution that might hold in deep replays of life. And the development of “seeing”, whether with eyes, echolocation or electrolocation, is intimately linked to the development of minds. The book concludes that even if evolution on a cosmic scale would be radically contingent, then that would not undermine the evolution of cognition, of minds. What remains unknown, however, is how rare the emergence of cumulative culture and cumulative technology would be. The universe could be filled with creatures with minds, yet could display little technological progress.