video
Episode
What is Evolutionary Developmental Biology?
Two big ideas in biology: the evolution of species via mutation, fitness and natural selection; and the embryological development of individuals, from fertilized egg to whole organisms. How do these two big ideas—“evo-devo,” as it’s called—relate? What novel ideas emerge?
Contributors
- Robert Lawrence Kuhn
- What is life, its essence, and nature, its role in reality? Is life contingent, I’ve wondered? A bizarre accident, in our particular universe of chance and time? Or is life deeply fundamental, perhaps inevitable, even a kind of necessity? How to discern life, at least life on Earth? Classically, I see two big ideas. The first is evolution, mutation-producing variation. Fitness yielding natural section. The second is development. The prenatal transformation from conception to birth. The stunning embryology from fertilized egg to whole organism. Recently, I’m watching the two big ideas – evolution and development – joining forces to form a remarkable new field. It’s called, “Evolutionary Developmental Biology” or, informally, “Evo-Devo”. Can Evo-Devo elucidate evolution? What is Evolutionary Developmental Biology? I’m Robert Lawrence Kuhn and Closer to Truth is my journey to find out. Evolution is the foundation of life as we know it. And the claim, I hear, is that Evolutionary Developmental Biology, Evo-Devo, provides fresh insights into what evolution is and how evolution works. How can what happens in prenatal and postnatal periods, when organisms form, grow, and mature, enhance understanding of the vast history of life on Earth? Can what happens in months help reveal what happened over billions of years? What are the principles of Evo-Devo? Its evidence? Its applications? To get the scope of Evo-Devo, a first sense of the field, I begin with a Philosopher of Biology. Who focuses on naturalistic and reductionist explanations, Alex Rosenberg. Alex, within evolutionary theory, evolutionary developmental biology has been an increasingly important way of thinking. I’d like to understand that the process by which this has become important, and then follow its development.
- Alex Rosenberg
- So, Developmental biology is the study of the way in which the organism emerges from the fusion of the ovum and the sperm, through the embryo into the infant creature, whether it’s drosophila or humans, okay. And for literally thousands of years, there was nothing in developmental biology but the taxonomy of the various species, and the stages of development, as observed by the scientist in the growth of an organism from one cell to multi-cellular. And only with the advent of molecular biology, and the reductionist successes associated with molecular biology, were we able to convert that entire discipline into one which substituted real explanation for a mere description of stages. So, now we’re at a point where we need to understand how the details of the development within lineages in generations of traits get selected for across generations, in lineages.
And, in so far as the developing embryo lays down the fundamental structure of the organism which then gets worked on by environmental filtration doing the selection, okay, we have to incorporate the developmental scenario in the detailed mechanism of evolution, in order to see how, in this huge design space, within which natural selection operates, it takes the particular trajectory that is taken. - Robert Lawrence Kuhn
- So, what is the developmental part of this?
- Alex Rosenberg
- So, evolution is going to have to work on traits, okay? And the traits, if we’re talking about biological evolution are going to be the hardwired traits, the traits that are written by – in the genome, okay? But the remarkable fact about the biological domain is the incredible diversity and complexity of the metazoan organisms that characterize our planet, of which we are among the most complicated examples, okay, and the material that evolution is going to have to work on to produce this incredible diversity and complexity, that material has got to somehow be written in the developmental program of the individual organism. And that makes developmental biology, once we understand it as a mechanical process, indispensable to understanding the trajectory of evolution.
- Robert Lawrence Kuhn
- Now, let’s go to what’s happening in the field of so-called Evo-Devo, evolutionary development.
- Alex Rosenberg
- So, one of the traits that’s important to the evolution of organisms on our planet is something called evolvability, that is to say, what looks like the foresighted character of a property of an organism that enables its descendants to respond differentially to a variety of different environments, some of which may be inimical to survival, and require the presence of a mechanism that encourages or drives variation because it’s only by variation that lineages can survive in these hostile environments, by producing a solution to a design problem that didn’t exist when the evolution of this particular lineage began. And the mystery that Evo-Devo has to solve is how could you package it into the instruction manual for the construction of an organism. This idea of evolvability must ultimately be cashed in by something that we – that we can understand about the nature of the developmental program of organisms.
- Robert Lawrence Kuhn
- Developmental Biology, Embryology, is the study, primarily, of prenatal processes. And Evolutionary Developmental Biology, Evo-Devo, is the application of Developmental Biology to discern mechanisms of evolution. Is Evo-Devo up for the challenge? I ask a Philosopher of Biology who specializes in Evo-Devo, Alan Love.
- Alan Love
- I think there’s several reasons why evo-devo has risen to prominence in the past several decades. Most importantly among those was a discovery in the early 1980s of a particular set of genes called the homeobox genes. And when those were discovered, it was surprising that they were conserved across wide distances of evolution and kind of triggered an interest in examining development and its potential role in evolution. Additionally, there was a number of researchers in the late 1970s who had become dissatisfied with some of the current theoretical orientation of evolutionary biology that also thought development broadly construed was important. So, a combination of these two things kind of signaled the beginnings of a ground swell. It’s important to recognize that there’s these two different dimensions to evo-devo. There is the evolution of development where researchers are simply trying to understand how do developmental processes change over evolutionary time. The other dimension of evo-devo is how does the developmental sort of structure of a living organism potentially have an impact on the evolutionary process. This is where things get more controversial because many people might be willing to acknowledge that there’s interesting questions about the evolution of development but might be more resistant to whether or not development has to change or somehow provokes us to rethink how evolution works.
- Robert Lawrence Kuhn
- It’s fascinating because it’s really reversing the arrow of causation in a sense. The first development is sort of a – a passive observation. And then, trying to infer what evolution is from that development. And the other is saying that development has an active process in the process of evolution. Now, as a philosopher, how do you interact with the biology of evo-devo? What does a philosopher bring to that, that a biologist does not?
- Alan Love
- So, a philosopher’s interest in evo-devo could be at many different levels. Some of the levels that were of interest to me had to do with the fact that it was challenging some of the standard ways of thinking. And there’s an interest in trying to understand why do people think that this challenges the standard way of thinking.
- Robert Lawrence Kuhn
- What was the current way of thinking and how does this challenge us?
- Alan Love
- So, the current way of thinking in the – especially in the 1950s and 1960s, primarily concentrated on the sort of population genetic orientation to evolution, which of course, is important but did not at all attend to how are those changes in say, allele frequencies and population making a difference in the variation we see at the anatomical level or at the organismal sort of level.
- Robert Lawrence Kuhn
- I remember the first lecture I had in gross anatomy in medical school at Johns Hopkins, where this phrase was imprinted in my mind, ontogeny recapitulates phylogeny. I mean, three words, and you have this gigantic area.
- Alan Love
- It originally goes back to Ernst Haeckel, a German biologist in the 19th century. The basic idea that he had was that the developmental process was somehow a –
- Robert Lawrence Kuhn
- Ontogeny.
- Alan Love
- Ontogeny – a recapitulation of what you saw in evolutionary history, phylogeny. And he thought then that allowed you to make certain inferences that might otherwise be tricky given that these evolutionary processes happened way in the past. The problem was that it was a bit too neat for its own good in terms of what it allowed. And people have once again thought, well, we don’t have to be committed to ontogeny recapitulating phylogeny in order to think that development might be relevant to evolution.
- Robert Lawrence Kuhn
- What are some examples? Simulate ways that ontogeny can, in development, can influence evolution and then, have a material proactive part in the – in making evolution happen.
- Alan Love
- Mm-hmm… digit patterns in the evolution of amphibians. And it turns out that when you compare frogs and salamanders, you end up with different ways of thinking of the development of the fifth and the first digit. And in the one, the first digit develops last and in the other, the fifth digit develops last. And as you look at the evolution of those two lineages, it turns out that one of them loses one digit, the other loses the other digit, and it’s the digit that corresponds to how it develops last. So, if you didn’t understand the development of the digits, you wouldn’t understand why the evolution –
- Robert Lawrence Kuhn
- That is very powerful because that’s a clear demonstration of the impact of – of development on evolution. So, what’s the future of the field?
- Alan Love
- The future of the field is whether or not more genetic detail in terms of genetic networks and other sorts of analyses is going to sort of give us everything we want or whether we really are going to have to look more at these additional levels of organization, cellular interactions, tissue interactions and the like. These are much harder to study but may be ultimately crucial to understanding how these developmental processes work.
- Robert Lawrence Kuhn
- Alan explains Evo-Devo’s two explanatory dimensions. First, The Evolution of Embryological Development. How embryology itself changes over evolutionary time. Second, the embryological developmental basis of evolutionary change. How embryology affects and drives evolution. I like how both dimensions endow Evo-Demo with philosophical potency, that extends and enriches evolutionary concepts and theories. In ways that the pure science of Evolutionary Biology cannot. So, can Evo-Devo’s ways of philosophical thinking be extended? I meet a philosopher of biology who interrelates diverse areas of biology. From human evolution to medicine, bioethics, and gender/sexuality, Rachell Powell.
- Rachell Powell
- The paleontologist, Stephen Jay Gould famously says, you rewind the tape of life to the time when the first animals evolved, and you let it play again, what would you get? And he argued that you would get some radically different set of forms. And you know, why would that be? Well, the idea is like well, just some really quirky differences would result in some very big differences in the outcomes. If it were the case that you know, you re-ran the tape of life and there was a different set of survivorship, why, in the intervening 500 million years, wouldn’t life just gravitate back toward the ideal states, right? The optimal outcomes? And the answer is that they become locked in place by developmental constraints.
- Robert Lawrence Kuhn
- Hmm.
- Rachell Powell
- And so, that’s why it’s critical for understanding the large-scale patterns of life on earth, that these kinds of contingent events get essentially frozen in place through developmental constraints. And then, that’s going to affect subsequent evolution tremendously.
- Robert Lawrence Kuhn
- So, when you talk about a constraint in developmental biology, what are we talking about? You obviously have the constraint of the genetic structure and its mutations.
- Rachell Powell
- Right.
- Robert Lawrence Kuhn
- You have the constraint of the physical environment in which the fetus develops.
- Rachell Powell
- Right. So, in the early stages of animal evolution, you had this, like, parade of bizarre forms, right? And the problem was that in the early stages of animal evolution, you had this great pliability because the developmental process, in this cascade of development that starts with the embryo, and then cascades through differentiation to these more fine-grained organs, and organ systems, and traits, hadn’t been fully laid down yet. And once they are, trying to change the fundamental framework that you’re dealing with, would require changing the early-acting genes in development. And that almost always is going to have catastrophic consequences for the phenotype. So, as a result, once you sort of connected up these so-called upstream components of the developmental process, with all of these downstream batteries of genes that guide differentiation of organs, and organ systems, and so forth, that once you have that, it becomes essentially impossible for selection to then go back and start from the beginning. And so, what you have is a bunch of extinction events that then just whittled away what, you know, the existing breadth of animal forms, and the ones that are remaining, are forever sort of confined to the developmental constraints of their own body plan. Some people have argued that convergent evolution, the independent origination of similar forms and functions is an argument that, in fact, if you rewind the tape of life and play it again, you’re going to get similar –
- Robert Lawrence Kuhn
- It will look similar.
- Rachell Powell
- Similar outcomes, right? But in fact, you know, if you look at this big data set of convergence, many of those convergences look like they are the result of shared developmental constraints that are essentially channeling the adaptive evolution of form in particular directions.
- Robert Lawrence Kuhn
- Right, so it really reinforces the radical contingency.
- Rachell Powell
- It does and it shows you that…
- Robert Lawrence Kuhn
- Rachell recruits Evo-Devo to challenge the “converging argument”, that evolution is not random but progresses toward pre-set or privileged structures. She opposes this “converging argument” which is used to undermine the radical contingency of evolution. Run the “Movie” of Evolution again, and you see a completely different “Film”. The view espoused by Stephen J. Gould, which Rachell defends. I love the debate – does evolution converge or not? Because it cuts to the core of life in our universe. Here’s Rachell’s argument. It’s not, she contends, that evolutionary traits converge, it’s that embryological development locks in traits so that they seem to converge. I’m not sure this argument succeeds. But, I am sure that such innovative thinking confirms the potential explanatory power of Evo-Devo. Are there other problems in evolution that Evo-Devo might address? I meet a neuro-anthropologist who explores the evolution of human cognition via developmental mechanisms that distinguish human brains from other mammalian brains, Terrence Deacon.
- Terrence Deacon
- So, if you want to explain something in evolution, you have to look at how the actual physiology of it develops. And interestingly in the brain, it turns out that there’s an almost sort of like a micro-evolutionary logic to it. Development is really different in the brain, in part, because cells in one part of the brain can’t just interact with cells next to them. They have to make connection with cells somewhere different. It’s the circuitry that matters, but the circuitry isn’t there to start with. The circuitry has to develop over time. And that means that there’s going to be this change in the way brains work over time. And to produce a new kind of brain requires that the developmental process has to be tweaked in some way. So, I became very interested in something that was obvious about human evolution, and that is brains are bigger, with respect to their bodies. But what’s interesting is that that’s also the case for monkeys and apes compared with most other mammals. It’s not because brains are growing faster. It turns out that when we looked across many, many species of brains and how they develop, what we found out is that brains in all mammals, they grow their brains at the same pace. And that means it’s not because brains are growing faster, or bigger. What actually happens is that the post-cranial body from the neck on down, actually is growing slower in primates. And that’s true for us too. So, what looks like we have bigger brains for our bodies, it’s because we have smaller bodies. It’s like being a Chihuahua compared to a Great Dane, for example. Now, it turns out that the human story is more complicated than that. We’re like other primates, in the sense of growing our bodies a little bit slower. But then, we continue to grow our brains longer. So, that they actually, in fact, grow beyond what you find in a typical primate. It turns out that unlike a Chihuahua, when its body growth is slowed down later in life, after birth, and therefore it looks like it has a big brain because it has a small body, in primates, and in us, that starts right at the beginning of gestation. Bodies are growing slower from the very start. The result is since brains have to be connected to the body, the body has to be connected to the brain, those connections and the competition for connectivity is changing right from the beginning. That means that primate brains and human brains are going to have a different connectivity pattern because of this change in proportions. And human brains are going to be different from other primates because we continue this longer. So, what we see in humans, is when we’re born, we’re very, very immature neurologically. And in fact, in comparison to most other primates, we should be born at about one year of age.
Where instead, our brains because they’re just continuing, staying immature longer, and growing longer at the typical fetal pace of brain growth, what we see is the brains continue to grow, into a couple of years of life, where they mostly slow down in other species. But as a result, the connectivity has changed radically because of this. And so, this is how development has influenced evolution, and how evolution has tweaked development, to develop these different kinds of connectivity patterns that make us unique and make primates unique from other species. - Robert Lawrence Kuhn
- About how long does brain development occur in a child?
- Terrence Deacon
- Brain growth begins to slow after about three years. And one of the major changes is our memory system doesn’t actually mature completely until we’re about five to seven years of age. The result is something that Sigmund Freud noticed that most of us don’t remember much of what happened before we’re five years of age. He thought it was because we had these repressed memories, it’s now apparent that the part of the brain critically involved in consolidating those kinds of memories, it’s not quite mature yet, that structure called the hippocampus. Slow to mature compared to many other systems. And so, probably not until about age five or seven, are we actually able to lay down these kinds of memories that we can actually make use of. Even though we’ve been remembering all kinds of skills before that. We learned our language well before that, really well-established, but the memory for those details, those episodes in our life are just inaccessible.
- Robert Lawrence Kuhn
- Yeah, and this shows a remarkable example of how the developmental process affects evolution because that process would then make certain individuals fitter to reproduce in the future.
- Terrence Deacon
- Right, so what this amounts to is that I think that, in our case, being able to communicate about things that are not present, might happen, could happen, shouldn’t happen, was part of what drove this change. Because those individuals were better at doing so, better at acquiring symbolic communication about this, and therefore better at organizing their behavior and cooperating. How a particular way of development was what was being selected for because it had this consequence.
- Robert Lawrence Kuhn
- Evolution explains life. And to explain evolution, in addition to all the classic mechanisms, from variations via mutations to molecular genetics of natural selection, Evolutionary Developmental Biology, Evo-Devo, makes a fresh contribution. Evo-Devo, applies Development Biology to discern the mechanisms of evolution. Evo-Devo works in two explanatory modes, how embryological development changes over evolutionary time. And how embryological development elucidates evolutionary mechanisms. Evo-Devo accounts for evolutionary contingency by explaining similarities among organisms via constraints of embryological development not “Converging Evolution”. The evolution of human cognition and indeed human uniqueness itself, depends directly on brain development, pre-, and postnatal. With Evo-Devo, I see evolution in a new light, revealing hidden mechanisms and processes. This enriches my understanding of life in all its grand diversity. For example, to me, figuring out how our human cognitive capacities developed is an essential step, in getting…. Closer To Truth. For complete interviews and for further information, please visit closertotruth.com