These seem like two very disparate ideas: the embryonic development of a single specimen and the evolution of a whole species. How did they get connected?
At first paleontologists were studying evolution on vast timescales through fossils. Then geneticists came on the scene, and they were studying small-scale differences within species based on mutations in genes. What has been called the modern synthesis of the two fields emerged in the 1940s with the idea that the sorts of genetic differences you could observe in populations, right out your window, when compounded and extrapolated over vast periods of time, could account for the large-scale changes we see in the fossil record. So the modern synthesis was a harmonization of those two scales.
But the modern synthesis did not explain evolution in full. It was still just a theory. Where was the empirical evidence? Darwin’s theory of descent was a black box. You could not see exactly what kinds of changes were taking place to account for the differences in forms. But the study of embryonic development has allowed us to peer into the machinery of making these creatures. We can study their DNA text and their developing embryos and ask, where do the differences arise? That gave us the empirical data for the theory. You can’t necessarily see the change happening in the adult, but you can see that if you change that nucleic acid base right there in that gene, at that particular point in embryonic development, that animal is darker. If you change those three bases over there, that limb is longer. This is the fundamental basis of evolution: changes in DNA. By experimenting with it and visualizing it all the way up the ladder of differences, we now understand that the modern synthesis is correct.
You’ve said evolution is like compounding interest. How so?
Just like a good money market account, evolution works through incremental change. If variants within a species provide an advantage, no matter how slight, then that form, that capacity, will be favored. If evolving spots on wings makes you more attractive to mates or more evasive to predators, those patterns will dominate. Those varieties will have more offspring. Added up over centuries, millennia, and longer periods of time, natural selection—the competition that takes place in nature between variant forms—is powerful enough to forge all the changes that we’ve seen on the face of the earth.
It is hard for most people to wrap their brains around such vast stretches of time.
A century ago, Teddy Roosevelt was president and cars were barely in use. That seems like an unimaginable amount of time ago, but biologically and geologically speaking, it was a split second. A million years is just a fraction of the time that upright hominids have had to evolve. It takes time for sea levels to rise, for rivers to cut their course. As temperatures change, as rain forests grow up or deserts emerge, the creatures that live in these regions are adapting and changing too.
You call the combination of evolution and embryonic development evo devo. What is that, exactly?
It is just shorthand for “evolutionary developmental biology,” a mini-syllabic description of this field that’s concerned with the evolution of development. It’s related probably to Devo, the new-wave band of the early 1980s—those were the guys who played with dog dishes on their heads. Before then you could describe evolution as change over time, but we did not have any grip on that process until the 1980s.
And that’s when you entered the scene?
Right. I was in graduate school doing research in immunology at Tufts University in Boston. I would just hop on the subway and go to seminars at three or four different schools. It’s stimulation, right? It’s hard to know how all the dots got connected, but I kept hearing that things were not well understood in evolution and things were not well understood in development, and I started thinking: How can I get at the meat? I was looking around for insights when I came across the very thin literature on the genes that sculpt fruit fly bodies, including the study of spectacular mutants. In these mutants, or Frankenflies, a single gene could put legs on the head in place of antennae. Other single-gene mutations gave the flies an extra set of wings or removed its eyes or wings completely. The fact that single-gene mutations could have such dramatic effects raised the question: What were these genes, and what were they meant to do? The quest was to figure out how these genes sculpted the fruit fly body form.
You saw the fruit fly as a window into evolution and development. How did you make the connection?
It was not an obvious call, because the expectation was that fruit flies didn’t have anything to do with the development of furry creatures. But in 1983 I found a laboratory where I could do the work, with Matt Scott at the University of Colorado at Boulder. Just as we were getting started, it became clear from our research and others’ that these body-building genes were not restricted to fruit flies; they were shared throughout the animal kingdom. It was a real jolt. All of a sudden we could do deep experiments at the most fundamental level to understand how form actually evolved.
So scientists were seeing the same master genes at work in many different species?
Yes. One shocking discovery was the relationship between our eyes and bug eyes. You wouldn’t think they had anything in common, right? Bug eyes, with 800 facets, work by different optical principles than human eyes. For almost a century and a half, biologists thought that they had evolved independently, from scratch, and that eyes had been invented many times in the animal kingdom by completely different means—different recipes in different groups of animals. We have now discovered that these eyes are formed by what is recognizable as the same gene, even though those animals have been evolving separately for 500 million years. When we took the mouse version of this gene—the same gene we find in the human—and put it in the fly and tweaked it, we induced fly eye tissue.
