Applying Embryology to Dinosaur Studies

We are going to interview Dr. Egawa who is a dinosaur researcher. I can’t wait! But, why dinosaur research not at a museum but at RIKEN? Since RIKEN BDR is a biological research institute, rather than a geological one, I’m curious to find out what kind of research he is doing…

How long have scientists believed that birds are descendants of dinosaurs?

I was excited to hear that there is someone who’s doing research on dinosaurs at BDR. I was particularly looking forward to this interview. Do you specialize in a certain species?

I’d say I focus more on the ones that eventually evolved to become modern birds such as theropods like Tyrannosaurus. Since dinosaurs are extinct, we can’t analyze them from a biological perspective. Therefore, it is easier to discuss the lineages that left their descendants today since they provide more clues to us. In other words, we can learn a lot about theropods by looking for clues in birds, which are their direct descendants. In a nutshell, I’m doing research on dinosaurs through birds.

Isn’t that the most fun part of the research?

Yes, it’s a catchy topic and stands out a lot.

Were you always interested in dinosaurs since you were little?

(Laughing) I was totally into dinosaurs! But I wasn’t so interested in collecting fossils. I preferred a more biological angle over that and I was the kind of kid who’d dissect chicken from the store.

I remember about 20 years ago or so that it was just speculation to say that the birds are descendants of dinosaurs. But now it’s accepted as a fact. When was it confirmed as a fact in the academic world?

I see. I think what’s accepted as common knowledge on that topic differs between generations. For the kids today, they are taught that it’s a fact.

My daughter is one of those kids.

In the academic world, Yale University led the study that found solid proof around the mid-70s which was further supported by modern phylogenetic analyses in the 80s. I think most researchers agreed with that fact by then, but there was residual opposition for a little while. Before the end of the 20th century, the debate had basically ended and it was accepted as a fact in the academic world. Having said that, it’s not that I witnessed the debate personally so I’m just following the whig historiography (the attitude that views history as the story told by the winners of the debates).

Oh, it wasn’t as recent as I thought!

The connection between birds and dinosaurs was first suggested by Thomas Henry Huxley, who was called Darwin’s Bulldog. Hence this link has been pointed out since the dawn of the theory of evolution.

In the 20th century, the wrist joints became the center of focus because those joints bend differently between lizards and birds. And it was pointed out that some dinosaurs had the joints that bend the same direction as that of birds. These types of modern ideas led to a revival of paleontology, which we often call the “Dinosaur Renaissance.”

I can see how the hand of Deinonychus resembles that of birds.

Reconstruction images constantly change

It seems to me that the reconstruction images change every time I look at it. For example, I used to see Spinosaurus standing on four legs but I now see it standing with two legs.

You mean the opposite—it’s now depicted to walk on four legs. There have been some recent publications on this topic, too.

Is it true that it does change a lot?

It definitely does. These days, Tyrannosaurus is even depicted with fluffy fur.

I’m gonna diverge a little bit here, but personally, I love the reconstruction images of dinosaurs because it’s like a snapshot of the popular theories of each era during which they were created. I can see what ideas were trending among researchers throughout history by looking at the reconstruction imagess. This is fascinating to me.

It’s interesting to me that even the idea of whether Spinosaurus stood with four legs or two legs can be up for debate. It would seem that debates like that can be settled by looking at the skeletons which have a defined structure, unlike features like the color or texture of the skin which require some speculation.

That’s because complete skeletons are unrealistically rare. A lot of them are reconstructed from only about ten percent of the bones of an entire skeleton.

That is definitely impressive! That means that you can even reconstruct a dinosaur from a single bone. It seems impossible to me, but how is it done?

We use closely related species as a guide. That also means that those images are drawn only when we have a known relative species.

How do you determine which species are related?

If there is a specific feature that’s unique to a lineage, then we can group them together. Since it’s not possible to analyze the genome of fossils, we have to rely on their morphology to draw a phylogenetic tree. In simpler terms, to draw a phylogenetic tree, we first look for a feature that we don’t see in any of the ancestors or that is found in only a few species, and those species sharing the novel feature are grouped together. We basically keep grouping based on the shared traits and features to create the phylogenetic tree. If we find an animal with the same unique feature as a known species, we categorize it into that group.

Fossilized proteins?

These days, a lot of phylogenetic trees are drawn based on genome sequences, but I suppose it’s not that simple for dinosaurs.

Right. It’s not feasible for dinosaurs.

I’d imagine it’s nearly impossible to find their genetic information unless you get super lucky and miraculously find a fragment of DNA.

Even though genetic information can be traced back several hundred thousand years, it’s still not that simple. However, proteins can remain for a very long time.

You mean re-construct a DNA sequence from an amino acid one?

Well, it’s difficult to do that from an amino acid sequence, but there has been an interesting study recently. It appears that proteins can preserve their three-dimensional structure, replacing the amino acid molecules making up that protein.

Do you mind elaborating?

Sure. It’s really just like a fossil. When bones become fossilized, it means that the calcium phosphate and collagen of the bones are mineralized. The same basic thing appears to happen with proteins—the protein shape remains while its contents are replaced. There is a method called immunostaining, which uses antigen-antibody reactions to detect proteins with a certain shape. Because protein structures are preserved, there have been studies in recent years showing this method is applicable to fossils, too . So these days, we can use immunostaining on the Tyrannosaurus. Having said that, it’s only been several years that this has been going on, so there is also a possibility that it’ll be disproved. The story about the discovery of DNA sequences in dinosaurs was also disproved after a few years, too.

I didn’t realize that there was such a discovery.

There was a paper reporting this that was published in the 1990s. But it ended up being disproved later.

Was it from contamination?

That’s exactly it. Therefore, likewise we need to carefully monitor fossil immunostaining research, although I think it’s possible that the data may really have biological significance.

That’d be interesting, but I guess we can’t just take this at face value…

Embryology for Dinosaurs

What comes to mind when I think of dinosaurs is science museums and such. But, why did you choose to come to RIKEN?

Because the information we can obtain from dinosaur fossils are limited to their morphology, discoveries about dinosaur evolution have been mainly made based on such morphological data. Therefore, I figured it’s easier to do research based on their morphology even if I’m doing biological research without fossils. As an undergrad, my thinking process for my future was dinosaur studies based on morphology; I prefer biology, especially anatomy, over geology; I want to learn something more related to their life sciences; and how and where can I use my strengths? I eventually concluded that the answer was developmental biology of birds. I came to RIKEN almost as an extension of that path. When studying developmental biology, it helps a lot to have the proper experimental environment set up with the proper equipment.

In speaking of developmental biology, it’s easy to understand when you are studying existing living creatures today, how do you link that story to talk about it for creatures that went extinct millions of years ago?

It requires a certain amount of logic when we are debating on things that old. Incidentally, what drew me to RIKEN was that I could have discussions on evolution and development, which also requires having a bit of philosophical mindset about science.

If we take birds as an example, they display unique traits and features that differ from reptiles and mammals. Those traits and features were acquired at different times in history and accumulated over time. If we apply developmental biology to some features of birds that appeared further back in the past, we can infer on what kind of developmental changes occurred during the dinosaur era.

I didn’t quite follow…

I see. To explain it differently, when you look at the fossil record, you can see the changes in morphology during the evolution that led to the emergence of dinosaurs. Since all animals start from a fertilized egg and it changes its form while developing into a matured state, the evolutionary changes must have occurred sometime during this developmental process. I’m interested in finding out more about this kind of developmental change. To do so, we can compare developmental processes among the animals having ancestral and novel morphologies and infer where in the developmental process the change occurred.

Let me see if I understand this. For example, if there is a three-legged bird like Yatagarasu and you find another species from 80 million years ago with similar traits, then you can trace back in evolutionary history to find when they became three-legged. Is it something like that?

Basically, yes. Suppose we were to compare the developmental processes of two-legged and three-legged birds that are alive today, and discover embryologically why and how the Yatagarasu develop three legs. Then, looking at the fossil record, we might find a point in the evolutionary history when the bird acquired a three-leg morphology, from which we can infer that the developmental process for three legs was acquired around this point in evolution. That’s the kind of logic my research is based on and it’s the most basic and naive way of thinking in evolutionary developmental biology. The relationship between development and evolution is probably more complex and there’s even more evolutionary phenomena involved. What I’m working on right now takes it a step forward from this.

Research goes on from skeletons to muscles

What plans do you have now to expand your research?

Up until now, I’ve been focusing my research on the thigh bone of birds and dinosaurs where it bends into the L-shape. I’d next like to study the muscles that move the thigh bone.

Right. Without muscles, you can’t move the bones…

When we broaden our perspective to include muscles, we can start to think about functionality instead of just forms. Since the law of physics such as gravity is the same in the dinosaur era as it is today, we can use the mechanical simulations to speculate the way the muscles might have moved. I’m more comfortable with this kind of approach since it’s more about biomechanics than developmental biology.

Development and functionality were considered to be phenomena before and after birth, respectively. But, I’d like to challenge that idea and push for discussing both of these phenomena on the same plate. I’d like to integrate the knowledge from those other areas and illustrate dinosaurs as vividly and naturally as possible. I believe the two aspects of dinosaurs, the making and motion of the body, are in a congruent relationship with each other.

It seems that as the technology develops further, restoration illustrations will be more realistic.

I think the animation of dinosaurs, in particular, might become much more realistic. The approach is a bit different but I sometimes wonder if I’m aiming for the same thing as dinosaur illustrators or movie artists.

I remember when Tyrannosaurus used to be depicted as being heavy and stalking like Godzilla, but now it seems quite agile despite its size.

Oh, that’s probably from the period when the Jurassic Park movie came out. By the 2000s, there were some theories saying that they couldn’t even run…

Are we reverting?

Yup. Now, the popular theory is that their posture might have been horizontal rather than upright like Godzilla and walked fast.

It might be interesting to see this history organized chronically.

There might be such books. I’m not sure…

I’d love for you to write that book!

OK. I’ll do that when I’m about 40.

Postscript

Since the topic was dinosaurs, I was excited as I listened to him speak. The reconstruction images in the museums and movies are so different now compared to what I used to remember seeing when I was growing up, since they just reflected the changes in the theories in the academic world. Regardless, the dinosaurs these days are so colorful. I’m curious to see what they are seen to look like 30 years from now.