I often take my walk to work for granted. After having lunch with Earth and Planetary Science PhD Student, Miranda Margulis-Ohnuma, I gained a newfound respect for the way my legs attach to my body.
As Miranda explained over lunch, “Everything that makes you what you are is a product of really complicated and old history.” The first vertebrates in the ocean were essentially tubes; a straight digestive tract supported by a spine. At some point, these tube-like fish crawled out onto land, eventually evolving to become humans.
Before they could do that, they needed some type of leg, and to have legs, they needed something to support those legs. Enter: The Pelvis. Though very diverse, all 4 legged animals (tetrapods) have the same main building blocks in their bodies: a head, a spine, and front and back limbs, which are attached by the shoulders or pelvis to the spine. Through her research, Miranda is discovering if the pelvis develops the same way in all animals.
Paleontologists have been at a standstill in figuring out where the pelvis came from for a century. While there are fossils from the time when fins were starting to look like legs, there are very few from when the pelvis and fins first showed up on the scene. Miranda brings renewed interest in answering this ancient question, combining paleontological and developmental biology techniques to explore this complex evolutionary past with a focus on the pelvis of the zebrafish.
All previous work studying the developmental origin of the pelvis has been done on chickens because their embryos (eggs!) are large and easy to manipulate. From an evolutionary standpoint, zebrafish and chickens are quite different. If Miranda finds that the development of their pelvises is similar, it would present compelling evidence that all animals form their pelvises in the same way.
Many modern fish have some of the earliest building blocks of what will become limbs. Fish have a large fin on their backs that does most of their swimming, but some also have sets of paired fins. In the movie, Nemo’s “Lucky fin” is one of his pectoral fins (yes, like our pectoral muscles in our chest), and he also has two pelvic fins that are further back and lower (pelvic, like pelvis).
Pelvic Fins of the Zebrafish. Image Credits: Miranda Margulis-Ohnuma
Like Nemo, zebrafish have small back fins that don’t do much in the way of swimming, but are supported by a pelvis on the inside. The pelvic fins are circled in the image above. There is a special breed of zebrafish modified with CRISPR gene editing so that their cells glow. This labeling allows cells in the embryo to glow in different colors, and that indicates where those cells will end up in the body of the zebrafish. Miranda’s fish have cells that glow in one color if they are a part of the back of the embryo (spine and skull), or the side (everything else). In the lab, Miranda dissects zebrafish whose pelvises have just formed to get a closer look at what types of cells from the embryo are coming together to form this tiny pelvis. In lieu of a fossil, she can use the tiny, glowing bone to learn about the structure, development, and function of the pelvis in what is thought to be an “early” evolutionary state.
A zebrafish pelvis, highlighted by the white box, and the attached pelvic fins. Image Credits: Miranda Margulis-Ohnuma
The zebrafish pelvis is tiny and unconnected from the spine, supporting fins that barely function for swimming, while the chicken pelvis supports the legs and allows the feathery biped to walk, run, and roost. Once her zebrafish study is complete, Miranda will be able to compare the development of the pelvis in chickens and fish and determine whether the differently functioning pelvises have a single common origin or if they evolved several times independently.
This work has the potential to solve a long-standing question in paleontology and deepen our understanding of land animals and the emergence of specialized bones that enabled walking. Miranda’s focus in Paleontology stems from a childhood passion for animals and a deep drive to explore the ancient past through the geologic record. As an applied scientist, I always ask about the bigger picture, how this will affect humanity. Miranda countered that scientific exploration to satisfy curiosity is just as important, saying, “It’s really interesting to know one’s place in evolutionary history.” After lunch, I found myself agreeing with her. It is cool to marvel at the incremental changes in evolution that transformed my ancient tubey ancestor into lizards, chickens, dogs, and my own body and legs carrying me back to my desk.