Neil Shubin, a professor of anatomy and organismal biology at the University of Chicago, has devoted his career to understanding the transition of marine to terrestrial life in the history of life on Earth.
What is known is that 300 to 400 million years ago a marine creature began to explore the possibilities of earning its living both on land as well as in the water.
The textbook version is pretty confident in telling us that it was this animal that became the ancestor to the whole class of mostly terrestrial animals called tetrapods. The membership of this very large group of animals includes amphibians, birds, mammals, and reptiles.
However, in the real world, certain facts create a difficult puzzle.
As Dr. Teksuya Nakamura, who works in Shubin's laboratory, explains in a research report he and colleagues published in Nature, "Understanding the evolutionary transition from fish fins into tetrapod limbs is a fundamental problem in biology." The problem results from the fact that the fish fins and tetrapod limbs are quite different from one another in their structure, in their development, as well as in their differing cellular properties. (Nakamura, et. al., Nature, 2016)
Where a fish's fins are based on a small cluster of endochondral bone (bone formed from cartilage and possessing a blood supply) with rays that are composed of dermal bone (not derived from cartilage and no blood supply), the tetrapods have three part forelimbs (humerus, radius-ulna-wrist/ankle with digits) all composed of endochondral bone.
While there are fossils from the transition time that show animals that have the characteristics of animals adapted for life in water as well as on land, until the research reported by Dr. Nakamura, a mechanism connecting fins to limbs has been unavailable.
Over two decades Shubin's work has approached finding such a mechanism from two directions. The first has been to search for fossils from the period. The search led to the 2006 discovery of Tiktaalik, a fish (the name is the Inuit word for "large shallow water fish") with some of the characteristics of tetrapods (such as tetrapod-like limbs composed of endochondral bone).
The second path has been through the comparative study of the development of embryos representing fish (zebrafish) and tetrapods (mice).
It has been this second pathway that led to remarkable insight into the evolutionary connection between fins (of fish) and limbs (of tetrapods).
While Shubin and his team knew for a number of years what experiments they would like to conduct, they also knew that they lacked the technology to undertake them. This changes in 2015 with the advent of the Crispr/Cas9 gene-editing technology. Crispr "exploits the tendency of bacteria (and of archaea) to insert short sections of DNA...into their genome in order to acquire immunity to certain pathogens.
You can read more about Crispr here.
Since the gene contains the information (coded into DNA) that is needed to create the various parts of an organism, the ability to edit (change or even disable a gene) allows investigators to decode that information by tracing the effects of such edits on the development of an organism. Using embryos of fish (zebrafish) and mice, the investigation began with a focus on the Hox family of genes. Other work has shown that when Hox-13 and Hoxd-13 genes are disabled in mouse embryos, the long bones of the limbs develop but the wrist/ankle bones don't. (Zimmer, 2016)
In one experiement in Shubin's lab, Dr. Tetsuya Nakamura created defective Hox-13 and Hoxd-13 genes in zebrafish embryos. The fully developed fish had deformed fins which lacked rays suggesting that Hox genes carried the information for making the dermal bone out of which the rays are constructed.
In another experiment, Andrew R. Gehrke, one of Shubin's graduate students, made an alteration in zebrafish embryos that caused the cells that switched on the Hox-13/Hod-13 genes to glow when the genes were on. These undifferentiated embryonic cells would continue to glow as they moved to the place where they would become specialized cells in the rays of the fin. When the fish were fully developed the rays themselves glowed!
When Gehrke engineered the mouse embryo cells to illuminate like those of the zebrafish, he found that when the mice were fully developed the wrist bones glowed.
The results are striking because they show that there is an equivalence between the rays of fish fins and the digits on mouse paws "at the level of the cells that make them." (Zimmer, 2016)
The development of fins and limbs are using the same information source rather than different sources as the differences between fins and limbs might suggest. The development of fins and limbs follows the same set of rules. As Matthew P. Harris, a geneticist who was not involved in the experiment, explained that "in both cases, the Hox genes tell a clump of embryonic cells that they need to end up at the far end of an appendage...the molecular address is the same." (Zimmer, 2016)
During the development of an embryo each undifferentiated cell is given an "address" where it will differentiate into whatever function it is to take in the animal. In a zebrafish, the cells that go to the address where rays form are specialized as dermal bone, while in the mouse embryo the same address is given to cells that will become endochondral bone.
The discovery of the evolutionary connection between fins and limbs provide an example of how understanding the gene "as the least divisible unit of a larger form" helps us to understand the larger whole, the organism as well as the relationships among other organisms.
Note: Organismal Biology: "Organismal biology, the study of structure, function, ecology and evolution at the level of the organism, provides a rich arena for investigation on its own, but also plays a central role in answering conceptual questions about both ecology and evolution." (http://ecologyandevolution.cornell.edu/research/organismal-biology.cfm)
Doudna, Jennifer (2016). The Doudna Lab: Exploring molecular mechanisms of RNA-mediated cell regulation.
Nakamura, T. et. al. (2016). Digits and fin rays share common developmental histories. Nature. 20 July, 2016.
Shubin, Neil (2009). Your Inner Fish: A Journey into the 3.5 Billion Year History of the Human Body. Teaching Materials for the book.
Shubin, Neil. (2009). The Tiktaalik
Zimmer, Carl (2016). From Fins to Hands. The New York Times, July 16, 2016.
Dr. John Holton
Dr. John Holton joined the S²TEM Centers SC in July of 2013, as a research associate with an emphasis on the STEM literature including state and local STEM plans from around the nation.