By Bonnie Kircher
MBL Embyology Course
When you look across the fossil record at the evolution of life, an instinctual response is one of awe. How did natural selection, an entirely passive force, direct monkeys onto their hind legs or fish onto land? When comparing fish and tetrapods (limbed vertebrates) that are alive today, it’s almost unimaginable that the two share a common ancestor.
These transitions are so spectacular that they typically lie at the center of the evolution vs. creation debate (think bumper stickers of Jesus fish vs. Darwin fish [with legs]). As well they should. The history of life lays out a testable hypothesis: A fish with legs should, in fact, have existed at some point in time. However, specimens of this intermediate nature are often elusive.
For Shubin, testing the hypothesis that an animal transitioning between fins and limbs lived at some point required a multidisciplinary approach. Shubin started his search for a fossil like this by using what we know about evolutionary transitions in climate to predict where it might be found. (Indeed, a paleontologist shouldn’t look for a fish with legs in a rock layer that was at the bottom of a deep ocean 375 million years ago, when the fish’s transition to land is hypothesized to have taken place.) After years of work, Shubin found exactly what he was looking for, exactly where it was supposed to be: a fish with appendages transitioning between fin and limb in a rock layer that used to host a shallow stream. Shubin’s find is the now-famous “fishapod” fossil named Tiktaalik roseae.
Shubin’s next question was, what is the essence of a limb? Are there structural factors that are exclusively limb-like, structure that you will never find in a fin? And is the fossil record a morphological readout of these genetic differences? By examining the developmental genetic programs of diverse fins and limbs, Shubin has been able to demonstrate that specific changes in spatial gene expression correlate with the evolution of digits, as well as with the morphological changes seen in the fin/limb transition fossil record.
Neil Shubin’s work demonstrates how using approaches from distinct fields can lead to an amazingly comprehensive understanding of the processes that shape the world we see today. Integrated research from the fields of paleontology, ecology, and developmental genetics has the potential to redefine what we know about major evolutionary transitions, as well as the genetic programs that shape living animals and the way they interact with the world.
Bonnie Kircher is a graduate student with Martin J. Cohn at the University of Florida and a student in the 2016 MBL Embryology course.
Top photo: Neil Shubin of the University of Chicago holds a model of Tiktaalik, the late Devonian period “fishapod” fossil he uncovered at Ellesmere Island, Canada, in 2004. Credit: Jellyfish Pictures, Ltd.