- Written by Steve Bratteng
- Published: 18 April 2014
Your Inner Fish is the first part of a PBS series by Neil Shubin, a paleontologist at the University of Chicago. The video series is based on his book, Your Inner Fish, and will include episodes, “Your Inner Reptile” and “Your Inner Monkey.”
In the first episode Professor Shubin recounts and re-enacts the monumental discovery of Tiktaalik, a transitional form linking fish and tetrapods (land vertebrates). Four hundred million years ago in the Devonian period the only vertebrates known were fishes. By 360 million years ago there were vertebrates walking about on the land. Shubin and a colleague set out to find an intermediate (transitional) form.
Finding fossils requires knowing the time frame when the forms you seek would have lived as well as where fossil-bearing rocks of that age might be exposed at the surface of the Earth.
Shubin had explored Devonian rocks in Pennsylvania, and he knew that Jennifer Clack had thoroughly explored similar aged rocks in Greenland, but no one had explored another prominent outcrop of that time period: Ellesmere Island in arctic Canada.
Hunting fossils in the arctic meant having only a few weeks each year in which to have access to the land without an ice covering. So, in July 1999 an expedition was set to spend time searching for a missing link between fish and land vertebrates.
The fourth such expedition, which was carried out in 2004 finally paid off when they discovered a snout protruding from the rocks. Complete exposure of fossils is seldom conducted in the field. The process involves excavating around the area of the fossil and then encasing the entire bit of rock and other debris in a plaster case.
Once they returned to Chicago expert preparators using delicate instruments slowly exposed what turned out to be a truly remarkable specimen. It was a fish with a flat head, a neck, and front fins that contained bone structure that showed a resemblance to what is seen in modern land vertebrates: a single bone (at the shoulder), two bones, some little bones, and finally digits. This is a pattern you share with all other land vertebrates that have limbs.
The fossil’s name was decided upon by asking indigenous Inuits what they thought, and arrived at Tiktaalik, which mean large fresh water fish.
Parallel to the fossil studies Shubin’s lab also explored the developmental genetics of how forelimbs are formed, and it turns out we have a gene called “Sonic Hedgehog” that is responsible for creating digits on limbs. And, we share this with birds and fish. There is even a version of this same gene in fruit flies.
It is a fascinating look at how we came to be what we are, and I look forward to the subsequent episodes.
You might want to check it out for yourself at this link.