Our distant ancestry lurks in strange places. Our skull, for example, holds secrets of relationships dating back to our distant fish ancestors. Our skulls are made up of two kinds of bones: cartilage bone and dermal bone, with the dermal bone forming the encasement that guards our vulnerable brain. Cartilage bone is preformed in cartilage in the embryo and only replaced later by bony tissue. Most of our internal skeleton forms in this way, including some skull parts. Cartilage grows by internal expansion, unlike bone, allowing the internal skeleton to expand with the body and later be replaced with bone. Dermal bone is formed directly in the deep skin, and growth is limited to adding new bone tissue on the outside rather than by expansion.
So what does this have to do with our ancestry? Dermal bone is
traceable back to the external plates over the internal head skeleton of our fish
ancestors. Through time, these plates became integrated into the skull as the brain
expanded beyond the original cartilage bone. A fishy story, true, but welcome to
reality.
Contributor: Arthur H. Harris, Laboratory for Environmental Biology, Centennial Museum, University of Texas at El Paso.
Desert Diary is a joint production of the Centennial Museum and KTEP National Public Radio at the University of Texas at El Paso.