Vertebrates built new heads from old parts, study suggests
An amphioxus in the Daniel Medeiros lab is seen with most of its body burrowed into sand and its mouth exposed, as it waits for food to drift by. Photo by David Jandzik.
The findings suggest that the appearance of the vertebrate head skeleton ‘did not depend on evolution of a new skeletal tissue, as is commonly thought, but on the spread of this tissue throughout the head’
During the evolution of invertebrates like amphioxus into vertebrates like fish, a remarkable structure appeared: the head.
How, exactly, the head evolved has long been a mystery, but scientists postulated that skulls were built from fundamentally new tissue. Now, research at the University of Colorado Boulder suggests that skull tissue was actually built from existing tissues never before found in invertebrates.
The findings by a team led by Daniel Medeiros, assistant professor of ecology and evolutionary biology at CU-Boulder, were published last month in the journal Nature.
Daniel Medeiros in his CU-Boulder laboratory.
“This suggests that the appearance of the vertebrate head skeleton did not depend on evolution of a new skeletal tissue, as is commonly thought, but on the spread of this tissue throughout the head,” the editors of Nature write.
The main difference between vertebrates and our closest invertebrate relatives— known as cephalochordates—is the vertebrate skull and head.
Amphioxus is a tiny animal—no more than three inches long—that lives in warm coastal waters. With only its mouth exposed to the water, it burrows into sand, mud or gravel, where it waits for food to pass through its gills.
“I’ve heard people describe amphioxus as a fish with its head cut off. And, really, it does look a lot like that,” Medeiros says. “Its body looks like a sardine filet all the way along its whole extent. It’s really hard, especially if you’re not used to looking at them, to know which end is the front.”
However, “If you look carefully, you can see the amphioxus has a mouth at one end.”
"I’ve heard people describe amphioxus as a fish with its head cut off. And, really, it does look a lot like that.”
As a biology student, Medeiros was taught that vertebrate-type skeletal tissue, like cellular cartilage and bone, is unique to vertebrates.
The idea that the vertebrate head is unique is called the “new head hypothesis,” and it proposes that vertebrates are distinguished by their new (in evolutionary terms) skulls, brains, and sense organs.
Medeiros’ research suggests an alternative conclusion. “When you look really carefully at amphioxus at just the right developmental stages, you can see what is basically the same as vertebrate cartilage in that invertebrate.”
Skull tissue, in short, “didn’t just appear from nowhere.” Early invertebrates similar to amphioxus likely had a little bit of the tissue, which is detectable only in the larval stage. “Basically, as vertebrates evolved, that stuff just spread into the head and proliferated along with the brain and sense organs” like eyes and ears.
Another implication of the research is that the development of vertebrate-style skeletal tissue would date to about 600 million years ago, 100 million years earlier than previously thought.
Amphioxus larva with cartilage (fluorescent green) forming around the mouth. Photo by David Jandzik.
Medeiros adds: “It makes vertebrates a little less special, as far as their fundamental, cellular differentiation programs.”
As scientists look more carefully into genomes and development, many body parts that seem unique to particular animal groups “really have very clear roots that are fairly deep in the evolutionary tree,” he adds.
The “cool implication” is that the very first animals on the planet were quite complicated. “Evolution has really been playing with tissue types and cell types that were already there.”
Medeiros’ puts it this way: “We have a new structure, but it’s using old parts.”
Other CU-Boulder researchers on the project were David Jandzik, Aaron T. Garnett, Tyler A. Square and Maria V. Cattell. Also collaborating was Jr-Kai Yu of Academia Sinica in Taiwan.
Funding for the research came from the National Science Foundation, CU-Boulder, the National Science Council of Taiwan and Academia Sinica.