The findings suggest that the appearance of the vertebrate head skeleton âdid not depend on evolution of a new skeletal tissue, as is commonly thought, but on the spread of this tissue throughout the headâ
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 journalÌęNature.
Employing new techniques to study the development of larval amphioxus, Medeiros and his colleagues identified a tissue âvirtually indistinguishable from vertebrate cellular cartilage.ââThis 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,â the editors ofÌę±·ČčłÙłÜ°ù±đÌęwrite.
The main difference between vertebrates and our closest invertebrate relativesâ known as cephalochordatesâis the vertebrate skull and head.
Amphioxus is a tiny animalâno more than three inches longâthat 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.
âIâve heard people describe amphioxus as a fish with its head cut off. And, really, it does look a lot like that,â Medeiros says. âIts body looks like a sardine filet all the way along its whole extent. Itâs really hard, especially if youâre not used to looking at them, to know which end is the front.â
However, âIf you look carefully, you can see the amphioxus has a mouth at one end.â
"Iâve heard people describe amphioxus as a fish with its head cut off. And, really, it does look a lot like that.â
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 ânew head hypothesis,â and it proposes that vertebrates are distinguished by their new (in evolutionary terms) skulls, brains, and sense organs.
Medeirosâ research suggests an alternative conclusion. âWhen 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.â
Skull tissue, in short, âdidnât just appear from nowhere.â Early invertebrates similar to amphioxus likely had a little bit of the tissue, which is detectable only in the larval stage. âBasically, as vertebrates evolved, that stuff just spread into the head and proliferated along with the brain and sense organsâ 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.
âIt gives us a better understanding of the kind of genetic and developmental events that made a vertebrate out of an invertebrate. It becomes more of a story of reorganizing and repurposing tissue that was already there, rather than having to create somethingÌęde novo.âMedeiros adds: âIt makes vertebrates a little less special, as far as their fundamental, cellular differentiation programs.â
As scientists look more carefully into genomes and development, many body parts that seem unique to particular animal groups âreally have very clear roots that are fairly deep in the evolutionary tree,â he adds.
The âcool implicationâ is that the very first animals on the planet were quite complicated. âEvolution has really been playing with tissue types and cell types that were already there.â
Medeirosâ puts it this way: âWe have a new structure, but itâs using old parts.â
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.
Ìę