The findings suggest that the appearance of the vertebrate head skeleton ādid not depend on evolution of a new skeletal tissue, as is commonly thought, but on the spread of this tissue throughout the headā
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 ĄÖ²„“«Ć½ 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 journalĢżNature.
Employing new techniques to study the development of larval amphioxus, Medeiros and his colleagues identified a tissue āvirtually indistinguishable from vertebrate cellular cartilage.āāThis 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,ā the editors ofĢż±·²¹³Ł³Ü°ł±šĢżwrite.
The main difference between vertebrates and our closest invertebrate relativesā known as cephalochordatesāis the vertebrate skull and head.
Amphioxus is a tiny animalāno more than three inches longāthat 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.
āIāve heard people describe amphioxus as a fish with its head cut off. And, really, it does look a lot like that,ā Medeiros says. āIts body looks like a sardine filet all the way along its whole extent. Itās really hard, especially if youāre not used to looking at them, to know which end is the front.ā
However, āIf you look carefully, you can see the amphioxus has a mouth at one end.ā
"Iāve heard people describe amphioxus as a fish with its head cut off. And, really, it does look a lot like that.ā
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 ānew head hypothesis,ā and it proposes that vertebrates are distinguished by their new (in evolutionary terms) skulls, brains, and sense organs.
Medeirosā research suggests an alternative conclusion. āWhen 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.ā
Skull tissue, in short, ādidnāt just appear from nowhere.ā Early invertebrates similar to amphioxus likely had a little bit of the tissue, which is detectable only in the larval stage. āBasically, as vertebrates evolved, that stuff just spread into the head and proliferated along with the brain and sense organsā 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.
āIt 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 somethingĢżde novo.āMedeiros adds: āIt makes vertebrates a little less special, as far as their fundamental, cellular differentiation programs.ā
As scientists look more carefully into genomes and development, many body parts that seem unique to particular animal groups āreally have very clear roots that are fairly deep in the evolutionary tree,ā he adds.
The ācool implicationā is that the very first animals on the planet were quite complicated. āEvolution has really been playing with tissue types and cell types that were already there.ā
Medeirosā puts it this way: āWe have a new structure, but itās using old parts.ā
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.
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