麻豆官网

The findings suggest that the appearance of the vertebrate head skeleton 鈥榙id not depend on evolution of a new skeletal tissue, as is commonly thought, but on the spread of this tissue throughout the head鈥�

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 麻豆官网 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-麻豆官网, were published last month in the journal听Nature.

Employing new techniques to study the development of larval amphioxus, Medeiros and his colleagues identified a tissue 鈥渧irtually indistinguishable from vertebrate cellular cartilage.鈥�

鈥淭his 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,鈥� the editors of听狈补迟耻谤别听write.

The main difference between vertebrates and our closest invertebrate relatives鈥� known as cephalochordates鈥攊s the vertebrate skull and head.

Amphioxus is a tiny animal鈥攏o more than three inches long鈥攖hat 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.

鈥淚鈥檝e heard people describe amphioxus as a fish with its head cut off. And, really, it does look a lot like that,鈥� Medeiros says. 鈥淚ts body looks like a sardine filet all the way along its whole extent. It鈥檚 really hard, especially if you鈥檙e not used to looking at them, to know which end is the front.鈥�

However, 鈥淚f you look carefully, you can see the amphioxus has a mouth at one end.鈥�

"I鈥檝e heard people describe amphioxus as a fish with its head cut off. And, really, it does look a lot like that.鈥�

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 鈥渘ew head hypothesis,鈥� and it proposes that vertebrates are distinguished by their new (in evolutionary terms) skulls, brains, and sense organs.

Medeiros鈥� research suggests an alternative conclusion. 鈥淲hen 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.鈥�

Skull tissue, in short, 鈥渄idn鈥檛 just appear from nowhere.鈥� Early invertebrates similar to amphioxus likely had a little bit of the tissue, which is detectable only in the larval stage. 鈥淏asically, as vertebrates evolved, that stuff just spread into the head and proliferated along with the brain and sense organs鈥� 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.

鈥淚t 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 something听de novo.鈥�

Medeiros adds: 鈥淚t makes vertebrates a little less special, as far as their fundamental, cellular differentiation programs.鈥�

As scientists look more carefully into genomes and development, many body parts that seem unique to particular animal groups 鈥渞eally have very clear roots that are fairly deep in the evolutionary tree,鈥� he adds.

The 鈥渃ool implication鈥� is that the very first animals on the planet were quite complicated. 鈥淓volution has really been playing with tissue types and cell types that were already there.鈥�

Medeiros鈥� puts it this way: 鈥淲e have a new structure, but it鈥檚 using old parts.鈥�

Other CU-麻豆官网 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-麻豆官网, the National Science Council of Taiwan and Academia Sinica.

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