Fossil fish reveals how jaws evolved

Thursday, August 25, 2011

Scientists have shown for the first time the reorganisation of the brain and sense organs that needed to happen for jaws to have evolved in our distant ancestors.

They used high energy synchrotron X-rays to look inside the heads of 400 million-year-old fossils of a long-extinct fish called a galeaspid from China and Vietnam.

Their results, published in Nature reveal the exact shape of the brain and sense organs this ancient creature had.

Galeaspids are a type of bony fish with no jaws. Scientists think they're more closely related to today's mammals, birds and fish - and other vertebrates with jaws ? than jawless vertebrates like hagfish and lampreys, and so represent an evolutionary intermediate.

Almost all vertebrates, such as mammals, birds, reptiles and other animals with backbones, have jaws. But some, like hagfishes and lampreys, don't.

How our ancestors went from having no jaws to having jaws ? and a mouthful of teeth ? is one of the biggest mysteries in evolutionary biology, and fossil evidence has provided few insights.

'Evolutionary developmental biologists have said for decades that you'd need a major reorganisation of the head for jaws to develop,' says Professor Phil Donoghue from the University of Bristol, co-author of the study.

'People have looked for evidence of this reorganisation of the head, but haven't found it, so they've suggested maybe there had been an evolutionary jump. But we found that did not happen,' he adds. Instead, for our ancestors to have evolved jaws, the way their brains developed would've had to be reorganised.

Donoghue and colleagues from China, France and Switzerland realised that they would've had to have two nasal organs at the front of the brain instead of one. Indeed, after scanning nearly 40 fossils, they discovered that galeaspid has one huge nostril, but two nasal organs, just like modern jawed vertebrates, while lampreys and hagfishes have just one nostril.

The stem cells that go on to create the jaws have to move forward from the back of the brain towards the front during development of the embryo for jaws to develop. But if you've got your nasal organs clumped in the middle of the face, that can't happen: the nasal organs act like a physical barrier to the movement of these stem cells.

But because galeaspids have two nasal organs with a gap between them, there's room for the stem cells to migrate all the way forward, meaning that it's possible for jaws to evolve in the space available.

Donoghue says this probably happened tens of millions of years before the origin of jaws.

'Our results show that the reorganization of the head to accommodate jaws wasn't a one-time event, the sense organs in the head were being rearranged for other reasons. Nostrils might have separated for feeding and breathing purposes,' he says.

Galeaspid's huge nostril in its head would have let it get more oxygen out of the water, because it would have increased the flow of water across its gills.

'This technology let us see the paths of all the veins, nerves and arteries that would've plumbed the brain of these amazing fossils,' explains Gai Zhikun also from the University of Bristol and the Institute of Vertebrate Palaeontology and Palaeoanthropology in China, lead author of the study.

Importantly, the technique is completely non-invasive, which means that valuable fossils don't need to broken up - unlike using traditional methods.

'This work would have been impossible without using synchrotron technology and our collaboration with the Chinese,' says Donoghue. 'You need access to technology and access to fossils.'

Zhikun Gai, Philip C. J. Donoghue, Min Zhu, Philippe Janvier & Marco Stampanoni, Fossil jawless fish from China foreshadows early jawed vertebrate anatomy, Nature, published online 17 August 2011, doi:10.1038/nature10276

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Natural Environment Research Council: http://www.nerc.ac.uk/

Thanks to Natural Environment Research Council for this article.

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