New Analysis of Fossil Reveals It Came From First Branch of Vertebrates

Enlarge / Yes, those are gills on this Cambrian animal. Dinghua Yang

Because we are members of the group, it is easy to see vertebrates as the pinnacle of evolution, a group capable of producing bats, birds, and giant whales in addition to us themselves. But when they first evolved, vertebrates were anything but a sure thing. They separated from a group that lived in the mud and didn't need to distinguish its top from its bottom or its left from its right, and so ended up losing an organized nerve cord. Our closest non-vertebrate relatives restored a nerve cord (on the wrong side of the body, naturally) but couldn't be bothered with niceties like a skeleton.

Exactly how vertebrates came about is unclear, and the likely lack of skeletons in our immediate ancestors has helped ensure that we don't have many fossils to help clear things up.< /p>

But in Thursday's issue of Science, researchers re-evaluated some enigmatic fossils that date back to the Cambrian period and settled several arguments about the exact characteristics of yunnanozoans. Answers include cartilaginous structures that supported gills and a possible ancestor of what became our lower jaw. In the process, they show that the yunnanozoa are probably the first branch of the vertebrate tree.

Yunnanowhatans?

You can get an idea of ​​what a yunnanozoan looks like from the picture above. The soft tissues of its flanks were divided into segments, a feature of our two closest living non-vertebrate relatives (amphioxus or lancet) and are present in vertebrate embryos, but are usually lost as their development into adults. Near the animal's head - and it has a clear head and mouth - there are also an array of arching structures which closely resemble the similarly located gill arches found near the head of fish modern.

If this interpretation is correct, it would mean that yunnanozoans look a lot like an amphiox but have a feature otherwise only found in modern vertebrates. This would mean that it preserves features essential to understanding the origin of vertebrates.

But the "if" from the previous paragraph is important. Many people in the field disagreed with this interpretation and placed the yunnanozoa elsewhere. Or rather several elsewhere, depending on who exactly was arguing. Some put them in the same group as amphioxus. Others have moved them away from vertebrates and placed them in the group of mud dwellers who lack two of the body axes found in vertebrates. Yet others have suggested they were the ancestors of a huge group of organisms that include things like sea urchins.

A small Chinese team has now attempted to settle these arguments. It does this in part by imaging more than 100 new fossils of the species. But a big part is that they used some of the most sophisticated imaging techniques available. This included three-dimensional X-ray imaging, electron microscopy, and a technique that bombards microscopic regions of the sample with electrons and then uses the emitted light to determine which elements are present.

I show one of the images from the article below to give an idea of ​​the detail provided by these imaging techniques.

Zoom in/Ignore labels - the level of detail in the new imagery helps us understand the structure of features that look like gill arches. Tien and. Al.