Because we are members of the group, it’s easy to see the vertebrates as the pinnacle of evolution, a group capable of producing bats, birds, and giant whales in addition to ourselves. 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 reestablished a nerve cord (on the wrong side of the body, naturally) but couldn’t be bothered with niceties like a skeleton.
It’s unclear exactly how vertebrates came about, and the likely lack of a skeleton in our immediate ancestors has helped ensure that we don’t have many fossils to help clear things up.
But in Thursday’s issue of Science, researchers reassessed some enigmatic fossils that date back to the Cambrian period and settled several arguments about what exactly characterizes the yunnanozoa had. Answers include cartilaginous structures that supported gills and a possible ancestor of what became our lower jaw. In the process, they show that yunnanozoa are probably the first branch of the vertebrate tree.
You can get an idea of what a yunnanozoan looks like the picture above. The soft tissues of its flanks were divided into segments, a characteristic of our two closest non-vertebrate living relatives (the amphioxus or the lancelet) and are present in vertebrate embryos, but are usually lost as they mature. 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 yunnanozoa look a lot like an amphioxus, 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 on the ground disagreed with this interpretation and placed yunnanozoa somewhere else. 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 ancestral to a huge group of organisms that include things like sea urchins.
A small team from China 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.