Because we’re members of the group, it’s easy to see 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 far from a sure thing. They branched off from a group that lived in the mud and didn’t need to distinguish their top from their bottom or their left from their right, and so ended up losing an organized nerve cord. Our nonvertebrate closest relatives recreated a nerve cord (on the wrong side of the body, of course) but couldn’t delve into such intricacies as a skeleton.
Exactly how vertebrates evolved from this wasn’t clear, and the likely lack of a skeleton in our immediate ancestors has contributed to the fact that we don’t have many fossils to clarify things.
But in Thursday’s Science, researchers have re-evaluated some enigmatic fossils dating back to the Cambrian period and settled several disputes over exactly what properties they have Yunnanozoa would have. The answers include cartilaginous structures that supported gills and a possible ancestor of what became our lower jaw. They show that Yunnanozoa are probably the earliest branches of the vertebrate tree.
You can get a feel for what a Yunnanozoa looks like the picture above. The soft tissue along its flanks was segmented, a feature of our two closest living nonvertebrate relatives (the Amphioxus or lanceolate) and is present in vertebrate embryos but is generally lost as they progress through adult development. Near the animal’s head – and it has a clear head and mouth – there are also a number of arched structures that look very similar to the similarly arranged gill arches found near the head of modern fish.
If this interpretation is correct, then that would mean Yunnanozoa look very similar to an amphioxus, but have a characteristic that is otherwise only found in modern vertebrates. This would mean preserving traits critical to understanding vertebrate ancestry.
But the “if” from the previous paragraph is a big one. Many people in the field disagreed with this interpretation and placed it Yunnanozoa elsewhere. Or rather, several others, depending on who exactly was arguing. Some place them in the same group as the Amphioxus. Others removed them further from vertebrates and placed them in the group of mud-dwellers who do not have two of the body axes found in vertebrates. Still others suggested they were ancestors of a vast group of organisms that includes things like sea urchins.
A small team from China has now tried to settle these disputes. This is done in part by imaging more than 100 new fossils of the species. But a big part is that they’ve used some of the most advanced imaging techniques available. These included three-dimensional X-ray imaging, electron microscopy, and a technique that bombards microscopic areas of the sample with electrons and then uses the light emitted to determine which elements are present.
I am showing one of the images from the paper below to give a sense of the detail these imaging techniques offer.
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