Because we’re a member 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 anything but a sure thing. They branched off from a group that lived in the mud and didn’t need to tell 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 re-established a nerve cord (on the wrong side of the body, naturally) but couldn’t be bothered with niceties like a skeleton.

How exactly vertebrates came out of this hasn’t been clear, and the probable lack of a skeleton in our immediate ancestors has helped ensure that we don’t have a lot of fossils to help clarify matters.

But in Thursday’s issue of Science, researchers have re-evaluated some enigmatic fossils that date back to the Cambrian period and settled several arguments about exactly what features the yunnanozoans had. The answers include cartilaginous structures that supported gills and a possible ancestor to what became our lower jaw. In the process, they show that yunnanozoans are likely the earliest branch of the vertebrate tree.