The first species having circle of Willis (circulus arteriosus cerebri)?

Question

I know mammals are not the only species that have this anatomical unit (i.e. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3982101/). But I was wondering what was the __first species__ that had a similar structure in their brain (or brain-like organ). Do we know this?

Answer 1

I would say that we do not and probably cannot know the answer to that question. Even assuming (and it is definitely an assumption) that such a precursor existed, the brain and its blood vessels are "soft tissues." Thus, they are extremely unlikely to be preserved in fossilized organisms. You may be able to find papers that propose such evolutionary precursors based upon currently living animal species with the same or similar structures, but those proposed past evolutionary ancestors would be based upon various assumptions, suppositions, and extrapolations from modern organism. Speaking of which, proposing evolutionary precursors between species for any anatomical structure is always more problematic than many people can admit, because there are always significant assumptions, suppositions and extrapolations involved.

So, the short answer is I don't know for sure, but I am not sure the precursor exists anyway, and I don't think we can find it if it does. I am including some links if you wish to keep looking:

https://www.sciencedirect.com/topics/medicine-and-dentistry/circle-of-willis

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3982101/