icakeov said:
Is it known how many genes the contemporary comb jelly would have with the very first "comb jelly" ancestor that is our ancestor too?
I don't know how many genes this would be, but a genome comparison of humans and ctenophores (comb jellies) should be able to answer that question.
Since I could not find that quickly, I found
this article, that says: "genes shared between bilaterians and ctenophores (
7,771)" (see fig. 3).
Bilaterians would include humans and therefore be somewhat representative of the comb jellies comparison with them.
If humans has about 20,000 genes, then the percentage should be >50%, but could be smaller, since not all bilaterian genes will be found in humans.
Presumably, these genes would be things involved in cell structure and function, as well as things like cell-cell communication, some of which are present in choanoflagellates and which are involved in the development of higher metazoans.
The article also has interesting information about when specific traits like collagen, miRNAs, Hox genes, neurons, and muscle cells arose, based on the phylogeny they are using.
Alternative phylogenies would have a different sequence of these events.
These features are all involved (as well as molecular genome studies) in determining how the most primitive phyla that arose at the base of the metazoa (animals) are related and the order in which these metazoan traits arose.
The commonly assumed closest outgroup to metazoans are the
choanoflagellates.
This (or actually their ancestors in the distant past) would be the group from which metazoans arose.
This was originally based upon the great similarity of the choanoflagellates with the choanocytes of
sponges (with are often considered the original metazoan), but has been confirmed by molecular studies.
How exactly the most primitive metazoan phyla, the
placozoans, ctenophores,
cniderian (jellyfish, anenomes, and corals), and
Xenacoelomorphs relate to each other, still seems to be in dispute since recent studies have not yet settled on a common answer.
Some phylogenies have neurons and muscles arising independently (which seems unlikely to me) or that they were lost in some metazoan lineages. Other phylogenies have other issues.
Personally, before I am have too much confidence in any particular hypothesis, I want to see more studies converging on a common phylogenetic hypothesis.
This has not yet happened. The game is still afoot.
Most Ediacaran fossils have not been related to modern groups of animals, but several of the Cambrian fossils have been.
Molecular studies have indicated that the first animal groups arose well before the Cambrian.
icakeov said:
But they also pointed out that 65 million years is quite recent, which I guess means that they are perhaps still very different from the earliest comb jelly ancestors.
Yes, it seems that many earlier Ctenophores died out since they split from the rest of the metazoans.
I like
@berkeman's post since I did not realize there was such a diversity of extant Ctenophores.