I wish it kept going a little more after fish. Vertebrates evolved from chordates, similar to sea squits. Their larva have all the characteristics of early fish. A digestive system, heart, vanadium based blood, gills, notochord, and even the earliest brain.

And that organism would have evolved from deuterostomes, similar to starfish, sea cucumbers or sea urchins. Infact sea urchin anatomy is fairly representative of early deuterostome anatomy. These organisms would have evolved the earliest true digestive system, alongside protostomes, the ancestors of mollusks and arthropods.

And deuterostomes and protostomes would have evolved from a bilateran ancestor similar to cnidarians, or jellyfish. This is where our specialized tissue layers and early multicellular embryology would have evolved. Protostomes and deuterostomes use different organizational approaches when forming a zygote, but they still differentiate into 3 tissue layers and all your cell types differentiate further from there. Cnidarians only have 2.

And that organism probably evolved from a coral like ancestor with undifferentiated tissue. Interestingly enough, coral has a nymph stage, which is impressively mobile and even has tiny light sensing cells so they can navigate for up to a day after conception in order to find the ideal spot to spend the rest of their lives. Fish, arthropods and mollusks all have nymph stages reminiscent of this period of development. Even humans do, except in placental mammals, that stage takes place entirely within the womb during the third trimester. Marsupials on the other hand don't have a third trimester, and have to be born much earlier because they don't have a blood barrier separating the child and the mothers, who's immune system will begin attacking the fetus.

It's interesting how steady the rate of evolution has been over the last 650 million years since the first multicellular eukaryotes evolved from single celled colonies of protists. So many vital function we take for granted that steadily emerged one after another, and still perfecting the process as recently as 80 million years ago.

Sorry about my rant. I'm obsessed with the beginnings of things. And there's been so much good research coming from comparative genome studies, but so few good sources for conveying that information to the public. The era of quality nature programming is over, and has been reduced to shark week and cheap thrills of deep sea aliens. We need a spacetime or kurtzgesat for comparative genomics and early evolutionary history.