I chose the comparison to Drosophila for two reasons. First it is an intensely-studied model organism. This means that Drosophila geneticists are constantly trying to convince us that their flies are virtually identical to humans (due to evolutionary conservation of molecular pathways and so forth) and therefore highly relevant to biomedical research. Second, many of the obvious physical characteristics of arthropods like Drosophila confound their true evolutionary relationship to humans and vertebrates in general. Until recently, I had a vague notion in my head that flies belonged to some sort of phylogenetic group of things with limbs, bodies, heads and eyes. It was just that flies don’t have backbones, and we do. I remember being shocked a couple years ago when a visiting speaker declared that sea urchins were a better model for studying development because they were so much more related to us than D. melanogaster. Turns out she was right. According to phylogeny, it would seem that many of the macroscopic similarities between humans and flies are examples of parallel or convergent evolution.
So this brings me to the main point of the challenge and the key to solving most of the questions. Drosophila melanogaster and Homo sapiens belong to phyla from opposite sides of two major branches of the animal tree, the deuterostomes and the protostomes. These lines split off from a common bilateran ancestor around 670 million years ago. The main features that distinguish these groups of phyla are embryological, like which hole becomes the anus and so forth.
At any rate, D. melanogaster belongs to the arthropoda (insects), which share a common protostomian ancestor with a diverse crowd of phyla that includes the molluscs (ie clams, snails, squid, octopuses) the nematodes (roundworms) and annelids (earthworms, leeches). On our side (the deuterostomes) we have the chordates (includes all animals with notochords like the tunicates and the lancelet, as well as all vertebrates) as well as the echinoderms (includes the sea urchin, starfishes and sea cucumbers).
So with this in mind most of the questions can be answered. Deuterostome species share a more recent common ancestor with each other than they do with the protostomes, so we are more similar to each other than to D. Melanogaster (a protostome). Falling in to this category are 1) starfishes, 2) the spotted salamander 7) the tunicate, Botrylloides violaceus, 8) the lancelet and 10) the yellowfin tuna. On the other hand, sharing a more recent common ancestor with the arthropods, other members of the protostomians are likely to be (roughly) equally similar to us as D. melanogaster is. So equally similar are 4) the common snail and 9) the roundworm C. elegans.
So this leaves: 3) the
1) more related to humans D. melanogaster (starfish)
2) more related (salamander)
3) less related (jellyfish)
4) equally related (snail)
5) less related (Trichoplax adherens)
6) less related (sponge)
7) more related (tunicate)
8) more related (lancelet)
9) equally related (roundworm, C. elegans)
10) more related (yellowfin tuna)
11) equally related (squid)
And the winner is…
A three way tie! Between
Anyway these are the correct answers as I (a lowly “biomedical researcher”) understand phylogeny based mainly on my interpretation of Mayr (What Evolution Is, Mayr 2001). The most recent molecular data as discussed a current Nature paper, (HT windy and Nimravid) seems to support them. (A beautiful animal phylogenetic tree from that paper is available here. Great poster for your wall!) Nonetheless, if anyone has a grievance and wants to make their case, bring it!
Stay tuned, as I’ll be adding a breakdown of the “class’s” answers. This should be interesting. Also, I hope to discuss what this all means with respect to the relative value of Drosophila as a model system for human biology in a future post. Are there organisms we're not paying much attention to that would be better? If you have comments on this topic they would be interesting to hear so feel free to kick it off below.
PS – If you want to look at the answers from commenters on the original post, you’ll have to click on the post title to see them all. Or click here. Seems our comment script is bugging out and spitting out incomplete comment listings in the main page.