Cosma Shalizi:

One reason papers like this gladden my heart is my basic intellectual cowardice: the sheer endless proliferating detail of biology overwhelms me, especially when something drives home the fact that we keep finding utterly new stuff everywhere we look. Here we are, looking at our own guts, and coming up with stuff like this: "Three sequences from two subjects ... appear to represent a novel lineage, deeply branching from the Cyanobacteria phylum and chloroplast sequences." See? There are organisms whose closest relatives are the stuff that turns ponds and leaves green living inside us, and until now we had no idea. And when we eventually look inside them, they're going to turn out to be weirdly complicated and uniquely strange, exactly like everything else. And of course the damn things will have histories, again exactly like everything else. Biology just doesn't stop, and at some point the details and special cases make me wish my head would explode.

William Tozier:

See, not only are all those new organisms everywhere, and not only (as Cosma points out) do they have histories, but they form an ecology or a society or a something-or-other-that-is-organized. They engage in a combinatorially intractable number of interactions with one another, and the nature and frequency of those interactions will change over time, and so it depends on what you had for fucking dinner what they “say” (chemically) to one another. And for that matter what they “say” (chemically) to one another and to you probably helped direct what you had for dinner. Plus there’s the ones on your skin, in the air you breathe, in the other tissues throughout your body, on your food… everywhere you think you can draw a boundary around “you”, it is crossed by some organism that you missed. And vice versa.

Heck, don't stop there. It's not just that there is an inextricable relationship between diverse gut flora and us, but that our concept of "us" is too limited. Not only does pond scum live within and upon us, but we are pond scum that got its act together and joined together under the most intricate and demanding neighborhood association contract ever. Yeah, it's odd to contemplate ourselves as a residence for cyanobacteria…but what about those guts themselves? Slithery slimy tubes oozing enzymes and slurping up sludgy goo from their lumens—is that me? Why should I feel more astonished at their contents than at the astounding weirdness of all of my constituent parts? Alga aren't any stranger than goblet cells or erythrocytes or pyramidal neurons.

I think Cosma's right that the intimidating thing is that it doesn't end. We have this illusion that we are discrete and bounded, but we're really just temporary skeins of sensations and chemicals that enwrap some complicated chemistry, and interact with everything around us. "I" am something that changes with the length of the day, the bits of organisms that I'm constantly stuffing into the anterior end of my digestive tract, the viruses that sneak in and diddle my chemistry, the scents of the people around me. The universe is one big stewpot, and we just happen to be somewhat lumpier bits…but we're still thoroughly infiltrated with the broth, and we're mistaken if we think we're somehow independent of the stew.