The Tangled Tree

Add some DNA from viruses, bacteria, chimpanzees, and, for some of us, Neanderthals, and pretty soon— well, ok, after a couple billion years — you have human beings. Let’s stress that plural for a second: we really are beings, our bodies covered inside and out with microbes. Some studies say we’re one-to-one bacterial to human cells, others that we’re three-to-one bacteria-to-human. Maybe thirty-seven trillion bacterial cells in each of us. Bacteria being so small, this turns out to be from 1-3% of our body mass. That’s two to six pounds for a 200 lb adult. We can’t live without these lifeforms; “humanness” very much incorporates these other lifeforms, who are also dependent on us. Of course, in other circumstances, these very things can also kill us.And don’t forget the archaea, the viral particles, the fungal cells, living with, or as, us. All these naturally come up in composite creature (like all us us) David Quammen’s eye-opening new book, The Tangled Tree: A Radical New History of Life.

What the book is really about, though, is the discovery over the last fifty years of the chimerical nature of our very cells. Not the microbiome, but the very structure of the cell, which incorporates what were once free-living lifeforms. Mitochondria in animals, chloroplasts in plants: these were originally separate lifeforms. They were absorbed by, or they invaded, cellular life-forms. They didn’t kill their hosts, their hosts didn’t kill them. All together they all became a new thing, mighty things, and they continued generationally.

“This is a profoundly consequential process: the transit of DNA from organelles of bacterial origin into the chromosomes; alien genes becoming incorporated over millions of years into the deepest cellular identity of plants, fungi, and animals.” This is now known as endosymbiotic gene transfer.

Horizontal gene transfer, as opposed to the vertical gene transfer of reproduction, happens all the time between microbes. It’s probably happening in your nose right now. It’s how bacteria thrive; indeed, some suggest that there’s just one superorganism of bacteria in the world, constantly sharing genetic data. (One estimate has total mass of bacteria in the world exceeding the total mass of all plants and animals.) Less theoretically, it’s also how bacteria defeat our antibiotics. Indeed, these antibiotic-resistant “superbugs” are now a major threat around the world, especially in hospitals. (The constant low-dosing of domesticated animals with antibiotics to increase their growth sure and hell isn’t helping in the production of super-resistant strains.)

Another related story told by Quammen is the discovery of the Archaea, the third domaine of life, in the 1960s-1970s. Of course these had always been there, but we were confusing them with bacteria, another kind of single-celled lifeform without a nucleus. The nucleus is what distinguishes us, and the plants and the mushrooms (us being the Eukarya) from the archaea and the bacteria. Archaea include some funky little critters, extremophiles who can live at near boiling temperatures and gobble up sulfur like pie, but others are also part of our microbiome. Are they the direct ancestors of the first forms of life on the planet, the start of it all?

“[…] kingdoms of life are hard to define. The lines dividing one kingdom from another are inescapably blurry.” The same goes for species, especially of the microbial kind, but but but also “higher” up, too.

I highly recommend this book. Be sure to read the section on mammals, which suggests that we (we here including primates, rodents, bovines, even marsupials, who turn out to have transient placentas), incorporate the immunosuppression abilities of retroviruses to make a better placenta. Think about it: a fetus is an alien invasion, half of its genes from elsewhere (like maybe the mailman?). Why doesn’t this trigger an immune response from the mother? Did these retroviruses join the early mammal team to make the maternal/fetal interchange smoother?So the tree of life concept, which predates Darwin’s famous sketch of 1837 by millennia, is entirely too simple. We are all hybrids. The roots and the branches are all inosculated together, that is, merged (grafted, but without a grafter) via the sharing of genetic material across species, even across kingdoms.

Hell, even a tree isn’t a tree anymore: they’re communal lifeforms living in contact with each other and fungal associations, communicating through the roots and the air…

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