Of Humans, Chimps, Spiders and Bats

There was an interesting, front-page article in yesterday's New York Times that reported that no fewer than six different "tribes" of bacteria thrive in the crook of the human elbow, with more than one million bacteria in every square centimeter of flesh. The bacteria symbiotically moisturize the flesh by processing the raw fats it produces, and are therefore called "commensals" by biologists.

Nature abounds in examples of commensals. Bacteria in our stomachs and intestines perform vital services by breaking down complex sugars and converting hydrogen, a byproduct of bacterial fermentation, to methane. The mixotrich is a micro-organism that lives nowhere else but in the gut of a certain Australian termite, where it assists in the breakdown of cellulose. But the mixotrich is not itself a bacterium but a large protozoan, half a millimeter or more long, and large enough to contain hundreds of thousands of bacteria inside itself - a universe, or at least an ecosystem, inside a universe inside a universe.

The entourage of all microbes that live in an organism is called its "microbiome." The Australian termite's microbiome would include, at a minimum, the mixotrich and the bacteria that live inside the mixotrich. The human microbiome collectively possesses at least 100 times as many genes as the mere 20,000 or so in the human genome. Since humans depend on their microbiome for various essential services, including digestion, a person should really be considered a superorganism or a colony, consisting of his or her own cells and those of all the commensal bacteria. Since the bacterial cells also outnumber human cells by 10 to 1, if individual cells could vote, people would be a minority in their own body.

This is a startlingly different way of looking at our selves, not as an individual, but as a community. Babies get some of their essential life-sustaining commensal bacteria from their mothers, and other bacteria quickly inhabit the newborn body soon after it is born. Other bacteria move into the body after it dies. Between birth and death, we exchange our commensal bacteria with other human microbiomes during various states of intimacy, ranging from food preparation to sex, so we're constantly incorporating parts of other "selves" with our "self."

This is very consistent with many of the interesting views of life presented in Richard Dawkins' The Ancestors Tale (about which I'm finally able to blog). There were several interesting themes that weave through the book, but two really stood out for me.

First, it is incorrect to think of humans as "more evolved" than any other life form currently on the planet, and even more erroneous to think of ourselves as the pinnacle or apex of evolution. The entire succession of species wasn't a grand scheme to produce us any more than it was to produce any other species. In fact, every species existent today has been evolving to adapt to their selected niche for every bit as long as we humans have. The mere fact that a species exists now means that its evolutionary lineage is exactly as long as ours. Earthworms, say, have been continually adapting to a life foraging for nutrients in soil for as long as we've been adapting to our lives. The fact that they appear more "primitive" in structure and abilities is only a testament to their early success at achieving a solution to finding nutrients in soil. Improvements to the basic earthworm blueprint have continuously occurred, including cultivation of its own commensals, and the earthworm you might encounter in the garden is now superbly adapted to life in that garden, just as you are to life as the gardener.

Considering the complexity of the bacteria-within-the-mixotrich-inside-the-termite scheme, it is apparent that all life has been continuously evolving in complexity and specialization since the beginning of life on earth, and there is nothing special or sacred about the H. sapiens route, other than you yourself have probably followed that route if you're reading this.

My current boss once told me that, as proof of human's superiority over other species, if you gave paint brushes to 1,000 monkeys and left them for 1,000 years, you still wouldn't get a Sistine Chapel. True, I replied, but a Sistine Chapel is only important to human beings. A monkey would have no use for anything as superfluous as a Sistine Chapel and has no incentive to waste any energy creating any such thing. On the other hand, how many humans could survive naked in the forest living off the land? How many humans can shoot a thread out of its butt that's stronger per millimeter of diameter than steel, and then weave a web with it to catch supper? How many humans can engage in unassisted flight in the pitch-black dark of night, guiding themselves by echolocation? And so on.

The second lesson I learned from Dawkins is how arbitrary the concept of "species" is in the first time. Looking at life on earth, it is easy to distinguish humans from chimpanzees from spiders from bats, but it reality, these are not separate, distinct things but more like points on a continuum, like differences in height between tall and short. Let me explain.

Every individual animal born on earth is a genetic product of its parent, whether born of egg, born live, or otherwise. Each new generation of a species is capable or breeding with existent members of the previous generation of its species, as well as its own generation and, if its capable of living long enough, the following generation(s). This goes back all the way through evolutionary time. Each generation is the same species as its parents, but over time minute changes occur and as you go back thousands of generations, the sum of all these minute changes accumulate. In the case of humans, our 250,000th great grandparent was so different from us, that some of its other offspring have become, over the generations, chimpanzees.

That 250,000th great grandparent was not itself a chimpanzee, but the common ancestor of humans and chimps. But while it's not correct to think it was what we would call a Homo sapien, no distinct boundary exists in the generational record between our species and our 250,000th great grandparent, just as no distinct boundary exists between a chimpanzee and our 250,000th great grandparent. And if there is no distinct boundary between this ancestor and both chimps and humans, how can we say that there is a distinct boundary between humans and chimps?

If nothing ever died, that is, if every organism ever born went on living forever, we would see the continuum between all so-called species as plainly as we see the continuum in height between so-called "short people" and so called "tall people." But organisms do not live forever, and the fossil record has gaps due to the capriciousness of geologic processes, so life on earth has the appearance of separate species, but it only appears that humans, chimps, spiders and bats are all separate things.

And, as reported in the Times, what we called "humans" are actually microbiomic assemblages of commensals, supercolonies of so-called species sharing tasks of thinking, moisturizing, digesting, and so on. And since these commensals can jump ship from individual to individual (and possibly species to species, although I'm not sure of this), how individual are we really, and on what basis can we distinguish ourselves from chimps, spiders and bats?