August 7, 2012 | 3
…like the rest of us. Fish tales, Piles of Ants and the Difficulty of Measuring the World
We have made great progress in biology but we still struggle to measure the limits of life. The one-eyed poet Jim Harrison has written that he has a count of the number of birds he has seen in his life. The count is, according to Harrison, “precise & astonishing.” We have no such literary luxury when counting the life around us more generally, no matter how we might choose to estimate and hope. In lieu of real counts we seem easily contented with estimates, however crude 1 .
The biologist E. O. Wilson has said that forty percent of the animal mass in tropical forests is made up of the living bodies of ants, ants running out of holes, ants running up trees, ants eating everything, everywhere. Wilson’s often repeated estimate (I’m among the guilty) derives from a study published in the journal Biotropica by Fittkau and Klinge in 1973. Working in tropical biology’s wild and wooly early days Fittkau and Klinge measured the biomass of vertebrates by shooting and trapping as many big animals aas they could during an extended period of 1970. They then weighed them. The weight of these vertebrates (which had been collected over an unspecified area opportunistically) was then compared to the best estimate of the mass of the mass of other arthropods, which came from an article by Beck (1971). When it came to ants, the best Beck could do was to assume (not measure) “that three quarters of the soil fauna, in terms of biomass, are ants and termites in the central Amazonian rain forest.” And so by following Beck who had guessed about the ants and termites Fittkau and Klinge produced an estimate of the proportion of living animal mass in a single Amazonian forest is made up of ants. Fittkau and Klinge’s estimate is not credible in any specific way and yet no one seems to have done better. The estimate, even if wrong, captures the essence of something tropical biologists know is true, there are a lot of ants.
[Image. Reeling in a big fish. Seldom in science to we understand just how big the creature on the other end of our line actually is. Source: Flickr, photo by Vik Banerjee.]
More recently, the many, many, studies of the species living in human guts almost always mention that trillions of individual bacteria live in human guts, ten times as many bacteria cells as human cells. Most studies fail to provide a citation for this estimate. A few cite a paper by Rodney Berg from 1996. In it Berg states that “The large intestine (colon) is the primary site of microbial colonization in humans and animals… Consequently, the large intestine harbors tremendous numbers of bacteria(1010-1011 bactera g--1 intestinal contents)…” Later Berg asserts that there are a total of 1014 bacterial cells in and on the average human, compared to 1013 human cells, which is to say ten times as many bacterial cells as human cells. Berg does not cite any source for this estimate. For other similar estimates in his paper he refers to books on the gut flora from the 1920s. I could not find the original estimate there either. One paper from 1924 does mention trillions of intestinal bacteria, but even then without citation. I’m going to keep digging for the original trillions of microbes reference (shoot me an email if you know it), but I will speculate that it is old and comes from studies done before we realized that most of the microbes in the gut cannot be grown in the lab. As I dig, I think it is worth wondering why we so readily cling to estimates, however crude, of the magnitude of the life around us?
I suspect there are multiple answers. Surely one is that as biologists we have experienced the grandeur and consequence of the particular organisms we study. We have seen the ants. We have considered the great density of microbes. We have gone to the mountain and found it to be wriggling with life. We have experienced these quantities and want to convey how sublime they are to those who don’t see them everyday. It is not just the number of individuals or their mass where we want estimates and so go wrong. Biologists love to cite references for the number of species on Earth even though nothing remotely similar for a consensus as to the number of species on Earth exists. If there is consensus, it is just that more remains to be discovered than has been discovered. How much more? Well, there are rough estimates, but we will leave the next generations to plunge into that darkness and fill it with light.
At heart biologists are often fishermen. They/we want you to know about the one that got away, how big it was, how amazing it was, how indescribable life, be it fish, microbe or ant, seems to always be. Nor are biologists alone. Astronomers sometimes feel the same compulsion to describe the indescribable realms in which they find themselves spending their lives. Carl Sagan was fond of calculating the number of stars in the universe. He famously described “billions upon billions” of stars. His math was rough and yet the big estimates he offered made clear that the universe was large enough to hold more grandeur than could our minds or scientific papers, grandeur and perhaps a little bit of extraterrestrial life.
Considering Sagan is actually useful in coming back to ponder the number of bacteria in our bodies. When Sagan was a very young man he married an even younger woman named Lynn. While Sagan looked up, Lynn looked down and around at the life around her. She studied the details of life and as she did she made a discovery. Building on the results of earlier Russian work, Lynn, who after she divorced Sagan and remarried would become Lynn Margulis, discovered that the cells in his Carl Sagan’s body, her body and those of everyone else were all bacteria. She imagined that in an ancient soup of life one bacterial cell engulfed another, however accidentally. The cell on the inside became the mitochondrion, the one on the outside, the host. The chimeric result is the cell from which all plant, fungal and animal life descended and descend, including, of course, Carl Sagan.
Margulis’s discovery is not normally considered to have anything to do with modern estimates of the magnitude of life or the universe and yet it has everything to do with these quantities. Margulis’s discovery means that each and every one of your cells is, by descent, a bacterial cell. Early on Margulis realized we are not just covered in microbes, we are microbes. Carl Sagan like everyone ever to have lived was composed of nothing but bacterial cells filled with liquids and gases and held together by bits of biochemical glue and signaling into the whole.
[Image. Carl Sagan whose body was composed of nothing but bacterial cells, signals and biochemical glue. Source: Planetary Society, 1980]
You might argue that the bacterial cells of which you are composed are so different than their ancestors that the linkage is one of metaphor rather than meaning. And yet there is this. Sometimes if a cell is removed from your body and grown in culture, it can be fed and grown like a bacterial cell. It can even–given the right (or wrong, since such cells are cancerous) mutations–(re)gain a sort of independent life, so much so that it has been argued that some cell cultures, derived from human cells raised in the wild, should now be regarded as new species. Your cells may be closer than you might think to being restored to their wilder life.
All of your cells are microbial. This truth is no cruder than the rough estimates of the number of microbial cells in your guts, the ants in the forest or anything else having to do with the grandeur of life. There are the ancient microbial cells you think of as “you,” then there are the far more genetically diverse cells in your gut and on your skin, cells that are less well coordinated and more at the mercy of fate and the environment and yet just as much part of you. Scrub the bacteria cells living in your gut or on your skin away and you would die. Scrub all of the microbial cells in your body away and well, there would be no cells left.
Where does this leave us when it comes to measuring life, counting our proverbial birds? It leaves us in the same place we started,ignorant but full of awe. Each biologist has their own count of the life they have seen, the real measure of which is more astonishing than rough numbers. Carl Sagan looked up to the sky to calculate the odds of finding more life “out there.” With the rover Curiosity on Mars, we are invited to ponder that question again. But as we do, it is worth remembering we still stumble when trying to quantify what is down here on this pale blue planet where no one can yet tell you how many species there are in your forest, how many species there are in your body, how many ants there are in the jungle or very much else about the measure of life except that it is so much bigger than we imagine. But maybe we should give ourselves a break. After all, we have figured out quite a lot given we are nothing more or less than the sophisticated coming together of trillions upon trillions of star-shaped cells. Trillions upon trillions, or more.
1-”As a child, fresh out of the hospital with tape covering the left side of my face, I began to count birds. At age fifty the sum total is precise & astonishing, my only secret.” JIm Harrison, from the poem “Counting Birds.”