Do I contradict myself?
Very well then I contradict myself,
(I am large, I contain multitudes.)
—Walt Whitman, “Song of Myself”
Creationists who think themselves clever sometimes ask why, if evolution is true, we don’t see bacteria re-evolving into multicellular organisms. But, as is often true of creationists, this taunt gets the situation completely backwards. Evolution is not a teleological march of progress but a blind algorithm, concerned exclusively with reproductive success. Looked at in this light, a far better question is why multicellular life ever evolved at all. In the metrics of success that evolution is concerned with, bacteria are our undisputed superiors. They far outnumber us; they thrive in environments we could never survive in; and, if they were ever to disappear, all of the so-called higher life on this planet would swiftly follow them into extinction.
To drive this point home, consider this mind-boggling statistic: in the body of every healthy human being, there are ten times as many bacterial cells as there are human cells. If this seems impossible, realize that bacteria are prokaryotes, lacking the complex internal structure of, and therefore much smaller than, our own eukaryotic cells. Most of the bacteria in the human body live in the intestines, and the 15 trillion or so bacteria in a healthy gut would just about fill a ten-ounce soup can.
Our gut flora are not invaders or pathogens, but an important part of the body’s normal functioning. Far from being mere passengers, they live in an intimate symbiosis with the cells of the digestive tract. They play a vital role in extracting nutrients from food: mice raised in absolutely sterile environments need to eat 30% more to stay at the same weight as normal mice. (This partnership is more prominent in herbivores like cows or termites, whose resident bacteria enable them to digest the woody plant tissues that no animal could otherwise eat – but our natural bacteria do the same for us, helping break down complex carbohydrates and proteins that the body cannot digest on its own.) In fact, it’s believed that the gut flora are a vital barrier against disease: by occupying all the niches within the body, they leave less room for genuinely pathogenic species to get a toehold.
Jessica Snyder Sachs, in her book Good Germs, Bad Germs, quotes an unforgettable portrait of the microbial jungle that lives just in the human mouth alone:
…I was able in my mind’s eye to zero in on the little fleshy crevices around Tom’s and Jenny’s teeth as they ate their meal and to see the turmoil of microbic life there, the spirochetes and vibratos in furious movement, the thicker corkscrew-like spirilla and vibrios gliding back and forth and the more sluggish or quiet chains and clusters and colonies of bacilli and cocci, massed around or boiling between detached epithelial scales and the fibers and debris of cells and food particles. Like the great and beautiful animals in whose mouths they live, these too are organisms, living things; and I could imagine them, quite like Tom and Jenny, making the most of the sudden accession of nourishment after a long fast. (p.36)
All our cells are… stuffed with bacteria which have become so transformed by generations of co-operation with the host cell that their bacterial origins are almost lost to sight… [They] have become so intimately enmeshed in the life of the eukaryotic cell that it was a major scientific triumph to detect that they were there at all. (p.537)
Each of our cells contains organelles called mitochondria. Colorfully speaking, they are the cell’s power factories – the reason we breathe oxygen is so the mitochondria can use it in the production of ATP, an energy-carrying organic molecule that’s used as a common currency for many of the cell’s chemical reactions. Mitochondria also have their own DNA – not the complexly packaged chromosomes of our own nuclei, but simple circular chromosomes similar to those of bacteria – and are capable of self-replication, dividing autonomously within the cell.
The evolutionary biologist Lynn Margulis is famed for discovering the origin of mitochondria: as their separate DNA hints, they were once free-living bacteria. Long ago in the mists of evolutionary history, these bacteria merged with the cells that would ultimately become our ancestors. Both cells benefited from this symbiotic partnership, and so it persisted and thrived. Over long stretches of time, the mitochondria lost most of their DNA, as it was no longer needed in their new environment. Though they have become so intertwined with us that they are no longer capable of independent existence, they remain as traces of the ancient eukaryotic union that was the first flowering of life recognizably like ours. Incredibly, even today, we can guess at the identities of those species involved in this historic partnership. From Richard Dawkins again:
…molecular comparison tells us the particular group of bacteria from which mitochondria are drawn. Mitochondria sprang from the so-called alpha-proteo bacteria and they are therefore related to the rickettsias that cause typhus and other nasty diseases. (p.539)
Though their wild relatives cause us grief, the tame mitochondria are essential to life as we know it. And not just our life, either: chloroplasts, the organelles that give green plants their color and make photosynthesis possible, are, like mitochondria, the remnants of an ancient symbiosis. For both plants and animals, the seen depends on the unseen: each of us is not a single organism but a myriad, a kind of ecosystem made up of hundreds of different species. If it were not for our microbial partners, our kind of life could not exist. This insight, and the humility it brings, are worth remembering whenever we Homo sapiens are tempted to imagine ourselves the crowning glory of life on Earth.