The day before my baby was due to be born I was struck by the improbability of birth, of the clay of one body separating from the clay of another. I watched my wife sleep. Her heart beat once for every two beats of the baby’s heart, an ancient drumming of anticipation. As I waited, I comforted my wife and then, perhaps to comfort myself, imagined the broader story, all the millions of births it took to lead to this one. My child was to be the result of the next in the long line of amoebic splits going back to the tree-nesting ancestors, or farther still to the four-footed dog-kin, to the fish, to the eel-form, to the first golden cell. She was to be the tumbling culmination of the meeting of bacteria, of their descendant fish, of early mammals, of monkeys and then, in the last few humble generations, human couplings. Surely we could do this. If fish could do it…, and then I realized I must have said the last part aloud because my wife gave me a look that emphasized that one should not compare a pregnant woman to a fish.
Soon, my wife was in labor. We went the hospital, driving fast. Mild sedation was needed for my mother-in-law and then for me. None was available. In the ward, the nurses moved between rooms like cells in a body, mechanical and tired. The computer screen showed a baby born in room seven and the nurses could be heard shuffling in with advice. “You should put it on its back to sleep,” they told the mother. “Sign here to indicate you understand. Clip her toenails. Swaddle her, like this. Call if you need anything. Sign here. Do you have any pain? Let me show you how to pee, how to stand, how to walk, how to breast-feed. Does she have a name? We will need to write down a name.”
At some point between morning and Oprah (time marked by the TV no one thought to turn off in the background), our baby was born. The baby—our baby—came out and was, quite impossibly, alive. Our baby, so long incubating, breathed air. No one thought to check whether it was a girl or a boy, and for a long time no one except the baby seemed to breathe.
Our daughter was eventually handed back to my wife. She had been handed a mass of living, baby-shaped cells—cells composing a life separated from her by a metal paper clip hung on the umbilical cord. As for a name, “Olivia,” my wife, or someone said. My wife thought, through the stupefying ether of birth, that there should be a ceremony, a naming, a dance by a tree, a proclamation. She also thought that the father should do something, but I was pale, not with horror, fear, or nausea (though these feelings did not help), but with an unspeakable feeling of the impossibility of the process. My wife, Monica, had just been separated from another life and now there were two.
In the weeks that followed, we would both think many things—some profound, some ridiculous. At one point, perhaps after someone brought a celebratory six-pack of beer over to our house, it occurred to me to imagine the history of life like so many sausages connected to each other through the links of births and embryos (it is the inaptness of the sausage metaphor that makes me think that it might have been more than a six-pack). A string of pearls would probably have been more elegant, but I am recounting what I thought at the time, during that sleep-deprived week when even my metaphors were exhausted. Swammerdam thought he could see in the innards of a snail ever smaller snails, and this is what history offers us as a view of the telescoping of life. Through the embryo, we can see the beginning of life, all life. Framed by this view, whether it be of sausages, pearls, or snails, are Lynn Margulis and Carl Woese, neither of whom I was the least bit concerned about at the time, but both of whom now seem very relevant.
Birth and its indelicacies, in some very literal ways, are Lynn Margulis’s bread and butter. She usually thinks about the birth and origin of single-celled creatures, in which the process of reproduction is sometimes, though not always, similar to the version we know and love. Cells divide, just as my wife and daughter separated at birth, just as did Lynn Margulis and her daughter. Genetic material is apportioned to each of the two halves, now wholes. The process repeats for hundreds of millions of years, most lineages leading to more single cells, one lineage leading to us.
There are deeper, more interesting ways in which Lynn Margulis and, for that matter, Carl Woese, are birth’s somewhat startled midwives. Here it is worth reviewing the process of birth. The mechanics of where babies come from are simple enough. We all understand the bit about the stork, but let me fill you in on some specifics missing from the stork story. You may imagine that a baby begins half egg and half sperm. At some level this is true, but in other ways it is not. My daughter’s chromosomes, the DNA in the nuclei of her cells, come half from me and half from my wife. Those chromosomes met and elected, somewhat undemocratically, which genes would be expressed (in my daughter’s case a strong dose of blue eyes, curly hair, and whatever complex set of genetic machinery codes for stubbornness). The chromosomes are only part of the story, though. The cytoplasm that Olivia inherited, the sea within each of her cells, comes entirely from my wife. With that cytoplasm come the mitochondria, also all my wife’s. If the mitochondria and cytoplasm are important to who we are (they seem to be), my daughter is already more my wife than she is me. In fact, every human child ever born and nearly every animal ever born (with weird and kinky exceptions—blame the bugs), has been more her mother than her father.
One of the things that Lynn Margulis’s Serial Endosymbiosis Theory showed, or at least provided a context for understanding, was that the mitochondria in our cells remain distinct evolutionary units. Inherited only mother to child, they have many of their own important and ancient genes, but have lost the vast majority of the genes they would need to live outside of cells—in many cases thousands of genes. They are analogous to cave fish that have lived for millions of years in the dark, and so gradually lost the genes necessary to create eyes.* What remains is a subset of these genes for respiration. Your mitochondria, by respiring, are keeping you alive. Because all of our mitochondria date to a single instance in which one cell was engulfed by another, all mitochondria bear vestiges of the first mitochondrion on Earth. Of course, it is like the family ax that has been around for twenty generations. The handle has been replaced ten times and the ax-head five, but all the same, it is a very old ax. My daughter, in each of her cells, from the moment she was born, had a kind of split identity—part mitochondria, part all the rest. In the same way that it is difficult to think of my wife when she was pregnant as having been two individuals, it is difficult to think of my daughter, or any of us, as, on any given day, being made up of two sets of genes, each with their own subtly distinct prerogative. Our ideas of self are too strong to allow such dissembling. In a way, what Lynn Margulis offered was and is too strange for us to incorporate fully into our daily lives.
Our divided cellular house has implications for the stories we tell about ourselves. We now know that an ancient archaean cell engulfed, but did not digest, a proteobacteria cell that would become the mitochondria. From that day forward, there would be a dual and tangled story of the lineage. Although our story, as told by our cells’ nuclei and mitochondria, is a single story, those two partners do not tell it in the same way. They have, after all, traveled very different paths.
The genes in the nuclei of our cells are mixed in each generation (my wife’s and my genes mixing in our daughter’s cell nuclei, for example), but mitochondrial genes are not mixed. Each sperm cell has mitochondria enough to power it along its merry way, but not enough to pass them on to the egg. If we look back from our daughter, for example, into ancestral time, we see different stories depending on how we look. My daughter’s nuclear DNA will tell the mixed story of my wife and me and each of our parents and then, in turn, each of their parents. Go back just five generations and the nuclear DNA tells the story of no fewer than 64 separate lineages seeming to converge on my daughter. Those lineages and their stories wind around the Earth to all sorts of improbable places and eventually, going back far enough, converge on the small populations of our origin. But trace my daughter’s mitochondrial DNA and you will follow a simpler story. That story will take you solely via my wife’s lineage, to her mother’s lineage, to her mother’s mother’s lineage and then to that of her mother’s mother’s mother and so on. We know little of my wife’s mother’s mother, but it is from her that all of Olivia’s mitochondrial genes come. In Olivia’s genes are stories that we do not know from word of mouth, stories that go back to North Carolina, up to Michigan, and then to Nebraska, and from there we can only guess.
Each of our stories is sketched in its broadest sweeps in our mitochondria. Read these stories and you will miss the details of wars and love, but capture the broad drama of our lives. You will capture, among other patterns, that most of the human diversity on Earth (at least that encoded in mitochondria) exists in Africa. What left Africa were only a few mitochondrial lineages. Of those, one subset went on to Europe, a different subset went on to Asia, and then a further subset of those to the Americas. Other lineages, ferried in the shells of boats across the sea, made it to Australia, to the Pacific islands, and onward. This story, the story we would call the human story, is told uniquely by our mitochondria. It is the story of microbial evolution not obscured by the daily life of humans.
Our mitochondrial narrative eventually takes us back to the caves of our ancestry, those places where we knew only the animals around us and painted them on the walls. The narrative takes us still further to those earlier ancestors who climbed out of trees, back to a time before language when we howled at snakes and pointed, but did not distinguish much further the species we met. It takes us back to our monkey years, back to the time before that when we were big-eyed and nocturnal. It takes us back even further: dog years, mouse years, back to our amphibian slime.
Lynn Margulis divided us, but that we can tell the story of the symbiosis she discovered is in large part due to Carl Woese. It is now cheap and quick to do what Woese labored over in his office for so many years. You could do it for yourself in your basement, almost. Swab the cells of your cheek. You can send those cells in to have them decoded, your mitochondrial DNA or even your rRNA decoded to be read aloud, your one and only ancestral shout, “I am man-monkey, squirrel, fish, microbe.” Once decoded, those genes could be compared to the rest of life. You would map somewhere near me, somewhere near all vertebrates on a densely twigged branch on the tree of life. We are huddled in our branch of the tree, shivering in evolution’s winds, holding our babies up one by one, that our buds may continue to offer forth, flower, year after year, a blossom nearly invisible on the great tree but, all the same, lovely and intricate. We are on the big tree, small, nearly unnoticeable, much as the rest of life tends to be for us in our daily lives. For the record, Woese and Margulis would map close to you too, in those few inches of stem or leaf.
Woese, like Margulis, keeps working, keeps stepping boldly forward. Part of what he has dedicated the remaining years of his life to (and how he divined that the original host into which mitochondria moved was an archaea) is following rRNA back even further to the earliest common ancestor.
Woese already believes he sees some parts of the early history clearly. For him, there is evidence for aspects of Margulis’s story (that mitochondria and chloroplasts evolved through symbiosis) and for his own story of who was who in those early relationships. He sees the first consumption. An archaea consumed a non-archaea. The non-archaea would become the mitochondria. The resulting lineage would diverge. It would become one of the three major lineages on Earth. Lynn Margulis disagrees with Woese about what the host cell was and about how and whether we might ever know for sure. It seems, in some respects, a small debate, but it is sometimes overlooked that what Margulis and Woese and others are essentially arguing about are the most significant events in evolutionary history. We all descended from a bad case of indigestion. A cell consumed another cell, but could not complete the task. These steps were repeated. From those early gastrointestinal dilemmas came us, each of our cells alive with no fewer than two species, maybe more, their genes replicating as we walk and talk, and as they do, passing their separate stories on that they might be read, by scientists like Woese and Margulis, or simply by the machinery of the cell they were intended for.
Margulis’s theory of serial endosymbiosis is enough for one scientific life, but the symbiotic event that led to our mitochondria-bearing cells is just part of her theory. She has pushed symbiosis much further, pushed it close to breaking, pushed it until what we know gives way to pure speculation. In part, her bigger ideas have to do not with whether or not endosymbiosis occurred and occurs, but where it fits in our understanding of life. Endosymbiosis—and more specifically, endosymbiogenesis, that new lineages evolve through novel symbioses—is now accepted. It has gone from the margins to being a standard part of our view of life. It is discussed as fact in a few paragraphs in every biology textbook, but in those paragraphs something seems to be, at least to Margulis’s broad view, missing.
She offered her endosymbiosis theory as a general theory of how the biggest changes in evolution occurred. She offered it not as a particular story for our cells but instead as the big, central story of all evolution. Endosymbiogenesis is, for Margulis, the origin of new lineages. All the rest, from Darwin, Wallace, and the competition-mad scientists who followed them, is marginal. Natural selection whittled life down, but symbiosis engendered it in the first place.
This grander place for symbiosis in our understanding has not been realized. Biologists accepted the symbiotic origin of mitochondria and chloroplasts and then pushed symbiosis back out to the margins. Margulis was accepted into the National Academy of Sciences, lauded by her peers and then, when she kept offering broad visions of symbiosis, such as the suggestion that all speciation events occur due to symbiogenesis, ridiculed a little. Enough is enough with the crazy ideas, many seemed to say.
How important are symbiosis and symbiogenesis in evolution? I don’t know the answer. I do know, though, that if Margulis is right, we will have to do more in our textbooks than add for her a few quiet lines. Part of the resistance to the view of science that she offers may just be that it remains new or that in her most extreme statements, she is, simply, wrong. Another possibility is that her most extreme views, even if right, break down what we have known since Linnaeus and even before. They break down what we have long assumed about ourselves.
The dominant story of evolution that we tell today remains the story of an evolutionary tree, its branches ever more divergent and full. Lichens confuse this story in that they are a merger of algae and fungi. Their evolution wraps together two separate evolutionary stories so tightly that they become one story with complicated roots. In Margulis’s view of the world, all evolutionary trees go back to tangled roots, and the deepest histories of our lives are, in a way, both inseparable and indecipherable.
We are unprepared to consider this kind of story of our past. We are unprepared for it in part because it means we may not be able to trace back each of our lineages cleanly. We may not be able to reconstruct each bend and dip of our tree, as Woese would like. The story of early life, in Margulis’s view, depends in part on who is telling the story, whether, for example, we tell the story via our mitochondria or their surrounding hosts. But she also sees deeper problems. More and more there is evidence for additional complexity, for the frequent swapping of genes not only within the domains of bacteria and archaea, but also the sharing of genes relatively frequently across the domains. That microbes pick up genes from their environment, genes from other lineages, is accepted. What remains in question is how common such trades are, whether they are common enough to call into question our ability to see clearly the early histories of life, or just common enough to muddy the picture. If, as is beginning to seem likely, many genes have been swapped, then there may be different evolutionary trees for different genes—and for each gene a different story, each correct in its own way, of the early history of life.
Both symbiosis and early swaps of genes also call into question our sense of individuality. The symbioses in our own bodies means that we are not one individual with a single history and a single body, but multitudes. We find it convenient to consider ourselves to be composed of host cells aided by our cellular helpers—slaves, our mitochondria have even been called. If we believe Margulis’s story, though, this is wrong, and we are every bit as beholden to our mitochondria as they are to us. We could not survive without them. Each of our parts has split identity. Our words and ideas come not from the “eukaryotic self” but from the combined self. In the same way that my pregnant wife was, in some meaningful way, two, each of our cells is divided. This is no mincing of words. It is inescapable.