Widowmakers, Mitochondrial Eve, and Retrospective Coronations

45. WIDOWMAKERS, MITOCHONDRIAL EVE, AND RETROSPECTIVE CORONATIONS

A woman in New York City may suddenly acquire the property of being a widow by virtue of the effects that a bullet has just had on some man’s brain in Dodge City, over a thousand miles away. (In the days of the Wild West, there was a revolver nicknamed the Widowmaker. Whether a particular revolver lived up to its nickname on a particular occasion might not be settled by even the most exhaustive examination of the scene of the crime.) The example gets its curious capacity to leap through space and time from the conventional nature of the relationship of marriage, in which a past historical event, a wedding, is deemed to create a permanent relation—a formal relation, not a causal relation—of interest in spite of subsequent wanderings and concrete misfortunes (the accidental loss of a ring, or the destruction of the marriage certificate, for instance).

The system of genetic reproduction is natural, not conventional, but it works like clockwork, and that very systematicity permits us to think formally about causal chains extending over millions of years, causal chains that would otherwise be virtually impossible to designate or refer to or track. This permits us to become interested in, and reason rigorously about, even more distant and locally invisible relationships than the formal relationship of marriage. Speciation, like marriage, is a concept anchored within a tight, formally definable system of thought, but unlike marriage it has no conventional saliencies—weddings, rings, certificates—by which it can be observed. Speciation is also a curiously “long-distance” phenomenon, in both space and time. Species are fuzzy-edged collections over time, and we could only retrospectively (and arbitrarily) identify some individual organism in such a crowd as the Prime Mammal (so don’t bother). We can see this feature of speciation in a better light by looking first at another instance of retrospective crowning, the conferring of the title of Mitochondrial Eve, which is not arbitrary.

Individual organisms have rather crisper boundaries and hence clearer identities than species, but even with them there are plenty of fence-sitters. Let’s take the most striking instance: of the ten trillion or so cells that are crowded together inside your clothes, nine out of ten are not human! Yes, symbiont visitors in thousands of species outnumber your host cells—the cells that descended from the zygote formed by your parents’ union. There are not only bacteria but also eukaryotes, both single-celled microbes and multicellular organisms: fungi, mites in your eyelashes and elsewhere, worms microscopic and bigger, and who knows what else. You are a walking ecosystem, and while some of the visitors are unwanted (the fungi that cause athlete’s foot, or the bacteria that lead to bad breath or crowd around any infection), others are so essential that if you succeeded in evicting all the trespassers, you would die. Since these symbiont cells are generally much smaller than your human cells, by weight you are mostly human, but their combined weight is not negligible either—probably several pounds, and maybe as much as ten. Then there are the viruses, in even greater numbers.

Still, in spite of your porous boundaries, you—like other individual organisms—are readily distinguished from the others, and sometimes we can single out a specific individual organism for a specific role in evolutionary history. Mitochondrial Eve is one of the most famous. Mitochondrial Eve is the woman who is the most recent direct ancestor, in the female line, of every human being alive today. We all have mitochondria in our cells, and they are passed to us through the maternal line alone, so all the mitochondria in all the cells in all the people alive today are direct descendants of the mitochondria in the cells of a particular woman, named Mitochondrial Eve by Rebecca Cann, Mark Stoneking, and Allan Wilson (1987).

Mitochondria are tiny organelles in our cells that play a central role in metabolism, capturing energy from food to use in all the body’s projects, and mitochondria have their own DNA, a telling trace of their symbiotic origin several billion years ago. By analyzing the patterns in the mitochondrial DNA of the different people alive today, scientists have been able to deduce roughly how recently Mitochondrial Eve lived, and even where she lived. Early calculations suggested that Mitochondrial Eve lived in Africa about three hundred thousand years ago, but that number has been refined more recently: she lived (in Africa, almost certainly) only two hundred thousand years ago. Deducing where and when is a far trickier task than deducing that there was a Mitochondrial Eve, a conclusion that follows from tracing the surviving maternal lineages back to their common ancestor. Consider a few of the things we already know about Mitochondrial Eve, setting aside the controversies. We know that she had at least two daughters who had surviving daughters. (If she had just one daughter, her daughter would wear the crown of Mitochondrial Eve.) To distinguish her title from her proper name, let’s call her Amy. Amy bears the title of Mitochondrial Eve; that is, she just happens to have been the maternal founder of today’s line of people. It is important to remind ourselves that in all other regards, there was probably nothing remarkable or special about Mitochondrial Eve; she was certainly not the First Woman, nor the founder of the species Homo sapiens. Many earlier women were unquestionably of our species, but none of them, as it happens, was the most recent source of all the mitochondria in all the people living today. It is also true that although Mitochondrial Eve had daughters and granddaughters, she was probably not noticeably stronger, faster, more beautiful, or more fecund than the other women of her day.

To bring out just how un-special Mitochondrial Eve—that is, Amy—probably was, suppose that tomorrow, thousands of generations later, a virulent new disease were to spread around the earth, wiping out 99 percent of the human race in a few years. Suppose the survivors had inherited resistance to the virus and were closely related through their maternal lines. Their closest common direct female ancestor—call her Betty—would be some woman who lived hundreds or thousands of generations later than Amy, and the crown of Mitochondrial Eve would pass to her, retroactively. She may have been the source of the mutation that centuries later became a species-saver, but it probably didn’t do her any good, since the virus against which it triumphs probably didn’t exist in its virulent form then. The point is that Mitochondrial Eve can only be retrospectively crowned. This historically pivotal role is determined not just by the accidents of Amy’s own time, but by the accidents of later times as well. Talk about massive contingency! If Amy’s uncle hadn’t saved her from drowning when she was three, none of us (with our particular mitochondrial DNA, thanks ultimately to Amy) would ever have existed! If all of Amy’s granddaughters had starved to death in infancy—as so many infants did in those days—the same oblivion would be ours.

The same logical argument establishes that there is—must be—an Adam as well: the closest direct male ancestor of every man and boy alive today. We can call him Y- chromosome Adam, since all our Y-chromosomes pass down through the paternal line just the way our mitochondria pass through the maternal line.1 Was Y-chromosome Adam the husband or lover of Mitochondrial Eve? The definitions give us no reason to assume so. Paternity being a much less time- and energy-consuming business than maternity, what is logically possible is that Y-chromosome Adam lived very recently, and was very, very busy in the bedroom—leaving Errol Flynn in his, um, dust. If the oldest living man today is, say, 110 years old, it is logically possible that Y-chromosome Adam was his father, an early-twentieth-century Don Juan who is also the father or grandfather or great-grandfather, and so on, of all the younger living males. We males make billions of sperm, after all, hundreds of millions in each ejaculation, so it wouldn’t take Y- chromosome Adam more than a week to make enough sperm to father the whole human race (in principle)! But by counting all the genetic differences in men’s Y-chromosomes around the world, and calculating how long it would take for that much mutation to accumulate, we can estimate the actual date for Y-chromosome Adam as somewhat less than one hundred thousand years ago. And again, if a plague struck down half, say, the male population, in all likelihood the crown of Y- chromosome Adam would pass to a much more recent progenitor.2

A curious fact about every individual organism—you, or me, or your dog, or your geranium, for instance—is that it is a potential founder of a new species, the first of a long line of whatchamacallits, but it will be hundreds or thousands of generations before whatchamacallits stand out from the crowd enough to be recognized as a species, so the coronation would have to occur long after you, or I, or your dog, or your geranium had returned to dust. Your parents could thus turn out to be the most recent common ancestors of all the members of two humanoid species, but don’t hold your breath. Chihuahuas and Great Danes are members of the same species, Canis familiaris, but if civilization collapses and their respective offspring go feral, they could more readily achieve the breakaway of speciation than, say, beagles and Basset Hounds, since without human help, fertilization of Chihuahuas by Great Danes, or vice versa, is unlikely. In all likelihood, however, both lineages would go extinct, as have most lineages over the eons, before that happened.

It is estimated that well over 99 percent of all the organisms that have ever lived have died without having had offspring. And yet here you are: of all your billions of ancestors over the years, from single cells to worms to fish to reptiles to mammals to primates, not a single one of them died childless. How lucky you are! Of course every blade of grass has an equally long and proud heritage, and every mosquito, and every elephant and every daisy.

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1 Note one important difference between the legacies of Mitochondrial Eve and Y-chromosome Adam: we all, male and female, have mitochondria in our cells, and they all come from our mothers; if you are male, you have a Y-chromosome and got it from your father, whereas virtually all—but not quite all—females have no Y-chromosome at all.

2 Setting logical possibility aside and looking at actual history, we see that there have been men who tried to monopolize the siring in their populations, and apparently sometimes succeeded to an astonishing degree: powerful kings and warriors who took and impregnated hundreds of women. They probably have made a disproportionate contribution to our human gene pool, for better or worse.