It’s Diversity All the Way Down

“The most impressive aspect of the living world is its diversity. No two individuals in sexually reproducing populations are the same, nor are any two populations, species, or higher taxa [categories of organisms]. Wherever one looks in nature, one finds uniqueness.” So wrote Ernst Mayr in This is Biology, published in 1997.

Grains of sand under an electron microscope (wikipedia)

Grains of sand 
(wikipedia)

Part of his statement was a new idea to me. Clearly each species differs from the next. But I had not fully absorbed the notion that every organism, if it reproduces in pairs, is different from every other individual in its species. (Single-cell organisms like bacteria that divide into identical clones are the exception.) Every individual grass plant, every fish, every pure-bred dog, every ant is as different from another of its species as two human neighbors are. And, as Mayr adds, that makes uniqueness the order of the day.

But what about  diversity and uniqueness in the non-biological, inanimate world? “Nature” includes not only living things but also rocks, water, air, light and other forces and materials. They seem to be unique in their own ways. Snowflakes are famously singular. Clouds change constantly. So does the surface of the ocean. Air flows and spins. I’m pretty sure I’ve never seen two rocks that are identical. It’s a good bet that every asteroid, planet and star is different from others. Looking out over the desert, the ocean, or the skies, we always witness diversity in shape, motion, color and light if we look closely enough.

Diversity and fertility in grass (www.kvkcard.org)

(www.kvkcard.org)

Still, Mayr seems right that the diversity of living things  “impresses” us in a distinct way. Each organism succeeds at being alive, yet does so in a slightly different way from the others.

Moreover,  that booming variety, that hedge against species failure, comes on fast and strong. New life thrusts itself at us—in the new baby, in a puppy, among the trees springing up in corners of the yard, in the horde of ants and bees and birds of summer. In Origin of Species, Darwin wrote, “There is no exception to the rule that every organic being naturally increases at so high a rate, that if not destroyed, the earth would soon be covered by the progeny of a single pair.”

Diversity multiplied by fertility.

 

Our Actual “Eve”

She lived between 100,000 and 200,000 years ago in southern Africa. These days she is known as Mitochondrial Eve, but the “Eve” part is misleading. Unlike the Biblical Eve, she wasn’t the first woman nor was she the only woman alive at the time—and there were plenty of men around as well. Still, Mitochondrial Eve was an actual person. We don’t know much about her except that she is the most recent woman to whom every human today, male and female, can be traced back on his or her mother’s side—from daughter or daughters back to mother, back to the mother’s mother, and so on.

But interesting as such a linkage may be to scientists, how significant is Mitochondrial Eve for us? I’m not sure. See what you think.

Mitochondria produce energy. Originally independent cells themselves, they were engulfed by larger cells long ago and made themselves at home. They brought with them their own tiny DNA molecules that are unrelated to the DNA of the cell itself that make up our genes and reside in the cell nucleus.Mitochondria in a cell (Flickr)

Mitochondria in a typical cell. The long thread of genetic DNA in the nucleus is shown, but the unrelated DNA inside the mitochondria is not. (Flickr)

But the bits of DNA in the mitochondria, like genetic DNA, mutate over time; they change slightly as they copy themselves. These variations in a cell’s mitochondrial DNA were handed down through generations of pre-human primates and then early humans themselves, a trail of inheritance separate from our genes.

All of this is difficult to visualize. Here is a rough analogy. Automobiles have their own specialized energy component, the 12-volt battery that cranks the starter motor. Car batteries come in different brands and shapes with coded serial numbers and dates on them. Over the years, independently of yearly changes to cars themselves, battery manufacturers make changes to car batteries. Now imagine—it’s admittedly a stretch—that if you had no other way of knowing when a specific car model first went into production, you could get an approximate date by examining the style and code numbers on the car battery.

The variations in mitochondrial DNA serve a similar purpose. All humans inherit in only through their mothers. Males don’t pass theirs along. Why? Because the basic parts of our cells come from the woman’s ovum. Fathers deliver their genetic DNA by sperm to the egg, but the egg cell itself that divides into two cells, then four cells, and so on, is mom’s. Complete with her mitochondria.

Over the course of five thousand generations or so, women around the world passed their mitochondrial DNA, with its small but distinctive variations, to their daughters. Along the way, though, some mothers bore only sons and other women had no children at all. Gradually, all the variations of mitochondrial DNA fizzled out, except one. We all carry it, as did a woman a long time ago, Mitochondrial Eve. As if all lines of car batteries, in car models that changed or were discontinued, went out of production except one.

What to make of all this? Compared to the Biblical Eve and her list of firsts—first woman, first human to be curious, first mother—we have little to show for our ancestry from Mitochondrial Eve. And the merging of genetic DNA from our mother and father has by far a greater influence on who we are and what we’re like. By comparison, Mitochondrial Eve is just a woman a very long time ago whom we all happen to be linked with inconsequentially on our mother’s side.

Still, the biomedical historian Siddhartha Mukherjee writes in The Gene, “I find the idea of such a founding mother endlessly mesmerizing.” For Mitochondrial Eve is one of our Most Recent Common Ancestors—an MRCA. The MRCA for any group of organisms  is the individual after which later generations evolved in different directions. The MRCA of primates (humans as well as chimps, apes, monkeys, baboons) lived 65 million years ago. The MRCA of all animals lived 600 million years ago. And the MRCA of all living things, 3.6 billion years ago. For many people, interesting to know but not so easy to imagine.

But it is possible with some effort to envision the Most Recent Common Ancestor who looked a lot like us. Maybe Mitochondrial Eve’s value lies here: by thinking about her, we may be getting better at wrapping our heads around the reality of even older ancestors who seem impossibly ancient yet who made us what we are.