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.

The Body Electric

We are juiced. From head to toe, miles of membrane shuttle electric charges through the body. Impulses pour in to my brain from eyes, ears, nose, mouth, and skin as raw versions of what I see on this screen, the feeling of the keys at my fingertips, the tapping sounds; then out from the brain through the wires to the muscles in my hands and fingers to type the s e  l e t t e r s.

Simple nerve systems appeared in early jellyfish and other sea creatures about 500 million years ago.  Loose nets of nerves responded to light and the touch of other creatures as these swimmers captured smaller fish and dodged bigger ones.

Much earlier, in the first fully developed cells, neurons began to evolve from membranes. A membrane, in Wikipedia’s words, is “a selective barrier; it allows some things to pass through but stops others.” A cell’s membrane helped the cell manage the salt levels inside the cell as it floated through the salty ocean. And since the salts of sodium, potassium and calcium consist of atoms with a positive or negative charge, the pores in membranes became gates that opened and closed to control the electrical potential across the membrane itself.

As animals evolved, such membranes lengthened into neurons with conductive axons, the “wire” of the nerve cell. In us, the longest axon runs down the length of each leg, branching as it goes. The shortest axons, fractions of a millimeter, fill our heads by the billions.

Neurons in the brain (Wikipedia)

Neurons in the brain
(Wikipedia)

The axons don’t carry an electric charge in the way that a wire carries electricity or a lightning bolt of electrons crashes to the ground. Instead, think of the wave at a sports stadium, where groups of fans stand up, throw their hands in the air, and sit down in a spontaneous sequence that moves through the rows. A nerve impulse moves down the axon in a similar way, charged atoms crossing through opened pores from one side of the membrane to the other and then quickly back again while the “wave” of the electric charge moves along.

The impulse never varies in strength. It is either on or off, either moving or only ready to move. There are no drops in the current, no power failures, no biological surge protectors needed. If a muscle must contract to move a load, the nerve signal, always at the same strength, simply repeats rapidly enough so that the muscle cells remain contracted.

At both ends of the axon, where the impulse begins and ends, devices of various kinds translate between the electrical charge and other structures. In the ear, sound waves cause small hairs to vibrate and set off the impulses that we hear as “hello.” In our eyes, light causes molecular changes that trigger the impulses to the brain to form the image we recognize as a chair. Where a neuron terminates at a muscle cell, the final “wave” triggers chemicals that start the muscle’s contraction.

We barely notice all this wizardry. Compared to the breath that we can feel and the blood we can see, our circuitry is undetectable. But if we’ve been shocked by a faulty toaster or we suffer from numbness or irregular heartbeats, we’ve glimpsed what can go wrong.

In another way, though, we are always aware of the electricity in us. Notice the faint tingle that is always present in our limbs and head. It’s a sense of animation, a potential, an ability to move a muscle, look around or think a thought at any time. That tingly readiness is, essentially, our neurons at the ready. It’s a reminder that we’re alive.