We Are All Descended From an Actual “Eve.” So?

She lived between 100,000 and 200,000 years ago in southern Africa. These days she’s known as Mitochondrial Eve, but that’s a little 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 everyone alive today—male and female, all 7.6 billion of us—is connected through their mothers by a speck of DNA.

But as important as such a linkage may be to scientists, how significant is it for the rest of us? Frankly, I’m not sure. See what you think.

Every cell in any organism contains small particles that keep the cell alive. The  nucleus, with the genetic DNA masterplan of the body, is the cell’s control center. Smaller particles carry out other functions. Mitochondria produce energy for the cell. They contain their own, separate, bit of DNA because millions of years ago they were free-floating bacteria that were absorbed by cells, proved useful, and took a permanent place in the cell anatomy.

Mitochondria in a cell (Flickr)

Mitochondria in a typical cell. The long thread of genetic DNA in the nucleus is shown but not the bits of mitochondrial DNA, which are incidental and much smaller. (Flickr)

Over time and countless cell divisions, and separate from any mutations in the genetic DNA, the DNA in the mitochondria also changed in small ways. As a result, the early apes, then the pre-humans, then the earliest modern Homo sapiens all carried the slight variations in mitochondrial DNA that they inherited.

But they inherited them only through the females. Males couldn’t pass theirs along. Why? Because we inherit our cellular structure from mom’s egg. While men may deliver their genetic DNA by sperm to the egg, it’s mom’s egg cell itself that grows into the embryo and into all human cells. Complete with the mother’s mitochondria.

Over the course of five thousand generations or so, women around the world passed their variations of mitochondrial DNA 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.

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 the other Eve, 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, as Siddhartha Mukherjee writes in The Gene, without elaborating, “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, whether the same species or not, is the individual or type after which subsequent 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, 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’s a little less difficult to imagine in the case of the most recent MRCA, the one who looked a lot like us. Maybe Mitochondrial Eve’s value lies here after all: by thinking about her, we may be learning to wrap our heads around the reality of many 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 the brain from eyes, ears, nose, mouth, and skin as electric translations 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, 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.