Our Actual “Eve”

She lived about 150,000 years ago in southern Africa. These days she is known as Mitochondrial Eve. The “Eve” part is a little misleading since unlike the Biblical Eve, Mitochondrial Eve wasn’t the first or only woman alive at the time, and there were also plenty of men around. Still, Mitochondrial Eve was an actual person to whom every human today, male as well as female, can be traced back on his or her mother’s side—from mother to mother’s mother and so on.

But interesting as such a linkage may be to scientists, how significant is Mitochondrial Eve for us? See what you think.

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)

Mitochondria (my-de-KAHN-dree-ah) are particles inside of cells that produce energy for the cell. Originally independent cells themselves, mitochondria were engulfed by larger cells long ago, proved useful, and made themselves at home.

When they did so, mitochondria brought with them their own bits of DNA. These strands are not related to, and are much smaller than, the complex DNA in a cell’s nucleus that make up our genes. But like all DNA molecules, as mitochondrial DNA makes copies of itself, it sometimes mutates; copying errors occur and the DNA changes slightly. As a result, mitochondrial DNA, handed down through generations of female humans, forms a record of our ancestry separate from our genes.

If this is difficult to visualize, a rough analogy is the battery in a car. These 12-volt energy-units that power the starter motor come in different brands with serial numbers and other codes on them. Over the years, independently of changes in cars themselves, battery manufacturers make changes to car batteries. Now imagine that you had no other way of telling the age of a car that had, say, been crushed beyond recognition. One option w ould be to dig out what was left of the car battery to find its codes or numbers. The car battery would date the car.

But if a particular version of mitochondrial DNA is passed down through women, how is it that males also carry it?  Because this DNA comes in the cell that each human grows from, and that cell is our mother’s. Fathers, through their sperm, contribute some of the genetic DNA that creates the new person, but the cell that begins to divide and multiply is mom’s, complete with her formulation of mitochondria DNA.

At the time that Mitochondrial Eve lived, of course, other mothers were passing along their own mitochondrial DNA to their own children, to their daughters’ children, etc. What happened to all those versions? Why is it that today’s humans everywhere carry the same version, the same mutation, of mitochondrial DNA? Apparently all those other lines of mitochondrial DNA fizzled out. Some mothers bore only sons, with no daughters to carry on their cell line. Other women had no children at all. The single remaining “brand” of mitochondrial DNA has been traced back to an approximate place and time five thousand generations ago. It is as if over the years all brands of car batteries went out of production except one, and that one is now installed in all cars.

What are we to make of all this? Compared to the Biblical Eve and her list of firsts—first woman, first human to be curious, first mother—Mitochondrial Eve wasn’t a forerunner of any of our significant traits. It’s that other  DNA, the genetic DNA from our mother and father, that plays a role in the color of our eyes and our musical aptitude.

Still, as biomedical historian Siddhartha Mukherjee puts it The Gene: An Intimate History:  “I find the idea of such a founding mother endlessly mesmerizing.”

It is mesmerizing to know that a small identifier in each of us can be traced back to a single human mother long ago. In theory, any diverse group of living things has a common ancestor after whom its descendants branched off. But that common ancestor may be difficult to “relate” to. The ancestors of all primates (not only humans but also monkeys, baboons, and chimps) lived about 60 million years ago and looked something like a squirrel with large eyes. I don’t feel the warmth.

On the other hand, I like the idea of being descended from a mother a long time ago from whom all other humans today are matrilineally descended as well, and whom we could, if we saw her, recognize as one of us.

400 Million Years of Ferns

Ferns are all leaf, all the time—no celebrated flower, no seduction of the insect. At the tips of green sprouts, curled fiddleheads unroll while leaflets widen behind them like the wakes behind boats. My store-bought Boston fern, tended for years with no expertise, is a fountain of green, its dazzling fans of leaves arching up, out, and over.

Before ferns, plants were creatures of the water, with no need for roots or stems. But about 450 million years ago, Earth passed through the first of its five mass extinctions (so far). The climate cooled, water froze, glaciers grew, the sea level fell, coastal and ocean habitats disappeared. But new land was exposed, and when the climate warmed again, plants grew there.

The first of these land plants were mosses. Carpets of moss sprang up along lakes and streams. But without deep roots or firm stems, mosses grew no higher than a few inches. Thriving on land requires tubes that can carry liquids up and throughout an organism. Ferns were among the earliest plants with roots to dig deep for water, and with stalks and stems to transport it throughout the plant. Vascular tissue changed everything. Ferns reached the height of trees.

Devonian ferns wikimedia

wikimedia

And ferns grew very efficient leaves. They needed them to capture carbon dioxide whose levels fluctuated widely over the millennia. Whenever the atmosphere’s declining carbon dioxide led to cooling temperatures, the hundreds of leaves on even a small fern could soak up enough to keep it going. Today, healthy ferns withstand cold weather better than most plants.

Ferns have multiplied the world over in part because of how they propagate. They were and are old-fashioned and deliberate about sex. Instead of reproducing through miniaturized plants embodied in seeds, ferns spread through spores, single cells released from the underside of the fronds. When the spores settle on moist soil, they grow into a tiny intermediate plant that will create the male and female components for the start for a new fern. Spore propagation seems an overly complicated, two-step process, but the lightness of the spores lets them disperse on the wind and germinate on distant moist soil.

Some days I stare at a plant whose distinctive features date back nearly 400 million years. No other plant or animal that I see every day goes back almost to the beginning of life on land. A fern’s ancestors invented leaves and roots and the old method of reproducing.  They survived four mass extinctions, as well as the flowering plants that burst on the planet 125 million years ago and came to dominate the plant kingdom. Ferns watched the dinosaurs come and go. Today they regale us with tales of ancient climates and their ingenious survival skills.