Fossils, Microbes, and King Kong

Seeing may not always mean believing, but it helps. Especially when the object is big, important, and embedded in a good story. People who might otherwise doubt the reality of dinosaurs nonetheless have the bones and the movies to help persuade them.  As do those who may still doubt our descent from apes. The story of King King may not convince them, but it makes an argument: the giant reptiles are followed by the giant ape who succumbs to a woman and to the modern city. Popular culture loves fossils, giants, and a tragic romance

The trouble is that what we think of as fossils—old bones, mosquitoes trapped in amber, hardened bits of plants—date back no more than 600 million years. That leaves three billion more years before that, six times further into the past, with no familiar evidence to vouch for it. For there were no animals or plants during those billions. There were only microbes, single cells reproducing, clumping together sometimes, like their descendants today in our gut. Little wonder that those eons are never the backdrop for any saga of the bacteria and archaea, No bones from which to reconstruct giants, no fossils to serve as relics to fire the imagination.

Or almost none.

Stromatolites in Australia, probably looking much as they did 3.5 billion years ago. (www2.estrellamountain.edu)

U Microfossils from 3.5 billion years ago (www2.estrellamountain.edu)

 

 

 

 

 

 

 

Start by searching for the oldest rocks. Try Australia, Greenland, or South Africa to find the ones that formed four billion years ago. Slice them thin, put them under a microscope, look for microfossils, tiny creatures’ cell walls that have mineralized into tough material. And look for chemical smears of carbon or the products of the earliest photosynthesis.

Elsewhere look for petrified stromatolites, the layered habitats of colonies of bacteria that filtered sea water for nutrients as far back as 3.5 billion years.

Or look for bands of rust in ancient rocks. Such rust means iron and oxygen, abundant oxygen given off by bacteria as a waste product, enough of it two billion years ago to poison the atmosphere for bacteria that couldn’t tolerate it and then make it a necessity for those microbes that could.

banded iron (britanica.com)

Banded iron (britannica.com)

 

 

 

 

 

 

But could the tiny remains and the odd colors and chemicals that you might find, ancient though they are, actually become the stuff of museum exhibits and monster movies? Could they find their place in popular culture as both entertainment and subtle education, as dinosaurs and apes have?

I think so. It’s not difficult to imagine exhibits of interactive, oversized ancient microbes. Or Hollywood dramas about invasive bacteria from a billion years ago, defeated in the nick of time by oxygen saturation while the heroic Caltech professor of Biogeochemistry (an actual field) explains that the godzillas of 200 million years ago were softies compared to the early microbes that thrived in volcanic vents.

Then our sense of the marvels of our ancestors would reach back through the full history of life.

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.