Life Before Fossils

king-kong-killing-pterodactyl-1024x766Seeing may not always mean believing, but when it comes to living things from millions of years ago, it helps. A skeptic these days would have difficulty doubting the reality of dinosaurs given all the bones in museums and the reconstructions come to life in countless films. When embedded in an adventurous and romantic story, oversized reptiles and even King-Kong-size versions of our primate ancestors put persuasive passion and flesh on the cool scholarship of paleontologists.

The trouble is that the stuff of the usual fossil history—old bones, insects trapped in amber, hardened imprints of early plants–date back no more than 600 million years. Such a number may seem very old, but from another perspective it is not nearly old enough. For life has been traced back three billion years before that, six times further into the past. It’s not surprising that life from that long ago is not the material for theme parks or movies about entrepreneurs like Carl Denham who searched out Kong’s island. For life was small for the first three billion years, with no animals or plants as such. There were only microbes, single cells that gradually acquired the complexities of modern cell life—a nucleus, the hunger for oxygen, sexual reproduction. But there are no two-billion-year-old bones from which to reconstruct cellular giants, no fossils to serve as relics to fire the romantic imagination.

Or almost none.

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

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

To find them, you have to search for the oldest rocks. Try Australia, Greenland, or South Africa for those that date back almost four billion years. Slice them thin, put them under a microscope, look for microfossils measuring a fraction of a millimeter, their cell walls mineralized into tough material.

 

 

stromatolites layers pinterest

petrified stromatolite (pintrest)

 

 

And 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.

 

 

 

But could the tiny remains and traces of chemicals from billions of years ago become the attractions of crowded museums and movie fantasies? Could they find their place in popular culture as both entertainment and subtle education, as dinosaurs and apes have?

I believe they could.  It’s not difficult to imagine oversized reproductions of ancient microbes which kids could walk through while trying to avoid getting snagged on strands of DNA or thrown off-balance by the cell’s motion from its flagellum, its tail. And climate change sets the stage for a movie thriller about bacteria mysteriously resurrected from three billion years ago that thrive on carbon dioxide and for whom oxygen is poison.

Then our wonder at the marvels of our pre-human ancestors would reach back through the full history of life.

 

The Pioneers: Archaea and Bacteria

For many years I shared the common belief that living things fall into three or four basic categories. Besides plants and animals, there are, I thought, one or two others groups that consisted of creatures too small to see, with names that varied over the years—Bacteria, Protists, Prokaryotes.

phylogenetic tree wikipedia

In this evolutionary genetic tree, animals and plants, in the upper right corner, are not the main limbs.         (Wikipedia)

Today there are still three categories, called Domains, but they are all too small to see. The only familiar name is Bacteria. Plants and animals are now small print in another Domain called Eukaryotes (you-CARRY-oats), meaning cells with a nucleus.

The third Domain is the Archaea. Archaea are like Bacteria in that they have no nucleus and are simpler, smaller and older than Eukaryotes. I’ve known so little about Archaea that I wasn’t even sure how to say the word. Either AR-kee-ah or ar-KY-a is acceptable. That noun is plural; the singular is AR-kee-on, an Archaeon, sounding faintly of Star Wars.

So how are these Archaea so different from Bacteria that they get their own subdivision? Biologist Carl Woese in 1977 argued they are indeed a different form of life. He showed that in much of their chemical make-up and their genetic sequencing, Archaea not only are distinct from Bacteria but are in some ways closer relatives than Bacteria to the Eukaryotic cells that form plants and animals.

I’ll describe a few features that Archaea and Bacteria have in common and then some features that are unique to Archaea. The information, from Wikipedia and elsewhere, is quite specialized and my renderings of it are admittedly general and selective.

Both Archaea and Bacteria are small, unstructured, and simple compared to the Eukaryotes that came after them. But one achievement they both share has been to try out nearly every possible chemical or environmental source to get their energy. Sunshine, salty water, temperatures ranging from volcanic to polar, even radioactive settings—varieties of Bacteria and especially Archaea have found ways to draw energy from, and live off of, these and many other environments.

Another similarity is that Archaea and Bacteria don’t reproduce sexually; two cells don’t mingle their genes to form a new individual that is slightly different from the parents. Instead, individual cells just multiply themselves by two and then divide to form identical clones. But despite their reproductive sameness, they had—and have—a different trick for switching up their DNA. A Bacterium or Archaeon can pump some of its DNA into another cell. Or a cell can just pick up a bit of DNA floating near it. No merging, no swapping, just fresh ingredients. It’s one reason that antibiotic-resistant bacteria in hospitals can spread their immunity to other bacteria so quickly.

archaea hot springs yellowstone nationa park (earth-chronicles.com)

Archaea at home in a Yellowstone hot spring.       (earth-chronicles.com)

This gene-sharing is called lateral gene transfer, and it has an interesting feature. It doesn’t have to take place between members of the same species. For animals and plants, sexual reproduction, to be productive, almost always takes place within one species. But DNA can be transferred from any Bacterium or Archaeon to any other variety in those Domains if the conditions are right. If plants and animals could do that, the mind boggles. You might see squirrels transferring some of their DNA into dandelions. Or vice-versa. Such promiscuity makes it easier, I think, to imagine how Bacteria and Archaea have evolved in so many different kinds and colors in so many different environments.

Despite their similarities, though, Archaea are distinct from Bacteria in notable ways. Archaea were first discovered in extreme settings where Bacteria fear to tread: geysers, intensely salty water, even thermal vents at 251 degrees F, the hottest place any organism has been found living. Another feature is that, while some varieties of both Archaea and Bacteria get their energy from light, Archaea do it their own way, through a process unrelated to the photosynthesis that Bacteria passed on to plants. Importantly, too, only Archaea produce methane, essential to organic decomposition. Finally, while many Bacteria can make us sick—think Lyme, Cholera, Syphilis—Archaea may be nicer; no pathogenic Archaea have been discovered, so far.

Archaea and Bacteria had the Earth to themselves for well over a billion years. Then about 2 billion years ago, Eukaryotes appeared, evolving from their single-celled predecessors but larger and internally more developed. By then, Archaea, like Bacteria, had carried out much of the groundwork for living, pioneering what it takes to survive in different conditions, experimenting with energy sources, trying out each other’s genetic parts.

And they succeeded. They didn’t fade away after the sophisticated Eukaryotes began evolving into countless large species. Today, the total mass of Archaea and Bacteria on earth is at least equal to the mass of all the plants, animals and other organisms together. They got the basics right.