For many years I shared the common belief that living things fall into three or four basic categories. Besides plants and animals, one or two others groups with names that varied over the years—Bacteria, Protists, Prokaryotes— consisted of creatures too small to see.
Today there are still three categories, called Domains, but they all refer to types of cells. The only familiar name is Bacteria. Plants and animals are now small dots in the huge Domain of 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 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 of living things? 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 possible 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.
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, successful sexual reproduction 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 extremely hostile environments 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.