Right now there are three general types of life. There are probably more that we're completely missing.
In high school biology, we are taught that there are three types of life: eukaryotes (that's us, and most everything else we often think of as life), bacteria, and archaea (extremophiles and other very primitive life forms). But some scientists are pretty sure that there are entirely different, undiscovered lifeforms that could be prevalent on Earth, and they remain undescribed because we're not good at looking for them.
In a new paper published in Science, Tanja Woyke and Edward Rubin of the Department of Energy's Joint Genome Institute note that "there are reasons to believe that current approaches [to discovering life] may indeed miss taxa, particularly if they are very different from those that have so far been characterized."
In other words, there may be life out there that doesn't even use the four DNA and RNA bases that we're used to; there may be life out there that has evolved completely separately from everything that we have ever known to exist; there may be life that lives in places we haven't even looked.
maybe undiscovered life is using a genetic code we don't understand
Discovering a new type of bacteria or virus or, hell, even a mammal at this point isn't exactly news. It happens all the time, and there are tens of millions of species out there that aren't described. Scientists can at least place newly discovered life into specific categories, but that's because they've been using the same methods of finding it for decades.
New microbes are often discovered by doing what's known as environmental sampling, in which all the DNA and RNA from, say, a soil sample or an ocean sample is amplified and replicated so that it can be sequenced, and then researchers try to separate it out into species as best as they can.
But there are well-known limitations to that approach: Lots of the RNA that is sampled has to be thrown out, and lots of it simply can't be amplified in a lab.
"It is reasonable to speculate that undiscovered and highly divergent branches of life may exist, possibly represented by domains whose marker genes differ extensively from those of the bacterial and archaeal branches on the tree of life," the authors wrote.
There's already some evidence that something weird and undiscovered exists out there. Highly novel (and large) viruses, with weird strings of DNA (for a virus) have been discovered that seem to have strings of DNA from seemingly archaeal and eukaryotic genomes. The conventional thinking is that these viruses got that DNA in antiquity from unknown and long extinct eukaryotic and archaeal organisms. But that's not necessarily the case.
radically novel organisms designed in the lab could exist in nature
The thought is that those genomes are "now only present as a so-called parasitic fourth domain [in these viruses]," Woyke and Rubin wrote. "It is possible, however, that this cellular precursor has simply not yet been detected and still exists awaiting discovery."
Finally, the most trippy suggestion the scientists make is that maybe undiscovered life is using a genetic code we don't understand, and haven't, until recently, known how to detect and sequence.
"Sequencing to date has mostly been limited to the detection of the canonical four [DNA] bases," they wrote. New techniques "have the potential to allow the recognition and characterization of environmental organisms with base modifications and compositions distinct from the four standard bases and their currently described modifications."
We might even already know of some of these modifications—but that's only because we've created them in the lab, in wacky experiments designed to create synthetic life. Earlier this year, researchers created life that had six different bases, two of which were completely artificial. Thing is, maybe life like that already exists in the wild, the scientists muse.
"This quest of synthetic biologists to build radically novel organisms also offers possible models for unusual varieties of life that may be sought in nature," they wrote. "The discovery of new building blocks and organisms from a new domain would likely have major implications for biotechnology, agriculture, human health, and synthetic biology efforts."