Considering that scientists are only able to grow an estimated one percent of known microbes in the lab, many suspect that there are plenty of new forms of life yet to be discovered, the so called “dark matter” of life. Now, a new study has hinted at the possibility that some of these unknown forms might be residing closer to home than imagined – in our own guts.
Over the last 10 years or so, the realization that not only do we carry around a teeming ecosystem of microbes, but that they are actually an integral part of our biology, has radically altered how we look at the human body. Known as our microbiome, the community of microbes – made up of bacteria, fungi, and viruses – live in our guts, mouths, and even the air surrounding us, influencing and contributing to how we function.
Because the microbes that live in our guts are some of the best studied in the world, this was like a good place to start looking for the fabled dark matter of life. Using DNA analysis, researchers are usually able to identify new species based on their unique genomes, but when it comes to microbes such as bacteria, things are a little trickier. This is because bacteria are often able to share sections of DNA, or genes, with each other. Known as “horizontal gene transfer,” this has the effect of blurring the lines between where one species ends and another begins.
To get around this, the researchers of the new study, published in Biology Direct, decided to focus on 86 gene families that are not normally transferred between microbes. They reasoned that if any of these are not from any known form of life, then there is a chance that they are new to science. Currently, there are three accepted forms, or domains, of life: eukaryotes (which include animals, plants, and fungi), bacteria, and archaea.
The researchers found around 230,000 portions of DNA that are related to the already identified 86 gene families. That was their starting point, and in a second analysis, they found a further 80,000 sequences of DNA that belong to the same 86 families. They found that around one-third of these DNA portions shared only 60 percent or less of their bases with already known sequences. This degree of difference is what is normally found between bacteria and archaea, and so could potentially represent a fourth domain.
But the researchers warn that while this finding might be significant, we shouldn’t jump ahead of ourselves. The massive differences in DNA sequences might simply represent an unknown amount of diversity already present in living bacteria and archaea, rather than an entirely new domain of life. The next stage, say the researchers, is to try and identify some of the living organisms that contain these unusual sequences, and then sequence their entire genome.
[H/T: New Scientist]