Really small bacteria.
From Quanta Magazine:
At Tiny Scales, a Giant Burst on Tree of Life
DNA sequencing is at the heart of this current study, though the researchers’ success also owes a debt to more basic technology. The team gathered water samples from a research site on the Colorado River near the town of Rifle, Colo. Before doing any sequencing, they passed the water through a pair of increasingly fine filters — with pores 0.2 and 0.1 microns wide — and then analyzed the cells captured by the filters. At this point they already had undiscovered life on their hands, for the simple reason that scientists had not thought to look on such a tiny scale before.
“Most people assumed that bacteria were bigger, and most bacteria are bigger,” Rubin said. “[Banfield] has shown that there are whole populations that are very small.”
The researchers extracted the DNA from the cellular material and sent it to the Joint Genome Institute for sequencing. What they got back was a mess. Imagine being handed a box of pieces from thousands of different jigsaw puzzles and having to assemble them without knowing what any of the final images look like. That’s the challenge researchers face when performing metagenomic analysis — sequencing scrambled genetic material from many organisms at once.
“Looking at things from a different angle may offer that possibility of a fourth domain,” he said — an equal partner to bacteria, archaea and eukaryotes. “There will always be novel stuff that will teach us foundational info about how life operates.” More.
Could this guy be right? No life on Mars? So much life on Earth? Too bad. But why?
Also: The Tree of Life is just a circle now. Mathematical, not organic concept. Does that not make a conceptual difference?