A new view of the tree of life
Laura A. Hug, Brett J. Baker, Karthik Anantharaman, Christopher T. Brown, Alexander J. Probst, Cindy J. Castelle, Cristina N. Butterfield, Alex W. Hernsdorf, Yuki Amano, Kotaro Ise, Yohey Suzuki, Natasha Dudek, David A. Relman, Kari M. Finstad, Ronald Amundson, Brian C. Thomas & Jillian F. Banfield
There is the usual discussion about difficulties of resolving deep branches. But the really interesting bit concerns that big purple clade I'd never heard of.
Of particular note is the Candidate Phyla Radiation (CPR)7, highlighted in purple in Fig. 1. Based on information available from hundreds of genomes from genome-resolved metagenomics and single-cell genomics methods to date, all members have relatively small genomes and most have somewhat (if not highly) restricted metabolic capacities7,13,24. Many are inferred (and some have been shown) to be symbionts7,25,26. Thus far, all cells lack complete citric acid cycles and respiratory chains and most have limited or no ability to synthesize nucleotides and amino acids. It remains unclear whether these reduced metabolisms are a consequence of superphylum-wide loss of capacities or if these are inherited characteristics that hint at an early metabolic platform for life. If inherited, then adoption of symbiotic lifestyles may have been a later innovation by these organisms once more complex organisms appeared.
I guess if they are sequenced from single cells, we should believe that they aren't just fragments of other organisms. Cool!