Members each domain are either single cells or multicellular organisms. Members of eukaryota are either single cells or multicellular, but in all cases the cells have a complex structure, with a nucleus and a variety of membrane-enclosed organelles. Although bacteria and archaea are superficially similar to each other, with very little internal structure, their differing molecular biology justifies putting them in separate domains.
Until recently, it was thought that neither bacteria nor archaea contain any membrane-enclosed organelles. (Indeed it’s now generally thought (endosymbiotic theory) that many or most organelles originated as primitive bacteria or archaea that became symbiotically incorporated in more complex eukaryotic cells.)
Not long ago structures were found in some bacteria that closely resemble organelles (acidocalcisomes) that store phosphates and related chemicals in eukaryotes. Now very similar structures have been found in some archaea. The presence of versions of the same basic structure in all three domains suggest that the common ancestor of the three also had some degree of complexity, instead of the simplicity that had been supposed.
Scientists call it LUCA, the Last Universal Common Ancestor, but they don’t know much about this great-grandparent of all living things. Many believe LUCA was little more than a crude assemblage of molecular parts, a chemical soup out of which evolution gradually constructed more complex forms. Some scientists still debate whether it was even a cell.
New evidence suggests that LUCA was a sophisticated organism after all, with a complex structure recognizable as a cell, researchers report. Their study appears in the journal Biology Direct.
The study builds on several years of research into a once-overlooked feature of microbial cells, a region with a high concentration of polyphosphate, a type of energy currency in cells. Researchers report that this polyphosphate storage site actually represents the first known universal organelle, a structure once thought to be absent from bacteria and their distantly related microbial cousins, the archaea. This organelle, the evidence indicates, is present in the three domains of life: bacteria, archaea and eukaryotes (plants, animals, fungi, algae and everything else).
So, what is it, exactly, that the researchers have found?