Discovering life's last universal common ancestor, or LUCA for short, is one of the great unresolved quests of science. Researchers scouring for traces of the elusive LUCA look for shared traits that exist between all three of the major branches of life: archaea, bacteria and eukaryotes (the cells that make up plants, animals, fungi, algae and everything else).
Now scientists based at the University of Illinois think they may have uncovered a breakthrough: a primitive organelle that can be found within all types of organisms, reports Physorg.com.
"This is the only organelle to our knowledge now that is common to eukaryotes, that is common to bacteria and that is most likely common to archaea," said professor Manfredo Seufferheld, who led the study. "It is the only one that is universal."
The newly identified organelle is so primitive that it consists of little more than a cluster of polyphosphate, which is a type of energy currency in cells. It therefore likely functions as a polyphosphate storage site within the cell.
The surprising find does more than just link together the ancestry of all life; it also demonstrates that the great-grandparent of all living things may actually have been far more complex than previously thought. In fact, the LUCA may even have been more complex than many of its eventual progeny, such as bacteria.
The reason is that bacteria technically aren't supposed to have organelles at all. In fact, bacteria are typically defined as cells that lack organelles. The discovery of these primitive polyphosphate organelles within bacteria may thus represent the remnants of a more sophisticated ancestor.
"You can't assume that the whole story of life is just building and assembling things," said James Whitfield, co-author of the study. "Some have argued that the reason that bacteria are so simple is because they have to live in extreme environments and they have to reproduce extremely quickly. So they may actually be reduced versions of what was there originally. According to this view, they've become streamlined genetically and structurally from what they originally were like. We may have underestimated how complex this common ancestor actually was."
Researchers identified the common ancestry of the primitive organelle by tracking the evolutionary history of a particular protein enzyme (a vacuolar proton pyrophosphatase, or V-H+PPase) that is common within eukaryotic, bacterial and archaea cells. They were able to construct a family tree showing how gene sequences of the V-H+PPase from each of the three branches of life were related.
"There are many possible scenarios that could explain this, but the best, the most parsimonious, the most likely would be that you already had the enzyme even before diversification started on Earth," said Gustavo Caetano-Anollés, another of the study's co-authors. "The protein was there to begin with and was then inherited into all emerging lineages."