The Last Universal Common Ancestor is the great-grandparent of everything that has ever lived on Earth. Scientists had long assumed it was a crude, simple thing, more a chemical mishmash than anything else. But our earliest ancestor holds some surprises.
Known as LUCA for short, it's hard to even know how to describe this...thing. For a long time, scientists assumed the LUCA was incredibly primitive, a basic assemblage of organic molecules that couldn't even be considered a cell. That might sound like the LUCA wasn't even alive in any sense we would understand, and that's probably true - that's the problem with coming up with a single organism from which everything on this planet could be descended. You have to go very, very far back into our evolutionary past to find it.
But maybe not quite as far back as we once thought. While we still don't have any direct evidence of what the LUCA was, researchers at the University of Illinois have found a particular organelle inside microbial cells responsible for polyphosphate storage. That may not sound like the most crucial of tasks, but the important thing is that this specific organelle is found not only in all multicellular organisms but also bacteria and their cousins the archaea. That makes it the first universal organelle, something that is shared by every living thing on the planet.
And, if a particular structure is found in every form of life, then that means it has to have evolved in the common ancestor of all those organisms - in other words, it goes all the way back to the LUCA. Co-author Gustavo Caetano-Anolles explains:
"There are many possible scenarios that could explain this, but the best, the most parsimonious, the most likely would be that you had already the enzyme even before diversification started on Earth. The protein was there to begin with and was then inherited into all emerging lineages."
This particular organelle is the only known thing to be universal to all lifeforms, so it's the only feature of the LUCA that we can know about with any certainty. However, even this one organelle implies a surprising level of complexity in the LUCA, perhaps even outstripping that of its descendants. This original organism might actually have been more complicated than some of the bacteria and archaea that came after it.
Fellow researcher James Whitfield explains:
"You can't assume that the whole story of life is just building and assembling things. 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."