All life on Earth is related, which means we all must share a single common evolutionary ancestor. And now it appears that this ancestor might have been a single, planet-spanning organism that lived in a time that predates the development of survival of the fittest.
That’s the idea put forward by researchers at the University of Illinois, who believe the last universal common ancestor, or LUCA, was actually a single organism that lived about three billion years ago. This organism was unlike anything we’ve ever seen, and was basically an amorphous conglomeration of cells.
Instead of competing for resources and developing into separate lifeforms, cells spent hundreds of millions of years freely exchanging genetic material with each other, which allowed species to obtain the tools to survive without ever having to compete for anything. That’s maybe not an organism as we would comprehend it today, but that’s the closest term we have for this cooperative arrangement.
All that we know about LUCA is based on conjecture, and the most promising recent research has been in figuring out what proteins and other structures are shared across all three domains of life: the unicellular bacteria and archaea and the multi-celled eukaryotes, which are where all plants and animals evolved from. This isn’t a foolproof method — it’s possible that two extremely similar but not identical structures could evolve independently after LUCA split into the three domains — but it’s a good starting point.
Illinois researcher Gustavo Caetano-Anollés says about five to eleven percent of modern proteins could be traced back to LUCA. Based on the function of these particular proteins, it appears LUCA had the enzymes needed to break down nutrients and get energy from them, and it could also make proteins, but it probably didn’t have the tools necessary to make DNA. This fits with other research that suggests LUCA fed upon many different food sources, and that it had internal structures in its cells known as organelles.
The big difference between LUCA and everything that came after, of course, is DNA. Because LUCA didn’t have the tools to deal with DNA, it probably used RNA instead, and it likely had very little control over the proteins that it made. The research suggests the ability to precisely control protein manufacture only came long after LUCA split apart, which means that protein-making was probably always a big crapshoot.
That’s why LUCA had to be cooperative, with any cells that produced useful proteins able to pass them on throughout the world without competition. This was a weird variation on what we know as natural selections — helpful proteins could go from a single cell to global distribution, while harmful or useless proteins were quickly weeded out and discarded. The result was the equivalent of a planet-spanning organism.
So why did this paradise of cellular cooperation give way to the last three billion years of cutthroat competition? The simple answer is that some cells probably outgrew this arrangement, as they had finally developed all the structures needed to survive without help. We don’t know quite why that happened, but it appears to coincide with the sharp increase of oxygen in the atmosphere. Whatever the cause, cells began eking out their own independent existences, ending the reign of LUCA that had lasted hundreds of millions of years… while beginning a new order that is still going strong 2.9 billion years later.
BMC Evolutionary Biology via New Scientist. Image by fusebulb, via Shutterstock.