LUCA, this was the ancestral microorganism from which all current living beings emerged
As suggestive as its name sounds, the "last common ancestor" to all living things is a living being that inhabited Earth millions of years ago. More than being itself, this concept has many evolutionary questions associated with it.
And it is that this being is, as its name indicates, the only common point from which we probably start, all: bacteria, plants and animals, among other kingdoms. Revealing its secrets is not easy, nor is it trivial, since this organism could help us answer the question never asked: how life appeared.
What is LUCA?
Our planet was formed about 4.6 billion years ago. A billion years later we find the first remains of living organisms, a very primitive species of algae. However, in all that time what about life? It did not appear suddenly, without more. This is where we find LUCA.
LUCA is the acronym for "Last Universal Common Ancestor" or last universal common ancestor. More than an organism, it is a concept whose seed was once planted by Charles Darwin, the undisputed father of Evolutionary Theory. Going back in his footsteps, it took Darwin decades to shape his hypotheses and ideas, which crystallized in "The Origin of Species," among other publications.
In its corpus Hypothetical, Darwin made it clear that the emergence of new species does not occur suddenly or for what it does. On the contrary, it is a gradual process that occurs due to small changes that are transmitted between individuals. The result is that, in the end, the members of what was once a single species differ so much from each other that they become distinct species. In that case, they are said to have a common ancestor, which matches the starting species.
If we draw the lines that unite the species and their ancestors, what we get is a kind of tree full of forks. Gross mode, and being very simplistic, we can call this a "phylogenetic tree". As we go to the "root" of this tree we will find fewer and fewer species. At the end of the whole tree, the point from where all the evolution that has given the biodiversity that exists on the face of the Earth begins, would be LUCA.
It is important to understand that LUCA is not "the first living being" that ever existed. From the appearance of life to the appearance of LUCA there is a huge stretch, of millions of years. But what is it then? We know with almost complete certainty that it was a unicellular organism, primitive but self-sufficient. We also know it was extremophile, although we don't know how much. With almost certainty he was not alone on Earth, but it does seem that from there everything else arose, at least according to what we know to date.
And what was LUCA like?
It is lawful to ask ourselves what this organism really was like. The truth is that it is almost impossible to know. But we can imagine it. With all his intuition and the knowledge acquired after decades of observation, Darwin himself dared to predict that life would have originated in a "hot pond" long ago. He was not mistaken.
Let's travel 4 billion years ago. The planet's surface was a rocky mass with empty oceans. Or almost empty, rather, because in this environment the hot rocky outcrops provided a breeding ground full of salts and molecules; the context necessary for life to appear. We have no idea how it arose (although we did have some suspicions), but it is clear that at one point the molecules organized and gave rise to much more complex components, self-sustaining and capable of further evolution. In other words, life.
This scheme is from a prokaryotic microorganism. LUCA would look like this, but more primitive.
Now let's take a closer look at the bottom of this strange ocean. Next to a fireplace, probably with a significant amount of heat and salts, we find a small bacterium. If we compare it with the current microorganisms we will see that it is decidedly much, much simpler and more primitive. It appears that it obtains energy from the heat of the chimney itself using matter from the dissolved salts.
This small organism, which we already baptized as LUCA, needed an aquatic environment to survive, as is obvious for any cell that is not lucky enough to have tissues that protect it. It is of a simple, prokaryotic cell structure, with a cytoplasm and the organelles and molecules necessary to live in it. The latest research also suggests that LUCA may not have needed extreme environments in order to survive. Instead, perhaps, it could have survived in more temperate areas, always warm, yes, as in Darwin's romantic little pond. But let's continue to unravel this organism.
What do we share with LUCA?
In order to continue drawing LUCA's sketch, we are going to start taking information from his metabolism. Thus, if we look closely, we will see that this small organism is already capable of storing information in DNA. RNA is also present, but it no longer serves as the primary genetic medium, but rather translates information into protein. Furthermore, in LUCA we find, as in us, ribosomal, messenger and transfer RNA.
We will also find a complete set of proteins dedicated to this process of genetic transcription. If we continue observing we will be surprised to see that LUCA already uses the 21 essential amino acids that are necessary for life and all of them are alpha-levorotatory. The enzymes that act on DNA are also present in its tiny existence. Furthermore, to store and exchange energy, LUCA already uses ATP.
Another curious issue is that LUCA has a tendency to get sodium out of the body, outside of his body, while concentrating potassium in the cytoplasm. To do this, it uses ion channels and special proteins. All these characteristics, in case you haven't guessed it, are the ones that tell us, without a doubt, that LUCA has something to do with each and every one of the current and former living beings, up to LUCA. But how do we know all this?
In search of a phantom organism
We have found no traceable organic remains of 4 billion years. Even if we did, which is impossible, they would be so degraded that we could extract very little information from them. How do we know all of the above, then? In order to make this robot portrait, researchers have spent decades comparing the genetic underpinnings of a myriad of organisms. As more and more genes from bacteria, archaea, animals, plants, fungi and more have been collected, the more we have been able to travel "into the past".
This is relatively straightforward thanks to supercomputing. The analyzes allow us to obtain the genetic maps of the organisms and compare them with each other. Knowing the evolutionary history of species, something that we have been doing for two hundred years, we can trace an evolutionary path (that we see reflected in the phylogenetic trees that we were talking about).
To find "LUCA", something we don't have the least bit of, researchers are working on comparing all possible genetic maps. As a result, they have found 355 genes common to all existing living things. Of course, they will not be the only ones, but we do know that almost with all probability, they must have been present in a first organism, millions of years ago.
These genes and their metabolic pathways are what allow us to know everything that we have already mentioned. From here on, the most plausible reconstruction that we can do has been made. And, after all, we are chasing a ghost. Probably the oldest ghost in history, the ghost of our only common ancestor.
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