Humans do not come from monkeys, but from... rocks?

How did life on Earth begin? This is a question we have asked ourselves for centuries, and although we have a few formed ideas, the most accepted theories still have quite a few gaps to fill. The exploration of comets that have recently passed nearby has given wings to the hypothesis that suggests that the basic elements for building living beings could have arrived from outer space. Indeed, it seems that comets may contain some of these key elements, and perhaps in the past they brought them to our planet, who knows from where.

But let's move forward a little and focus on the next step. Regardless of the origin of these basic components, what happened next? How did the first living being appear? This original inhabitant of the Earth, from which all the forms of life that have emerged over time would have derived, has received a name: LUCA, the initials of last universal common ancestor [the last universal common ancestor].

Scientists have been wondering for years what this LUCA must have been like, and thanks above all to advances in genetics, which allow us to trace the history of genomes to practically their origin, we are beginning to get a clear picture of it. However, despite this, the work led by Dr. William Martin, an evolutionary biologist at the Heinrich Heine University of Düsseldorf, in Germany, has been received with quite a surprise, published a few weeks ago in the journal Science Advances, where he suggests that LUCA might not even have been alive, as we currently understand life.

More like a rock

After centuries of studies and discussions, we have finally concluded that the tree of life has three branches (or domains): that of bacteria, that of archaea (microbes with certain peculiar characteristics) and that of eukaryotes (where we figure, along with all animals, plants, fungi, and some microorganisms, and which would have appeared later). The three domains did not emerge independently, but rather evolved separately from a common trunk, a primitive organism that is what we have named LUCA. This organism would have only one cell and the capacity to reproduce and die, surely with some kind of rudimentary genetic material, and would be fueled by some form of basic metabolism that provided it with the necessary energy.

This is where Dr. Martin's group makes its radical proposal: the metabolism of this primeval cell would not resemble what we now share, to one degree or another, all living beings, but rather would be a series of chemical reactions that have nothing organic about them. LUCA surely was born more than four billion years ago in the extreme conditions of the seabed, where there was a varied mixture of chemical compounds in an environment of volcanic activity, and instead of using something similar to the enzymes we have now to obtain energy, it simply took advantage of the chemical reactions on the metals it had in the environment, because the little genetic information it possessed did not allow it to manufacture these enzymes. In other words, in some things LUCA would resemble a rock more than a living being.

The idea is quite revolutionary and, naturally, not yet shared by all experts. But if it were confirmed, it would mean that life began in a hybrid way, with the formation of an entity that certainly already had some biological elements, but which still depended on basic chemical reactions typical of the mineral world. We could consider, in a way, that our first ancestor was not entirely alive, which is still a philosophically interesting paradox and, at the same time, a good dose of humility. How LUCA found the way to finish defining life as we understand it now is another of the mysteries that remain for us to solve.

Life on other planets

We may never fully know what caused this planet, lost in a quiet corner of the Milky Way, to end up filled with such a spectacular variety of life forms. Going that far back in time and expecting to find conclusive answers is a practically impossible task, even with the fantastic tools that science has at its disposal today. But, in any case, the pieces of the puzzle we are finding are beginning to form an image that, although incomplete, is most fascinating. And perhaps one day it will bring us a little closer to having an answer to the other great question that follows: are we alone in this Universe? Must there necessarily be life on other planets, or are we the result of a lucky chain of coincidences too remote to repeat elsewhere?

If we understand how easy or difficult it is for a LUCA to appear spontaneously, and even what kind of forms it can take, we will be in a better position to calculate how likely it is that all the necessary conditions for this initial spark will occur. For the moment, everything seems to indicate that Earth should not be an exception and, whether few or many, there must be other lucky planets, especially considering how big – perhaps infinite! – the Universe is. It is another matter whether we coincide in time (and in a relatively close space) with a civilization that has survived long enough to be advanced enough to communicate or – even more difficult – travel between the stars, a fact which, despite what we like to see and read in works of fiction, seems statistically impossible. Let us settle, for now, for thinking that there surely are – or have been – other LUCAs roaming the galaxies, and let us hope they have been as lucky as ours.