In search of beauty

The emergence of eukaryotes (any organism with cells that have a clearly defined nucleus) probably corresponded to the most extraordinary event in the entire history of life, as at least two radically disparate creatures merged, giving rise to a chimera much more complex than any previously living being.

For billions of years, two groups of similar creatures, bacteria and archaea, comprised the two major branches of the tree of life. Distinguished more by their genealogies than by their characteristics, these beings remained circumscribed to the complexity limit that confines them to this day, until an unusual event opened the doors to the phenomenal occurrence that culminated in the emergence of eukaryotes.

The event occurred when, in some way, bacteria embedded themselves in certain archaea and startedto live inside them in such an inextricable manner that the two beings ended up merging into a single chimeric creature. This combined organism then began to benefit from the energy generated by the embedded bacteria, transforming them into one of its organelles, the mitochondria. This occurrence corresponded to an exchange of the cell’s energy source, previously distributed along the surface of the cell membrane, for an energy source distributed throughout the volume of the cell.

It turns out that surfaces correlate with the square of their spatial dimensions, while volumes correlate with the cube of these quantities. Consequently, chimeric beings equipped with energy sources distributed throughout their volumes – mitochondria – rather than on the cell surface, as is the case for all other beings, managed to surpass the complexity limit that had never been crossed during the previous billions of years. Brimming with mitochondria – energy sources distributed throughout their volume – chimeric cells could grow much larger than their predecessors without experiencing energy depletion. This led to the generation of an amount of energy much greater than that produced by any other organism then alive, rapidly resulting in cells with volumes 10,000 times larger than the previous ones and possessing an equally unprecedented complexity. Such a situation set the stage for what was probably the most extraordinary event in the entire history of life.

The great saga

The saga of eukaryotes truly began when a predatory bacteria, a tiny active hunter from the group of BALOs (Bdellovibrio And Like Organisms), started to take more and more time encysted in its host – a giant mitochondria-rich cell – weaving a coevolutionary relationship where parasite and host engaged in a lengthy arms race. This resulted in a continuous and mutual reduction of aggression, leading to increasingly extended periods within the host cell that allowed the parasite to wait for the most opportune times to hatch its offspring, while the host tried to take advantage of the time gained to reproduce.

The most spectacular episode of this long saga came when the host retro-parasitised its parasite by injecting its own genome into its parasitic partner in a masterful counteroffensive. The unusual counterattack transformed the parasite into the vector for spreading its host’s genome, which it began to carry and disseminate in each new infection. In a manner reminiscent of a zombie, the retroparasite produced a profusion of BALOs carrying its genome that, upon infecting a new organism, inoculated it with the chimeric genome. Thus, each new infection corresponded not only to the infection by the BALO but also by the retro-parasite, thus turning it into a kind of zombie.

At this stage, the chimeric BALOs unmistakably resembled spermatozoa, while infections began to acquire increasingly distinctive features of sexual reproduction! Note that, without exceptions, all eukaryotes, and only them, originated from sexual reproduction in at least some previous generation. And yes, spermatozoa are ultimate emdosymbionts (organisms that live in the body or cells of another)!

Genomic control

Subsequently, the cyst where the former parasite used to exert genomic control over its host ended up transforming into the nucleus that characterises eukaryotic cells, and only them.

Mitosis corresponded to a counteroffensive by the parasite in response to the host’s attempts to get rid of it by dividing into two. The counteroffensive involved the division of the cyst/nucleus in synchrony with the division of the host cell, thus infecting the two resulting halves of the cell division. Meiosis originated from the need for genome stabilisation, subject to uncontrolled polyploidy (a condition where a cell has more than one pair of chromosomes) resulting from successive infections.

Therefore, the emergence of sexual reproduction, the cell nucleus, mitosis, and meiosis is explained by the same event that unveils the origin of eukaryotes.

The greatest show on Earth is even more marvelous than we previously imagined!

The theory is published in: Gollo, G (2023) On the emergence of eukaryotes and other enigmas. Biosystems, doi.org/10.1016/j.biosystems.2023.104958.

Life is really beautiful!