Two billion-year-old predecessors of our cells could have been the most successful farmers on Earth

According to an article published in PNAS by István Zachar, András Szilágyi, Szabolcs Számadó and Eörs Szathmáry, it is possible that present organisms exist because bacteria “domesticated” the ancestor of mitochondria much like humans farm pigs today.

21 March, 2018

The living organisms we see around us are – almost without exception – eukaryotes, that is, they are made up of cells with a cell nucleus. However, the larger part of the biomass is made up of much simpler organisms, namely prokaryotes, that is, bacteria without cell nuclei and single-celled microorganisms (Archaea), which also have a similar structure. The most difficult evolutionary transition was the formation of eukaryotes from prokaryotic ancestors approximately two billion years ago.

We are sure that during this transition an archaeon and a bacterium species developed such a strong symbiotic relationship that the bacterium permanently got into the host archaeon (endosymbiosis). As a result of this merger emerged the highly successful (but never again repeated) eukaryotic cell line. The bacterium was the ancestor of an organelle, the mitochondrion. Mitochondria have maintained some independence; however, today they are unable to survive outside the cell on their own. At the same time, the existence of mitochondria is vital for eukaryotic cells: in exchange for nutrients and a stable home, mitochondria produce large quantities of energy for the host cell in the form of ATP (adenosine-triphosphate) molecules.

In this photograph of cow cells taken with a microscope, the mitochondria were stained in bright yellow to visualize them in the cell. The large blue dots are the cell nuclei and the gray web is the cytoskeleton of the cells. Source: Flickr/NIH Image Gallery/Torsten Wittmann, University of California, San Francisco

Smooth operation, deficient explanations

The above described symbiosis seems to be a perfect relationship. However, genetic tests have revealed that the protein responsible for tapping the energy stores of mitochondria evolved well after the merger – some millions of years, according to estimates.

Due to the lack of fossils, there is little physical evidence about this evolutionary process; there are competing theories which attempt to explain it. Some think that the mitochondrial ancestor may have been a parasite living off the host cell. Others hypothesise that the host predated on mitochondria and simply gobbled them up. However, the problem with both theories is that they do not explain how a one-sided relationship could exist for such a long time that made it possible for the energy-tapping protein to evolve.

The first farmers

A third hypothesis was tested in the study by István Zachar and his team. According to this theory, hidden advantages might have stabilised the originally disadvantageous relationship. This farming hypothesis was first formulated more than twenty years ago by John Maynard Smith and Eörs Szathmáry. According to the theory, the host was a predator which regularly consumed other cells – among them the freely existing ancestor of the mitochondrion. However, the host did not digest all the bacteria it had eaten. The ones that survived this process kept living inside the host (e.g. owing to photosynthesis) and might have been able to multiply as well. This means the host kept these organisms as a farmer does, in a similar manner to humans breeding pigs (or growing peas, where photosynthesis is also included). If there was not enough prey in the environment, it was beneficial for the hungry host to have reserves helping him to survive lean times. Consequently, farming meant an evolutionary advantage for these organisms over those cell types that lacked the ability to farm.

The authors of the article recently published in PNAS used evolutionary ecological models to prove that this head start could have been enough for the farmers to oust non-farmers on the ecological time scale, and thus for farming abilities to spread in the population. Moreover, farmers regularly proved to be stable compared to newly emerging non-farming mutants. In essence, farming means that a bit of investment in good times (i.e. storing prey for the bad times) can reduce the long-term loss or even the risk of extinction. In sum, it is better to have an egg both today and tomorrow than to have a hen today and starving to death tomorrow. This trick might have been useful not only in the case of the emergence of mitochondria, but may provide an explanation for other types of endosymbiotic relationships, such as the emergence of chloroplasts.

The research was funded by National Research, Development, and Innovation Office Grants NKFI-112788 (to I.Z.), NKFI-K119347 (to E.S. and A.S.), NKFI-K124438 (to A.S.), and GINOP-2.3.2-15-2016-00057 (to E.S.); European Research Council Project 294332 (E.S.); the Volkswagen Stiftung initiative “Leben? – Ein neuer Blick der Naturwissenschaften auf die grundlegenden Prinzipien des Lebens” under project “A unified model of recombination in life” (E.S.); and the European Research Council under the European Union’s Horizon 2020 Research and Innovation Programme Grant 648693 (to S.S.).

Further information

István Zachar, Department of Plant Systematics, Ecology and Theoretical Biology, ELTE, Budapest, Hungary
Tel: +36 1 209 0555 / 1707
zachar [dot] istvan [at] okologia [dot] mta [dot] hu