Per aspera Terra ad astra

(This is Latin. Don’t google it, go through the text and at the end you will understand the meaning.)

Maria Magliulo (University of Essex)


I don’t know the story behind this cartoon [2], nor the intention of the artist. I found it by chance. It made me think that, despite being funny, it has deeper meaning: God is actually annoyed, that life-form evolving in his “primordial soup” was not supposed to be there. If we look around us, the parallel is immediate: so far, Earth seems to be the only planet hosting life. Was it a mistake done without any purpose by someone who just had a bad day? In scientific terms: what accidental prebiotic environmental conditions could have given rise to such a wide palette of life molecules? Scientists have long discussed what is needed to make life, but life per se is undoubtedly a difficult concept to define. What then is fundamental to life, which aspects observed on Earth should we consider essential, and which is necessary for an origin? Finding an answer to those questions is crucial in the search for extra-terrestrial life.

In recent years the discovery of Earth-like planets tickled the minds of lots of people, as in the case of the TRAPPIST-1 system (Gillon et al. 2016). Among a total of seven Earth-sized planets, three of them are located in the “habitable zone” of the star (green boundary in Fig. 2), where the temperature is just right for liquid water to exist on the surface – and here is where I start thinking that enthusiasm is not always science’s best friend. “Habitable” = capable of hosting life/being inhabited; good luck then seeking to get 1600 calories a day drinking pond!

Figure 2: The TRAPPIST-1 system and the inner part of the Solar System as a comparison for their habitable zone. Whether or not an exoplanet could have liquid water depends a lot on its temperature. A planet’s temperature has a lot to do with how far away from its star it is and on the size of the star. Scientist use what they know about stars to calculate a habitable zone for each one. Credit: NASA/JPL Caltech.

What then is crucial to the emergence of life on a planet? What makes Earth conducive to life?

Spoiler alert: you won’t find any answers here, though I want to offer a different view. Instead of asking “Where are Earth-like planets in space?”, let me instead ask “How long has Earth been Earth?”. Imagine dropping back 3.8 billion years ago – when life first began. There was nothing to eat, no atmosphere shielding us from cosmic radiation, not even oxygen to breathe, and you don’t have to have a PhD in biology to know that that is a bad place to go looking for life. However, in some ways, that “primordial soup” was cooking itself; life was starting, feeding on a bunch of reduced smelly compounds, under harsh atmospheric conditions. In a must-read perspective published in Nature Ecology and Evolution by Judson (2017), a view of the path that evolving life followed on Earth was proposed. The main concept is what kind of energy was available and how life exploited it, modifying it in return, which led to the evolution of new organisms more suited to the new environment.


Figure 3: Key events during the energy expansions of evolution (Judson 2017). Life emerges using geochemical energy. Sunlight was shining all along, but oxygen, flesh and fire are all consequences of evolutionary events. No category has disappeared, indeed, this resulted in an expanding realm of energy sources available. Such expansions have mediated the transformation of the planetary environment, which, in turn, mediated the future course of evolution.

Let me give you one example: when microbes evolved the ability to harness energy from the Sun and release oxygen as a by-product, it completely changed both the biosphere and the geosphere.

From a biological perspective, oxygen is a very reactive atom, and in certain amounts also lethal – indeed, lots of organism at that time became extinct. Surprisingly, life not only evolved a way to avoid being killed by all that oxygen[3], but also, to actually exploit that oxygen to speed up their metabolism! Thanks to this, the production of the biosphere leveled out, and as a consequence, the biodiversity increased.

But that’s not all: such a dramatic increase in atmospheric oxygen led to an explosion of mineral diversity. This completely changed the chemical landscape of the surface of the Earth: that is just because life happened, and you can look for records in rocks (figure 4).

Figure 4. Banded iron formations: distinctive units of sedimentary rock composed by thin layers of iron oxides, alternating with bands of iron-poor shales and cherts. It is assumed that at the beginning Earth’s oceans were full of iron and nickel dissolved. As photosynthetic organisms evolved, oxygen started to react with all the available iron, forming insoluble iron oxides. The banding is assumed to result from cyclic variations in available oxygen. Credit: Wikipedia.

If you think this is an already super exciting subject to think about, let me be a bit more provocative.  How would you imagine extra-terrestrial life to look like? As a Star Wars fan I just can’t help but imagine a Naboo-like planet somewhere in the Universe. However, as a microbiologist, I can tell you, every Gungan living there would have microbes living all over their body. If I would ever have the chance to go there, believe me, the first thing to do would be swabbing microbial mats all around[4]!

That is simply because microbes are the least common multiple of life. I might have doubts about the chemistry of life (i.e. life based on silica: could it be possible?), but no doubt about “simple” unicellular organisms that make the bridge between the geosphere and the biosphere possible. They have been rightly defined as the “biogeochemical engineers of life on Earth” by Falkowski (2008) and here I cite:

“ …our current environment reflects the historically integrated outcomes of microbial experimentation on a tectonically active planet endowed with a thin film of liquid water.”

Life could have started somewhere else, and Mars has attracted the most optimism about the possibilities for extra-terrestrial life for a bunch of reasons. Perhaps in the past it could have sustained biology and maybe it could even do so today deep underground. Certainly, if you look at its rocky surface we do not think of it as habitable for life as we know it. Maybe “its current environment reflects the historically integrated non-successful outcomes of microbial experimentation on a, tectonically kind of active planet, once, endowed with a thin film of liquid water”.

Now, the key question is: “Are there any remaining life-form still alive buried deep underground?”[5]

This may sounds as a sci-fi plot (and it could be a good one actually), but the discovery of a deep biosphere lying underground here on Earth, makes this way more realistic. I know some microbes who have been trapped inside salt rocks 6.5 million years ago; they are still there, slowly reproducing, staying alive.

In my PhD, I’m going to ask them how do they do that. Plus, I’m going to ask them what would they do if they would live on Mars.

So, stay tuned.

“Per aspera ad astra” means “through hardship to the stars”, but I substituted aspera with Terra; it is only through deepening our knowledge about life on Earth, the logic that underpins this co-evolution between life and its planetary home, that we can look up in the sky and try to answer to the great question: “Is there life beyond Earth?”

That was just the best title I could have think of.



References:

Falkowski, Paul G., Tom Fenchel, and Edward F. Delong. 2008. “The Microbial Engines That Drive Earth’s Biogeochemical Cycles.” Science 320 (5879): 1034–39. https://doi.org/10.1126/science.1153213.

Gillon, Michaël, Emmanuël Jehin, Susan M. Lederer, Laetitia Delrez, Julien de Wit, Artem Burdanov, Valérie Van Grootel, et al. 2016. “Temperate Earth-Sized Planets Transiting a Nearby Ultracool Dwarf Star.” Nature 533 (7602): 221–24. https://doi.org/10.1038/nature17448.

Judson, Olivia P. 2017. “The Energy Expansions of Evolution.” Nature Ecology & Evolution 1 (6): 0138. https://doi.org/10.1038/s41559-017-0138.

[1] Vassily Kandinsky, 1926 – Several Circles

[2] Modified from: https://www.pinterest.ca/pin/417286721697683123/

[3] https://twitter.com/cmtribull/status/830265712744742912

[4] Modified from: https://www.starwars.com/databank/jar-jar-binks-biography-gallery

[5] Modified from: http://www.cartoonaday.com/mars-mission-curiosity-lander-searches-for-life/

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