Astrobiology Revealed #10: Iva Vilović
On Lessons in Habitability from Earth’s Recent Past
by Aubrey Zerkle
This week we asked Iva Vilović about her paper entitled “Variations in climate habitability parameters and their effect on Earth’s biosphere during the Phanerozoic Eon.” Originally a physicist by training, Iva is currently a final year doctoral student in the Astrobiology Research Group, at the Center for Astronomy and Astrophysics at the Technische Universität Berlin, Germany. Iva discusses how looking back through time at the history of life on Earth provides a valuable window for considering habitability outside our own solar system. (This interview has been edited for length and clarity.)
In your paper, you considered the Phanerozoic biosphere as a model for how planetary environments affect habitability. How did you become interested in applying the lens of Earth history to the broader field of astrobiology?
If we want to dare talk about life elsewhere, we must first understand life on Earth throughout its epochs, not because we necessarily expect to find life like ours, but because we need a basis for comparison! Earth is the only planetary body, to our current knowledge, where life has not only emerged, but also spread globally and managed to thrive. Earth's biosphere offers a diverse range of life forms and ecosystems, each leaving behind distinctive signatures in the atmospheric and geological record.
Not only that, but Earth's biogeochemical cycles, including the carbon, nitrogen, and water cycles, have played a crucial role in shaping the planet's habitability. Studying these cycles helps us recognize similar processes on exoplanets. Earth's history has also been punctuated by catastrophic events like asteroid impacts and volcanic eruptions. Understanding how life on Earth has survived and recovered from such events provides insight into the resilience of life and the potential for life to persist in challenging environments elsewhere. By studying these [past] biosignatures and their corresponding environmental parameters, we can develop techniques for detecting potential signs of life on other planets, helping guide future missions and observations.
As part of this study, you compared trends in biomass and biodiversity with proxies for various environmental variables, to see how they co-varied throughout the Phanerozoic. Why did you choose the specific environmental parameters you chose?
Life is sensitive to changes in its environment. Even the smallest changes on a local scale can greatly impact its abundance and distribution. There are, however, a few environmental parameters whose cycles have important global consequences. For example, atmospheric oxygen is vital for the respiration of most living organisms, including humans. It is used in the process of cellular respiration to generate energy. Earth’s atmosphere is composed of approximately 21% oxygen, and many organisms have evolved to thrive in this oxygen-rich environment. Increased oxygen levels can lead to higher metabolic rates in organisms. This means that organisms have more energy available to carry out essential functions, such as movement, reproduction, and growth. As a result, they can generally operate at faster speeds, including hunting, foraging, or escaping from predators, which plays a significant role in driving evolutionary processes.
Another globally essential parameter is carbon dioxide. Plants, algae, and some bacteria undergo photosynthesis, a process in which they use sunlight, water, and carbon dioxide to produce glucose (a form of sugar) and oxygen. This process is crucial for sustaining life on Earth. Another key aspect is that carbon dioxide is also a greenhouse gas. In moderation, it helps to trap heat in the Earth’s atmosphere, maintaining a suitable temperature for life. However, an excess of carbon dioxide can lead to global warming, causing detrimental effects on habitats.
Furthermore, the global average surface temperature of a planetary body determines whether it can sustain liquid water on its surface, which is essential for nutrient cycling and biochemical processes. Life on Earth has evolved to thrive within a specific temperature range. For example, most terrestrial life forms, including humans, flourish within a temperature range of roughly 0°C to 50°C (32°F to 122°F). Temperature therefore affects the rate of biochemical reactions, with extreme temperatures disrupting essential cellular processes.
Lastly, we focused on global runoff rates, which are an indicator of the aridity or humidity of a certain geological period. Adequate humidity levels are critical for maintaining water availability for organisms. For many organisms, especially those that respire through moist surfaces or engage in transpiration (like plants), humidity levels directly impact their ability to exchange gases and absorb nutrients. Humidity levels also impact nutrient cycling and biochemical processes in organisms.
Were the trends you found largely what you expected, or were there any surprises?
There were indeed a couple of surprises! One was the negative correlation between biodiversity and global average surface temperatures. It was even proposed by Heller & Armstrong (2014) and Schulze-Makuch et al. (2020) that a slightly warmer exoplanetary global surface temperature could be more suitable for life than the current global surface temperature of ~15°C. This rationale was based on the Carboniferous Era, between ~325 and 250 million years ago, during which the global surface temperatures were around 10°C higher than today. This period witnessed the highest biomass to date. In fact, most of the coal we harvest today actually comes from the Carboniferous Era. But it seems that warmer temperatures are only beneficial in combination with other parameters, like a more humid atmosphere, and only as long as no rapid or large temperature excursions take place, like they are starting to do now in the context of anthropogenic climate change.
Based on your results, which environmental variable would you say is the most important for sustaining planetary life? And can you briefly explain why that is?
If we had to identify one variable that is of paramount importance, it would likely be liquid water. Here's why: Water is often called the "universal solvent" because it can dissolve a wide range of substances. This property is crucial for the transport of nutrients and chemicals necessary for life processes. Not only that, but many biological processes, including those involved in metabolism and cellular reactions, occur in aqueous solutions.
Water also has a high specific heat capacity, which means it can absorb and store a significant amount of heat without a large change in temperature. This property of water helps stabilize temperature variations on a planetary scale, making it conducive to life. In terms of physical habitats, liquid water serves as a home for a vast array of life forms, from microorganisms in Earth's oceans to larger organisms like fish and amphibians. If we assume that extraterrestrial life also needs a liquid solvent, which stays liquid in an Earth-like temperature range that allows for relatively quick metabolic processes, then water could be considered one of the most important parameters for sustaining planetary life.
In the paper, you also state "there are likely other factors not included in the analysis affecting the environmental parameters as well as the biology of our planet." What are some other factors you think might be important?
Yes, there are likely many other factors impacting the climate and therefore also the biology of our planet! From a geological perspective, these could include volcanic activities. From an astrophysical perspective, these could include impacts from outer space, the change in solar luminosity and activity over eons, and Milankovich orbital cycles, which affect Earth’s climate every ~41,000 years. Another global factor might be where in the galactic year we are [the duration of time required for the Sun to orbit once around the center of the Milky Way Galaxy], which lasts about 230 million Earth years, as there could be different amounts of cosmic rays or other galactic radiation received at the surface of Earth [during different times of the galactic year].
When looking at life and environments back through geologic time, we are often limited by the available proxy data. Is there any environmental variable you'd love to test that currently doesn't have any clear proxy in the rock record? In other words, what “dream proxy” would most help you continue this work?
If I could, I would love to go back in time, equipped with a scale, and measure all the global biomass every 2 million years or so, to see how it changed throughout the eons. It would be especially interesting to measure the biomass during the Carboniferous era, when giant insects with wingspans the size of humans and mushrooms the size of trees, inhabited the Earth! During this time, Earth’s organisms grew so large because of the highest oxygen contents to date, which provided enough energy for reproduction and growth.
I would also bring a temperature and relative humidity sensor, since [proxy] measurements of relative humidity throughout the Phanerozoic are severely lacking, and temperature data is also reconstructed using a combination of proxies. Another “dream proxy” would be atmospheric pressure measurements. It has been proposed that higher atmospheric pressures might provide a denser atmosphere, which would support a larger biosphere by providing it with more mass and energy.
Based on what you’ve learned from the Phanerozoic, what would be your advice for astrobiologists looking for life on other planets?
I would say to look for a nice temperate world with signs of water vapor. As I discussed earlier, liquid water is a critical ingredient for life as we know it. When investigating exoplanets or celestial bodies, we should prioritize those within the habitable zone of their star where conditions might allow for the presence of liquid water. We should also consider temporal scales, and keep in mind that the evolution and persistence of life can occur over long periods of time. This is why investigating environments with stable conditions that could have supported life over geological timescales should be a priority!
Finally, is there anything else you’d like to mention that I haven’t asked about?
This work is the first of a three-part Ph.D. project on habitability! Understanding an exoplanet in the context of its stellar and environmental parameters is essential for assessing its habitability drivers. This is why we first focused on how Earth’s habitability has changed throughout its natural history.
Beyond environmental parameters, stars with about 45% to 80% the mass of the Sun, so-called K dwarf stars, have previously been proposed as optimal host stars in the search for habitable extrasolar worlds. For the second part of the project, we therefore calculated the electromagnetic spectrum of a K dwarf star transmitted to the surface of a hypothetical habitable zone planet and used a starlight simulator to expose photosynthetic Earth organisms to this modified light.
It is also vital to determine whether life that flourishes under such conditions can also be observable. This is why in the last part of my project we will use theoretical climate-chemistry models to determine, for example, how transmission spectra would look on such a habitable planet. By synthesizing insights from the Phanerozoic Eon's environmental parameters, experimental data on photosynthetic organisms’ adaptability to K dwarf radiation, and synthetic transmission spectra of habitable planets, we can discern potential life-sustaining environments beyond our solar system!