As discussed previously, outer space conditions are harmful to human beings. However, this does not necessarily mean that they are devoid of life. Indeed, there are very harsh environments on Earth that are not suitable for humans, and yet, there are life-forms that thrive in them. These are often microorganisms, such as bacteria and yeast, and represent the pinnacle of adaptation on Earth because of the extreme environments they inhabit, including very high acidity, both freezing and scorching temperatures, and exposure to UV radiation. Considering life can survive in such hostile environments on Earth gives us hope that we may also find life outside of the Earth’s atmosphere.
Whether our current knowledge and technology are sufficient to find life in space is still being debated. Although we have explored life in space by growing cotton plants, taking real-time pictures of Mars, and sending animals and humans to space, there are still important issues to take into consideration when searching for life in space.
First and foremost, assuming there is life outside the Earth, how should we look for it? Our current techniques rely on looking for DNA molecules because of their critical role as the blueprint of life as we know it. A second major approach is the detection of other molecules such as oxygen and water that could serve as building blocks of life. Intriguingly, there have been some reports of water on Mars and Europa, one of the moons of Jupiter, which hints at specific places where it would be interesting to search for life.
Searching for life in space poses many challenges for both scientists and funders. Fortunately, the diversity of environments on Earth provides extreme and stressful conditions that we can use to study life in space without leaving Earth. Given that these places exhibit characteristics similar to those in space, it is reasonable to expect extraterrestrial life to be rather similar. For instance, the dryness of the Atacama desert makes it similar to the Martian surface, so any biomarkers found in this desert would be interesting to consider when screening Mars. An example of such a biomarker could be specific lipids in regions of this desert containing fossils. These lipids degrade very slowly in very dry conditions, so they could have been produced by very ancient life-forms and still be detected today (Wilhelm et al., 2017). Therefore, we could use these lipids as a target to screen for Martian life-forms, whether they are contemporary or ancient.
Another option is to directly look at the organisms that inhabit these harsh environments. These organisms are called extremophiles and can also be used to study adaptation to extreme environments. Though growing them in the laboratory is quite a challenge since their living conditions are very hard to replicate, scientists have overcome this issue with metagenomics studies. Metagenomics refers to sequencing the genetic information of communities of microorganisms without needing to grow them in a laboratory (Cowan, Ramond, Makhalanyane, & De Maayer, 2015).
These kinds of analyses have attracted interest in extremophile communities, leading to increasingly comprehensive efforts like the Extreme Microbiome Project. This project focuses on sites that possess extreme environments such as the Dead Sea (high salt concentrations), the International Space Station (microgravity – weaker gravity than on Earth), and the Alaskan permafrost (extreme cold temperatures). The results of this project will allow us to know what genes are unique to extremophile communities, hinting at how they adapted to these conditions. As all the data will be publicly available (Tighe et al., 2017), many fields of biology will benefit from them. Astrobiologists, in particular, would be able to know what biomarkers to look for in space environments if they resemble any of the tested extreme environments.
The wealth of knowledge generated by the Extreme Microbiome Project could yield a lot of insight into life and evolution. By comparing the new sequences to our databases, we would be able to track the new sequences’ position in the Tree of Life and see if they are similar to any organism on Earth. Doing such a test would revolutionize the way we think about life as a whole. For example, if these organisms are similar to others on Earth, we would then have to consider whether life originated on Earth or came from elsewhere. Alternatively, if these organisms are indeed distinct from everything we know on Earth, they might come from a separate origin of life. If there are multiple ways to provide an origin to life, what are the commonalities and differences between them? Can evolution repeat itself and lead to Trees of Life with similar biodiversities?
The search for life outside the Earth is certainly one of the most exciting questions that new genomic technologies will allow us to start addressing. To the best of our understanding, if there is life in space it should be similar to that in extreme environments on Earth. Studying these organisms on Earth will help us know how and what to look for. Still, what if despite all the efforts, the results are lifeless samples? Such an outcome would not necessarily be disappointing since there are several factors that could explain these negative results. Besides, lifeless samples will also help us draw the line between extreme habitable environments and extreme uninhabitable ones.
Now that scientists are integrating the study of organisms on Earth to search for life in space, more and more discoveries will be made, leading to answers to these questions and the discussion of their implications. These answers will lead to new, exciting questions, particularly with respect to the origin of life, evolution, and even for human life outside of our planet.
We would like to thank Dr. Charles Cockell for insightful comments and perspectives on this piece.