Earth's biosphere, and all events that led to us, and the way we perceive the environment and the Universe are the result of over 4 billion years of reciprocal influences between environmental and biological processes Fabbro et al. An asteroid impacting instead of missing Earth would completely alter the course of evolution at any time.
Ultimately, biological evolution on Earth has been partially dictated by somewhat predictable cycles and events astronomical, climate but drastically more so by stochastic events, both geological and biological in nature Fig. It follows that, even considering panspermia and planetary exchange as primary seeding mechanisms e. Stochastic events will give each planet a unique fingerprint.
If life arises through abiogenesis of simple organic compounds Des Marais, ; Popa, ; Shapiro, ; Pross, ; Pross and Pascal, , this fingerprinting will be even more uniquely connected to its planet of origin, and cosmic chances are that both exogenic and endogenic processes are acting together in proportions specific to each planetary system. Coevolution of life and environment. Environmental perception and neural systems of the species living on Earth are intimately linked to the coevolution of life and environment, a process that has been subjected to random probability events all along its continuum.
This process started with accretion and the specific elements that contributed to planetary formation. As prebiotic chemistry transitioned to life, the period of heavy bombardment continued to spatially randomly deliver material to Earth, contributing equally to the destruction of a nascent life and the development of a new one. Since life took hold, stochastic events have continued in the physical world as environmental and cosmic catastrophes, and in the biological world as adaptive evolution and epigenetic changes. While the notions of habitable zone and environmental habitability are critical in trying to predict whether a planet could host life as we know it, the very random nature of the coevolution of life and environment renders each planet and the life it may bear a unique experiment, which questions the anthropocentric principle underlying the Drake equation.
Moreover, if Earth is any indication of universal planetary laws of evolution, nature produces simple systems in overwhelming numbers compared to complex systems Mandelbrot, Taking inventory, global species richness estimates suggest that up to 10 million species of eukaryotes and up to million to a billion prokaryote species exist today on Earth Colwell and Coddington, ; Gaston, ; Colwell, ; Mora et al. In this inventory of all terrestrial life, through competition, only humans have reached technology. However, this statistical snapshot alone does not provide an accurate estimate of a realistic ratio, as it does not reflect the unknown total number of species produced by our planet over time.
Those include evolutionary dead ends and species that went extinct as a result of climatic or catastrophic events but still played their part in shaping the global biosphere from which we, the human species, emerged. It does not account for the shadow biosphere hypothesis, either Cleland and Copley, ; Davies et al. A systemic approach to N shows that it took 4 billion years of symbioses, competition, biodiversification, and the development of all species that did—and did not—survive for us to be here.
Some key evolutionary leaps have yet to be completely understood. Only very recently, the common ancestral gene that may have enabled the evolution of complex life over a billion years ago was identified. Without this gene, life on Earth might not have evolved beyond the stage of slime Lai et al. We also have yet to understand what led one species of apes to separate itself from the others just a few million years ago Arsuaga, Understanding these transformative processes may bear considerable weight on N. It is certainly not necessary to wait for all the answers to start searching for ET, but we must acknowledge that these questions exist, and develop search strategies adapted to the complexity they underscore in order to augment the chances of success.
We are, indeed, the product of local astronomical and planetary factors. However, it would be unreasonable to suggest that similar evolutionary convergence never happened with seemingly so many planets already discovered in the small spatiotemporal window of the Kepler telescope.
Somewhere out there, based solely on numbers and probabilities, life may have evolved to bear some resemblance to us—if only fortuitously. It might interact with its planetary environment as we do, and evolve to produce biological forms with logical minds presenting similarities to us who may be willing to communicate in ways we can understand.
However, the numbers are unlikely to be in the billions or even the millions in our galaxy. There may be just a handful scattered across vast distances and time. Taking life's evolution on Earth as a guide, there is likely a universal probabilistic law of evolutionary convergence that is inversely proportional to life's complexity; that is, the simpler life is, the greater chances are that similar life-forms will be abundant throughout the Universe. The more complex life is, the more rare convergence is likely to be.
Complexity in life-forms is an integration of temporal evolution and probabilistic events. The longer life is around, the greater chance it has to adapt through regular cycles and, at any given time, to mutate through stochastic events. The longer evolution takes, the greater the chances are that species will be wiped out and ecosystems profoundly transformed e.
Conversely, human evolution shows that technology brings its own sets of risks: the natural dynamics are upset Holocene extinction: Barnosky et al.
At this point in time, humans have generated an environmental disequilibrium that reverberates across the biosphere globally and endangers the conditions of planetary habitability that were favorable to its emergence. The notion of self-engineered destruction is certainly present in the last factor of the Drake equation.
L reflects on how long a technological civilization might be willing and able to communicate. More than duration, this factor focuses on the odds of detecting a signal; that is, the longer an alien civilization broadcasts its presence, the more chances we have to detect it. Assuming the anthropocentric view of a technological civilization presenting similarities with ours, willingness to communicate may depend on a host of reasons e.
How long such a civilization would continue to communicate is a more complex issue. Duration can relate to a civilization's ability to avoid self-inflicted—or other—destruction, scientific advances, and interest. It could also relate to a cosmologic perspective we have not yet reached, including a sense of place and responsibility as a member of a universal community e. As expressed, L examines for how long such a civilization would broadcast a signal in ways we can detect, which are primarily focused on radio and optical astronomy. We could assume, however, that a technological civilization may communicate or broadcast in ways so advanced compared to ours that we simply cannot imagine what they are and are thus unable to detect its signals.
Or this civilization may have long since disappeared. These scenarios are implicitly present in the current definition of L. However, there is also an evolutionary dimension to this factor that transcends the existing formulation. The evolutionary pathways that lead to complex life on Earth strongly suggest that advanced life as we know it may be rare in the Universe and unlikely to be in a state of advancement that is temporally synchronous with us.
However, that does not mean that other types of advanced intelligences are as rare. Limiting our search to something we know and can de facto comprehend is, probabilistically, a constraining proposition, one that leaves no room for an epistemological and scientific foundation to explore alternate hypotheses.
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To find ET, we must expand our minds beyond a deeply rooted Earth-centric perspective and reevaluate concepts that are taken for granted. Rather than constraining the search, SETI efforts must involve the most expansive exploration tool kit possible. If we unbind our minds, it should not matter whether ET looks or thinks like us, has a logic that makes any sense to us, or uses familiar technology for interstellar communication. ET is likely to be very different from us and completely alien to our evolutionary processes and thought processes, which may be deeply connected see Section 5.
Ultimately, to find aliens, we must become the aliens and understand the many ways they could manifest themselves in their environment and communicate their presence. Such an intellectual framework not only moves the Drake equation forward toward the existence of drastically different probabilistic civilizations, it also brings us to consider alternate evolutionary pathways, including life as we do not know it and do not yet understand. Further, such a framework allows us to look at evolutionary pathways in our own biosphere and question the emergence of complex, intelligent life with a different set of eyes.
For that to happen, we must conceptualize something we do not know, which can be approached in a number of different ways. One is by trying to access unknown concepts and archetypes that are literally alien to us i. This is what science fiction attempts to do in its depictions of alien worlds and civilizations.
Not surprisingly, this process results in more or less elaborate versions of ourselves, since these representations are generated by neural systems wired to our own planetary environment. To conceptualize a different type of life, we have to step out of our brains. A path to finding life we do not know requires us to identify a common universal heritage , one that includes signatures and signals that can be recognized across different evolutionary tracks and across space and time. Advances in astrobiology, life science, and cognitive science are bringing new perspectives and depth to that concept.
Some of them are already being explored, while others belong to disciplines that have yet to be involved with SETI and represent a currently untapped potential. For instance, water and carbon are driving search strategies; the formation, preservation potential, and detection methods of biosignatures that could be similar to Earth's are being investigated for the exploration of extinct and extant life on Mars, Europa, and exoplanets.
In situ biosignatures are physicochemical, geological, morphological, and mineralogical in nature Summons et al. Remotely detectable biosignatures include gases in planetary atmospheres Pilcher, ; Segura et al. Given an environmental analogy, it is conceivable that alien biospheres presenting similarities with ours may have generated and left traces we could recognize.
However, none of these signatures are convincingly unambiguous evidence of life's presence as both biological and abiotic processes alike can produce them Schwieterman et al. Therefore, it might be difficult to use them as universal markers of life as we know it, let alone for life we do not know. Astrobiology and Earth sciences show that the systemic disequilibrium generated by the presence of life could be a promising candidate as a universal marker of life Schwartzman, ; Branscomb and Russell, , Russell et al.
Biological activity, from microorganisms to humans, utilizes and modifies its environment, producing traces physical, chemical, isotopic not otherwise found in nature in the absence of life. As long as we search for biology with a physicochemical support, such disequilibrium will be generated and measurable across species and planets—although we will have to start by learning how to untangle it from the planetary background.
The argument can also be made that some technological civilizations, or civilizations beyond technology, may be so advanced that they have returned to equilibrium and generate living conditions that do not betray their physical presence anymore—or they purposely hide their presence Kipping and Teachey, In such instances, they will remain stealth to this search method. Planetary biosignatures reflecting the presence of a biosphere will still be visible, but traces of advanced beings on that planet may no longer be detectable.
While this marker falls short of helping the detection of environmentally stealthy civilizations, it can help find those that have not yet reached that stage. It also offers a universal vision to life detection that goes beyond life we know, thereby vastly expanding the statistical planetary pool that can be probed today.
Further, and critical for SETI, this approach does not depend on the willingness of aliens to communicate. The coevolution of life and environment creates a physicochemical overprint that, regardless of the stage of life's development, will betray its presence. Learning the range of signatures produced by such disequilibrium from local to global scale should, therefore, become a priority in the development of techniques to search for life of all types, sizes, and advancement stages.
Even though each candidate signature might not be produced by an advanced civilization, this method will identify more potential targets for SETI. The current limitation is both the spectral resolution of instruments and the distance of possible targets, but this method should become a critical investigation strategy to survey our galactic neighborhood Seager and Deming, ; Deroo et al. A search for systemic disequilibria at a planetary scale is currently one of the most promising methods for detecting life beyond Earth.
Taken alone, it might not be enough to inform us of the stages of life's development, but in the coming years, analog missions to our own atmosphere might teach us how to identify technosignatures. But what are we searching for and listening to? In theory, SETI is searching for coded messages sent through controlled laser emissions or radio signals containing patterns that cannot be readily explained by known natural phenomena, e.
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