Astrobiology presents a unique framework to investigate both the distribution and probability of life in the universe and the future evolution of the human species as whole. Both of these aspects touch on the same idea, one that is a fundamental philosophical question that astrobiology seeks to understand and answer: Are humans a unique species in the context of the universe? It certainly seems that way based on what we know so far, but it would only take a single communication to change that view. A multitude of questions follow from this: Is human evolution, and its subsequent struggles, on our planet unique or do all species undergo similar stages of societal evolution, eventually either transitioning towards a completely sustainable energy model or crumbling under the weight of resource depletion? It is impossible to know the answers to any of these questions until a second data point is established, and that is why I believe that this is the most open question in astrobiology.
Currently, we do not have any understanding of how often or where life forms in the galaxy, although numerous models have been proposed to try and explain this (1,2). It may be the case that if the right conditions for the formation of life are present, however rare those may be, then life inevitably forms. It may also be that, despite ideal conditions, formation of life relies on a string of random chance events that eventually leads to a self-replicating chemical system. It may also be that there are no single ideal conditions for the formation of life and, so long as a string of low-chance events all occur regardless of environment, an organism will form. All of these are possible, but all seem to lead to a similar evolution as has been seen in humans throughout Earth's history. A multi-celled organism will not spontaneously form out of its necessary building blocks. If anything, then perhaps a small system will form that is capable of evolution and replication. This system would then increase in complexity until it reached a form requiring multi-cellularity for further increases in complexity. Such an evolution is exactly how life on Earth is speculated to have evolved and, indeed, it seems the only way an intelligent species could arise from its component molecular building blocks.
Based on such a predictable model for evolution of intelligent beings, where does that put the uniqueness of the human species on the "quirky" scale of the universe? If we abide by the same mechanisms and are forced by the same evolutionary drives as other intelligent species, are we are representative of the "typical" societal evolutionary pathways? Any species that becomes sufficiently technologically advanced would take rise over other species on a planet, as we have seen humans do over other animals, and would push to create renewable energy resources for eventual off-world exploration. As such a species became aware of their situation in the universe, perhaps by looking out towards the stars through telescopes or radio-based equipment, they would realize that they are not alone and begin the search for extraterrestrial life. Additionally, understanding of their own evolutionary mechanisms could push any species towards researching methods of defying said mechanisms, allowing them remarkable control over their own evolution and creating a postbiological society (3). Certainly, it can be expected that any alien species that wishes to explore the galaxy cannot be weighed down by biological factors such as aging, illness, or mutation and systems would likely be created to counteract these forces. Thus, it seems that any intelligent species that becomes sufficiently advanced would necessarily form a postbiological society, and it appears that humans are on the way towards this as well. Gene technologies such as CRISPR-Cas9 genome editing and research into telomeric replication in DNA have allowed humans an unprecedented understanding of the biological factors that lead to death and illness, a factor that will likely eventually lead to the formation of a human postbiological society.
It has already been stated that the evolutionary path of humans, especially with regards to the handling of the Earth's energy budget, is typical for an evolving society (4). The transition between a class IV and a class V planet, representing unsustainable and sustainable methods of harvesting the planets (and suns) energy respectively, seems to be the deciding factor in the development of a multi-planetary species. If resource constraints are not met before a critical point is reached, then there will not be enough resources to research the necessary technologies to advance to a class V planet and the society can be expected to never make it off the planet. If these needs are met, however, then the necessary technologies will likely be in place for long-term sustainable growth of the civilization and subsequent expansions that might take place into the cosmos. These goals can be thought of as typical for an intelligent species and thus humans can be seen as being at a typical stage in the evolution of intelligent species.
Therefore, because of the numerous stages of human evolution that would resemble the evolution of any intelligent species in the cosmos, it is unlikely that humans are alone or unique in the universe. It is much more likely that we simply have not joined the numerous other intelligent civilizations in galactic conversation or in death. The large degree of uncertainty in our understanding and the philosophical, scientific, and religious implications of this are the reasons that I consider the question of human peculiarity the most open question in astrobiology.
1. Ponnamperuma, C. (1992) The Origin, Evolution, and Distribution of Life in the Universe. Physiologist, 35(5), 273-278.
2. Lineweaver, C. H., Fenner, Y., Gibson, B. K. (2004) The Galactic Habitable Zone and the Age Distribution of Complex Life in the Milky Way. Science, 303(5654), 59-62.
3. Dick, S. J. (2003) Cultural Evolution, the Postbiological Universe and SETI. International Journal of Astrobiology, 2(1), 65-74.
4. Frank, A., Kleidon, A., Alberti, M. (2017) Earth as a Hybrid Planet: The Anthropocene in an Evolutionary Astrobiological Context. Anthropocene, 19, 13-21.