The question of whether we are alone in the universe is one that fascinates both scientists and the general public alike.
As a scientist, it is a question I think about often, and one I most want an answer to. But that answer depends a lot on what kind of life is favored in the cosmos – if any at all.
In this article, I explore the factors that may determine how common complex, intelligent life is beyond Earth. On our own planet, the story of life has been dominated by single-celled organisms for over 3 billion years. Earth formed 4.5 billion years ago, with the first fossil evidence of life appearing 3.5 billion years ago.
For nearly 2 billion years, all life consisted solely of prokaryotic single-celled creatures. While amazing biochemical processes evolved in these cells, multicellularity did not emerge until about 700 million years ago.
What can we deduce from the fact that our planet was inhabited solely by bacteria and other microbial life for so long?
It implies that evolutionary success does not necessitate multicellularity or complexity comparable to macroscopic animals. During this age, single-celled life developed incredible nano-scale equipment for a variety of tasks, including photosynthesis for energy production, intricate intracellular transportation, and ingenious metabolisms. Even in the absence of macroscopic species, this single-celled biosphere had advanced in terms of sheer metabolic complexity. Perhaps single-celled life is the most prevalent outcome when life appears on distant planets.
The propensity towards multicellularity may be more specific to Earth’s surroundings. We cannot draw any conclusions until we have more evidence from extraterrestrial biospheres. Another potential obstacle to sophisticated life is planetary atmospheres and oxygen. When life first appeared on Earth, the atmosphere was probably a combination of nitrogen and carbon dioxide. It was not until around 2.4 billion years ago that oxygen became a significant component, building up over time as a result of a new type of photosynthesis.
The introduction of oxygen into Earth’s atmosphere proved to be a critical role in the evolution of complex life. As animals developed ways to use oxygen, energy availability increased considerably, accelerating the evolution of larger, more complex creatures. Some astrobiologists argue that only oxygen provides the necessary chemical conditions to support the metabolic needs of motile, multicellular biological life. As a result, the late oxygenation of the Earth’s atmosphere may have been necessary for the formation of organisms like us.
Other factors that may influence habitability on planets include their atmospheres. It has long been assumed that stars have a “habitable zone” of distances at which liquid surface water can exist on orbits. However, astrobiologist Eddie Schwieterman and others have presented the idea of a habitable zone designed expressly for sophisticated life.
To keep the surface warm on planets farther away from their sun, substantial greenhouse gas blankets such as carbon dioxide would be required. However, the high CO2 levels tolerated by many bacteria would be poisonous to most known macroscopic life, potentially restricting animal life to a smaller range of orbits.
So we have numerous lines of evidence indicating that, while microbial biospheres are abundant on exoplanets, complex animal biospheres may be rare. If this is correct, it would imply that the galaxy is teeming in single-celled alien life but sparsely populated with worlds containing recognisable animals. However, caution is advised before making this judgement.
First, even conservative estimates suggest that our galaxy contains hundreds of billions of planets, allowing for significant evolutionary experimentation. Second, we know that nature finds answers to problems beyond our comprehension; perhaps systems for sustaining complex life in high CO2 conditions could arise after all.
Finally, we cannot make firm conclusions without further observational data beyond our unique biosphere. The good news is that our detection capabilities are improving rapidly. The coming decades may provide the first indications of how lonely or crowded our cosmic neighbourhood may be. As a scientist, I anxiously anticipate data that will inform extraterrestrial evolutionary theories by revealing whether complex macroscopic life is widespread or unique to our remote tiny planet.
