SCIENCE28 - Intelligent Alien Life in the Universe?

I’ve always been interested in human evolution and have studied and written about the subject.  I’ve also been fascinated about the prospect of intelligent alien life in the universe.  I’ve followed closely over the last few decades as we have built elaborate earth-bound antenna farms to “listen” for alien communications, and launched increasingly powerful telescopes to try to find “markers” or signs of advanced alien civilizations.  I’ve also noted, and sometimes been amused by, the imaginative fictional depictions of alien life forms in books and films.  

While ruminating on all this one day recently, I wondered what “science” says today about the possibility of intelligent alien life existing somewhere in the universe, and if it does, would alien life forms have evolved to look and act anything like humans (humanoids).  I realize that there are no answers to these questions yet, but I thought it would be useful and fun to explore these questions and summarize arguments for and against.

 

So, in this blog, after reviewing the key steps in human evolution for reference, I’ll discuss arguments for and against the existence of intelligent alien life, and arguments for and against any intelligent alien life being similar to us.

I warn the reader up front that my format to address these issues will be subheading-dominated so that I can most efficiently organize the data and my thoughts.

As usual, I will list my sources at the end.

 

Key Steps in Human Evolution

I thought it would be useful to review the key steps in human evolution on Earth as a basis for evaluating the possibility of similar evolution of intelligent aliens on other worlds.

Life began on Earth 3.5 to 3.8 billion years ago (bya) with single-cell microorganisms (that evolved from primitive protocells) that lived without oxygen, were heat-loving, and inhabited geochemically active environments like hydrothermal vents. 

Between 3.5 bya and 700 million years ago (mya), life on Earth was predominantly microscopic, evolving from single-cell microorganisms to include bacteria and more complex cells. The major transition during this period was the evolution of photosynthesis, which began to release oxygen into the atmosphere, and the eventual appearance of complex, multicellular life.

The slow pace of evolution between 3.5 bya and 700 mya was due to limiting environmental conditions and the complexity of developing multicellular life.  Key factors included the extremely low levels of oxygen in the atmosphere, which were necessary for complex animal life, and the immense genetic and cellular hurdles that had to be overcome to evolve beyond simple, single-celled organisms.  It took a long period for life to build the genetic tools and complexity needed to create the first animals. 

Around 600 mya to 700 mya, the first sea animals (sea sponges and comb jellies) appeared.

Up until about 500 mya, all life was sequestered in the sea.  Fish were the first vertebrates and introduced additional organs like stomachs, spleens, and body components like scales, teeth, blood, and more.   Bony fish arose, and over time their development brought about sophisticated changes to the skeletal system, eventually producing “proto-limbs” that would enable organisms to walk on land.

The first land animals (420 mya) were millipedes or other arthropods (segmented bodies, jointed appendages, and hard external skeleton).  These creatures eventually became tetrapods (“four-footed”), and they had features like four-legs, a backbone, and lungs which could absorb oxygen from air.  All the amphibians, reptiles, birds, and mammals that followed are descendants of the original tetrapods.

Around 225 mya, the first mammals emerged as small, shrew-like creatures that lived alongside the dinosaurs.  Over time, mammals evolved hair, specialized teeth, sweat glands to regulate body temperature, and a more efficient circulatory system.  Mammals also brought about features like nocturnality, mammary glands, external genitalia, and a variety of other features that distinguished them from other living species at the time, like birds or reptiles.

The first primates, which had features like larger brains and forward-facing eyes, appeared around 55-56 mya.  Around 7 mya, the first great apes emerged in Africa.  These apes, such as orangutans, gorillas, and chimpanzees, were highly intelligent and social creatures that lived in complex communities. 

About 2.5 mya, one lineage of apes gave rise to the first members of the genus Homo.  The main developmental changes during this time were the full-time bipedalism of apes, increasing brain size, and advanced bone development that enabled dexterity for tool construction and hunting.   Inventions like fire and clothing arose early in the Homo genus, and eventually complex language, hair loss, and dramatic facial changes would evolve.

Our human species, homo sapiens emerged about 300,000 years ago, with advanced cognitive abilities, complex language, art, and eventually, agriculture, and civilization. 

Key evolutionary stages for evolution of humans on Earth.

Human evolution occurred through the process of natural selection, where organisms with heritable traits better suited to their environment are more likely to survive and reproduce.  Over many generations, the advantageous, heritable traits become more frequent in the population, causing the population as a whole to become better adapted to its environment.  While the genetic mutations that cause variation are random, the selection process itself is not.  Traits that benefit survival and reproduction become more common, while those that don't are less likely to be passed on.  This mechanism drives evolution, leading to the diversification of species as populations adapt to their environments.

Evolution of life on Earth was interrupted by five mass extinction events between 444 mya and 65 mya, caused by severe changes in climate and ocean chemistry, volcanic activity, and an asteroid impact.  These events killed up to 96% of life existing at the time, resulting in opportunities for new species to replace the ones that had died off and, in some cases, accelerating the evolution of certain species, including humans.  See my blog on the subject at: https://bobringreflections.blogspot.com/2025/06/science22-mass-extinctions-accelerated.html.

 

Arguments for the Existence of Intelligent Alien Life 

The statistical likelihood of intelligent alien life is considered to be high by many scientists.  This is based on the vast number of stars and potentially habitable planets in the universe.   

Vastness of the Universe: The observable universe is estimated to contain 200 billion trillion stars.  Our Milky Way galaxy alone has hundreds of billions of stars and could have billions of Earth-like planets.  Even if the odds of intelligent life emerging on any single habitable planet are small, the sheer number of planets makes it seem highly probable that intelligent life has arisen elsewhere.

In 1961, astronomer Frank Drake created the Drake equation, a formula to estimate the number of active, communicative extraterrestrial civilizations in the Milky Way galaxy.  It's a probabilistic argument that multiplies seven factors, including the rate of star formation and the fraction of stars with planets, to estimate the number of civilizations that could be detectable.  While it's a valuable framework for organizing research, many of the variables are still unknown, making it impossible to get a precise answer.  However, for over 60 years, the equation has produced a lot of discussion and spurred the search for intelligent alien life.  

Astronomer Frank Drake’s equation has spurred the search for intelligent alien life.

Early Life on Earth: Evidence suggests that life on Earth began relatively soon after the planet formed and became habitable.  Some scientists argue that if life began so quickly here, it may be common throughout the cosmos, provided the right conditions exist.

Panspermia: This theory suggests that microscopic life forms, such as bacteria, can travel through space via comets or meteoroids, seeding life on other planets. The discovery on Earth of microorganisms that are adapted to thrive in environments with conditions typically lethal to other life forms, such as extremes of temperature, pressure, radiation, salinity, or pH (measure of acidic or alkaline), and the presence of organic molecules on meteorites and in space, lend some support to the possibility of life surviving interplanetary travel. 

 

Arguments against the Existence of Intelligent Alien Life. 

The Fermi Paradox, first articulated by physicist Enrico Fermi in 1950, highlights the contradiction between a high probability of alien life and the complete lack of evidence for it.

The Fermi Paradox leads to the central question: "Where is everybody?". 

 

Here are two potential explanations for this cosmic silence: 

The Great Filter: This theory posits that at some point between the initial spark of life and the development of a civilization capable of interstellar space travel, there is a barrier that is incredibly difficult for life to overcome. This could be the origin of life itself, the rise of complex cells, or, more ominously, a propensity for technological civilizations to destroy themselves.

Intelligent Life is Rare: It's possible that the specific conditions that allowed for intelligent life to develop on Earth are exceptionally rare, making intelligent life here a statistical fluke.  This Rare Earth hypothesis suggests that the evolution of complex, multicellular life to intelligent life requires a perfect and highly improbable combination of circumstances.

 

Arguments for Humanoid Intelligent Aliens

For an alien species to become technologically advanced, it is likely to inhabit a moderate-gravity, land-based environment with access to a rich energy source, like a star.  These conditions could favor the evolution of a body plan that is generally similar to ours.

An optimistic view that intelligent alien life might be humanoid is supported by the concept of convergent evolution and a recent study that alien life might be more likely to be humanoid that previously thought.

Convergent Evolution. This theory suggests that similar environmental and physical challenges can lead to independently evolved species developing comparable features and body plans, even on different planets. 

The principle of convergent evolution has been studied for centuries, with early ideas stemming from anatomical comparisons that revealed similar structures despite distant ancestry.  Naturalist Richard Owen, in the 19^th century, laid the groundwork for understanding convergent evolution. The concept gained further scientific momentum as evolutionary theory developed and was later confirmed through genetics, which demonstrated how unrelated species independently evolved similar traits in response to similar environmental pressures

On Earth, the phenomenon of convergent evolution has produced similar features in unrelated species.  For example, both dolphins (mammals) and ichthyosaurs (reptiles) evolved streamlined bodies and fins for efficient movement in water.  And both birds (reptiles) and bats (mammals) evolved wings for flight.

An optimistic view applies this phenomenon to planets outside our solar system.  This means that if another world shared certain environmental characteristics with Earth, intelligent life there could develop some "humanoid” features, including traits such as:

Bilateral Symmetry: A body plan with two mirrored halves offers superior maneuverability and is a common evolutionary strategy on Earth.

Energy Efficiency: Standing upright offers long-distance visibility with a relatively low body mass.  An upright posture also conserves energy during movement, allowing a species to cover more ground with less effort.

Tool Manipulation: A bipedal posture frees the upper limbs to become highly dexterous for creating and using complex tools.  Opposable thumbs, a key feature of the humanoid form, allow for fine manipulation, which is critical for developing advanced technology.

Sensory Arrangement: A head with bilateral sensory organs like eyes, ears, and nostrils provides stereoscopic vision and hearing, which is advantageous for perceiving and navigating a three-dimensional world.  This arrangement is also an efficient way to house the primary command center, or brain, of a complex organism.

Adaptability: A versatile, generalized body plan allows a species to thrive in multiple environments, rather than being limited to a specialized niche. This adaptability could be crucial for a species hoping to spread beyond its home planet. 

This interpretation imagines the humanoid form as a practical, and therefore common, solution to the challenges of developing advanced intelligence.  Alternatively, this perspective does not suggest that aliens would look exactly like humans, but rather that a broadly humanoid body plan is a plausible outcome of evolution.  Intelligent alien life would likely be defined by the unique environmental conditions of its home planet.  This view allows for a diverse array of variations, including different skin textures, extra eyes, or unique appendages. 

Humanoid alien concept.

Recent Study.  In a 2017 study, scientists at the University of Oxford concluded that aliens could be shaped by the same evolutionary processes as humans, making them potentially similar to us.  The research suggests that the theory of evolution, and the principle of natural selection, are universal and would apply to life anywhere in the cosmos.  Key points from the 2017 study:

Universal Laws of Evolution: The Oxford study argues against the idea that extraterrestrial life would be monstrously different due to the randomness of evolution.  It posits that because natural selection is a universal mechanism, aliens would be shaped by similar forces to adapt and survive in their unique environment.

Predictable Traits from Convergent Evolution: The study builds on the concept of convergent evolution, which demonstrates that similar environmental pressures on Earth often lead to analogous physical features in unrelated species.  Applied on a universal scale, this could mean that alien life may develop familiar traits, such as:

Bilateral Symmetry: Having two sides that are mirror images of each other.

Limbs: Appendages for movement.

Heads with Sensory Organs: A concentrated area for sensory perception, like eyes and ears.

Digestive Systems: Internal structures for processing nutrients.

The researchers argue that this predictable, "universal" form of evolution challenges the view that intelligence is a complete fluke of chance and is far-fetched on any planet.  While this does not guarantee human-like aliens, it increases the odds that intelligent extraterrestrial life might follow a recognizable path to complexity. 

The idea of humanoid aliens is deeply rooted in our culture for understandable reasons.  The humanoid form is familiar, and makes characters more relatable for human audiences.  For decades, science fiction has presented us with countless examples of humanoid aliens.  By portraying extraterrestrials as humanoid, writers make them more relatable, allowing for exploration of human themes like morality, ethics, and social dynamics.  This convention can be interpreted as an optimistic projection, assuming that alien intelligence would lead to a body plan capable of communication and interaction in a way that feels familiar and not overwhelmingly foreign. 

The humanoid form is a testament to the efficient combination of various features that have allowed humans to thrive and develop technology.  The optimistic view suggests that these same advantageous traits might logically evolve elsewhere under the right conditions.  The humanoid form, while not guaranteed, presents a highly effective and versatile design for an intelligent, tool-using species. 

 

Arguments against Humanoid Intelligent Aliens

While some human traits may appear, the odds of a truly humanoid alien are astronomically small, largely due to the sheer variety of evolutionary paths and environmental factors. 

Variety of Evolutionary Paths. The evolution of the human body plan was not a foregone conclusion, but rather the result of a specific, lengthy, and somewhat unpredictable sequence of events.  Reasons intelligent aliens would likely not be humanoid include:

Evolution is Not a Predetermined Path: Evolution works by adapting existing forms to changing conditions, not by striving toward an ideal form.  If life on another planet began with a different basic body plan-for instance, one more like an arthropod, its path to intelligence would be completely different.  Earth's own octopuses and squids offer a compelling alternative.  They possess high intelligence, excellent problem-solving skills, and a flexible, boneless body with multiple manipulating tentacles.  On another world, a similar lineage could develop greater manual dexterity or tool-making capabilities.

Concept of nonhumanoid intelligent alien life form resembling an octopus.

 

Another possible type of intelligent life form could evolve as a stationary organism, perhaps similar to a plant, that uses mobile, bio-engineered tools or symbiotes (an organism that lives in a state of symbiosis with another organism) to manipulate its environment.

Alternatives to Bipedalism: Walking upright was one of the most important factors in human evolution, freeing the hands for tool use and allowing for greater endurance running.  While bipedalism offers benefits, such as thermoregulation and efficient low-speed movement, it is not the only viable mode of locomotion for an intelligent species.  Other options could include:

·         Tripedalism or hexapedalism for increased stability.

·         Aquatic species that use fins or tentacles.

·         Arboreal creatures that retain tree-dwelling adaptations.

Alternative Paths to Tool Use / Opposable Thumbs: Our fine motor skills are a product of our arboreal ancestry and a complex set of evolutionary changes. Other species on Earth have developed dexterity in different ways.  The giant panda, for example, evolved a thumb-like structure from a wrist bone, completely different from human anatomy.  A non-humanoid species might develop manipulating appendages for tool use in a different way or in a different location on its body.  An octopus, for example, has highly dexterous tentacles and is quite intelligent, demonstrating an alternative evolutionary path.

Different Forms of Intelligence: Intelligence itself may not be the same.  An alien civilization could be a hive-mind, a distributed network where individual organisms possess limited intelligence but function as a collective superorganism with complex capabilities.  Or an alien species could evolve that doesn't rely on technology in a way humans would recognize. 

Alternative Biochemistry:  Several forms of biochemistry are agreed to be scientifically viable, but are not proven to exist currently.  Life on Earth uses carbon compounds for basic structural and metabolic functions, water as a solvent, and deoxyribonucleic acid (DNA) or ribonucleic acid (RNA) to define and control their form.  If life exists on other planet, it is possible that there are organisms with quite different chemistries - for instance, involving other classes of carbon compounds, compounds of another element, and/or another solvent in place of water.

Planet Identical to Earth. Even on a planet nearly identical to Earth, intelligent life would almost certainly not be humanoid.  The specific combination of evolutionary pressures that led to the human form is an intricate and improbable set of circumstances that would likely play out differently on another world, even under the same conditions.  The principle of convergent evolution suggests that similar environments can lead to similar physical traits, but it does not mean life will converge on the human form.  For example, the streamlined bodies of sharks (fish) and dolphins (mammals) are a result of convergent evolution for swimming efficiency, but they have distinct anatomical differences. 

Contingency Events: Evolution is a process full of chance and historical contingency.  The particular genetic mutations, selective pressures, and mass extinction events that led to our intelligence are highly specific to Earth's history. 

Environmental Factors. A wide variety of environmental factors could lead to the evolution of non-humanoid intelligent life.  Any factor that profoundly alters the selective pressures on a developing species could push evolution down a completely different path from the one that led to humans. 

Gravity. The gravitational pull of a planet would have a massive effect on the size, shape, and mobility of any complex life. 

High-Gravity Planets: Life would likely be short, stocky, and dense to withstand the crushing force.  A multi-limbed, quadrupedal, or even multi-legged body plan would be more stable and likely than a bipedal one.  Flight would be highly improbable, requiring either extremely dense air or a different method of locomotion entirely.

Low-Gravity Planets: With less gravity to fight, life could grow taller and slenderer. Flying creatures could be common, with larger wing spans and less muscle power required to get airborne.  A civilization could potentially develop and thrive among the clouds, giving them a completely different perspective and material culture. 

Atmosphere. The composition and density of a planet's atmosphere are critical to evolution, especially regarding respiration and the potential for flight. 

Dense Atmospheres: A thick atmosphere would make flight easier, potentially leading to airborne or buoyant intelligent species that never touch the ground.  This could lead to a less tool-oriented or physically manipulative society.

Different Atmospheric Gases: If a planet's atmosphere is not oxygen-based, life would need a completely different biology.  For instance, life on a planet with a hydrogen sulfide atmosphere would likely have a completely different metabolism and could evolve forms entirely alien to us. 

An alien planet’s environment would determine alien life forms.

Resource Distribution. The way resources are distributed across a planet affects where and how life, particularly intelligent life, develops. 

Subsurface Oceans: If life and resources are sequestered in a subsurface ocean beneath a frozen crust, intelligent life would likely be aquatic.  An octopus-like species that develops dexterous manipulator appendages and learns to use bioluminescence to communicate could emerge.  Their civilization might never use fire or develop terrestrial technologies.

Scarcity and Abundance: If key elements or minerals are only available in specific, dangerous locations-like deep-sea hydrothermal vents or volcanic regions-it could drive the evolution of a species to endure extreme conditions to harvest them.  Alternatively, if resources are very abundant, an organism might not develop the aggressive resourcefulness needed for tool use and technology. 

Type of Star. The type of star a planet orbits influences everything from the available light to the planet's overall climate. 

Dim or Red Stars: A planet orbiting a dim red dwarf star might lead to species with much larger eyes to gather as much light as possible.  This would influence the sensory organs and the species' perception of its world.

Unstable Stars: An unstable star, prone to frequent, powerful flares, might favor life that is hardy and capable of surviving underground or underwater to escape surges of radiation. 

Planetary Geology: A planet's geological activity, or lack thereof, could shape the evolution of its intelligent life. 

High Volcanism: A planet with very high volcanic activity might favor species that are resistant to heat and toxic gases.  Their technological development would be shaped by the readily available geothermal power.

Tidal Forces: A moon-bound species that experiences extreme tidal forces could evolve to adapt to a highly dynamic environment.  This could lead to amphibious life that can survive both under intense pressure and in harsh surface conditions. 

While the factors that drove human evolution might favor traits like dexterity and problem-solving, the specific combination that produced the humanoid form is highly improbable to be replicated.  The evolutionary path to intelligence is not a single, predetermined road, but a vast network of possibilities influenced by countless chance events. 

 

Concluding Observations

I found the arguments for and against the existence of intelligent alien life and the arguments for and against that such life, if found would be similar to us, reasonable, but far from conclusive.  Pretty complex questions with so many unknowns!

However, I believe that even if there’s a high statistical likelihood of intelligent life-forms having evolved elsewhere in the universe, and even if we find evidence of intelligent alien life out there someday, there is a very low probability that we’ll be able to communicate or interact with them.  The reasons are the vast distances, and travel or signal transmission times involved - based on our current understanding of the universe and its physical laws.  Therefore, I believe that we have not been contacted by aliens in the past, and that Fermi’s Paradox does not apply.  These important questions remain open in my mind.

What do you think?

 

 

“In the deepest sense, the search for extraterrestrial intelligence is a search for ourselves.” - Carl Sagan, American Astronomer

 

 

Sources

My principal sources include: “Humanoid,” “Fermi Paradox,” and “List of humanoid aliens,” Wikipedia.com; “Aliens may be more like us than we think,” ox.ac.uk/news/2017-10-31-aliens-may-be-more-us-we-think; plus, numerous other online sources, including answers to many queries using Google in AI Mode.