Would Life Really Develop Differently on
Another Planet? - Convergent Evolution Pointed ears, blue skin, abnormal head, or
reptilian jaws: there are a thousand ways in which science fiction literature, movies, and
alleged eyewitness accounts have depicted Aliens visiting Earth. So different in height, shape,
and somatic features, but still with one thing in common: a vaguely human appearance.
But if in the entertainment world, it is easier and cheaper to make up actors
as humanoid E.T.'s rather than as shapeless blobs or tentacled octopuses--in the scientific
debate the criteria are evidently different. Yet there is no consensus among researchers
as to what these possible neighbors from the next galaxy should look like:
assuming and not assuming they exist, are they similar enough to us or so different
that our minds cannot even imagine them? Try to follow along and we will
try to find out together. OK? Some argue that it is futile to
speculate on the nature of alien life; our imaginations would be too constrained
by our own experience to be able to embrace the amazing heterogeneity and unusual
possibilities perhaps realized in other worlds. Well, if I may express my
personal opinion... I disagree. In my opinion, science can help us
overcome such a pessimistic view, allowing us to make realistic assumptions about
how aliens might be structured morphologically. Many - like me - are confident that
the laws of physics and chemistry are unambiguous and universal. And that they work
on Earth just as they do on any exoplanet. If there is one thing we know for certain about
aliens, it is this: they too, just like us, are a product of evolution by natural selection.
Others, however, believe that biology is an exception. They find it hard to believe that
the biological laws underlying Earth's evolution are also applicable to exoplanets.
Carl Sagan, one of the most famous astronomers of the twentieth century,
was firmly convinced that intelligent life existed elsewhere in the universe,
yet he wrote, "To our knowledge, however, biology is a literally terrestrial and provincial
science, and we can be familiar with only one special case in a universe of diverse biologies."
When we face the unknown, there are indeed good reasons to be cautious. But why should biology
be "terrestrial and provincial" rather than universal? Shouldn't the laws of nature,
physical, chemical, and even biological, be common to the entire universe?
Fortunately, there is a strong component of scholars in the scientific community who
do not think like Sagan. According to them, aliens, if they exist, cannot be so
different from us--and to support this hypothesis they usually invoke a well-known
phenomenon, that of evolutionary convergence. Basically, species subjected to the same
environmental pressures tend to develop, by natural selection, similar anatomical features.
This is why a mammal like a dolphin looks similar to a cartilaginous fish like a shark. Even a
creature that developed and lived in the liquid ocean of a distant exoplanet would probably
develop a fish-like shape to move quickly. But just a moment... before we get into
the details, it will be good to clarify which aliens we are talking about... On our
Earth, there are currently some 20 million different species... from the whale to the
gnat... If our discussion were limited to ascertaining the differences between any animals
on Earth and any animals on an extrasolar planet, well... then we would already have the answer...
Indeed, one need only consider the extraordinary variety of shapes and sizes of our fauna to
realize immediately that an alien planet with environmental conditions similar to our own
would probably also be home to millions of species extraordinarily different from each other.
There would also be animals there large and small, terrestrial and marine, flying and terrestrial...
occupying every environmental niche available. Not identical in form to terrestrial ones,
but certainly similar in function dictated by environmental and evolutionary pressure.
So that, when we go out and actually discover a large number of Earth-like sentient worlds, we
will find that most of the creatures on them took forms designed by physical and biological
laws: fish-shaped swimming organisms, bird/bat-shaped flying creatures, four-limbed
terricolous creatures... and perhaps largely upright, two-armed, large-headed bipedal
sentients... "humanoids" that use tools. It should therefore be clear that when we speak of
comparisons between aliens from different planets, we must mean only morphological comparisons
with sentient beings who have arrived to express a technologically advanced civilization!
It matters little whether an extrasolar planet is inhabited by millions of very strange species...
the point of this discussion is, to answer the question: If aliens from other star systems came
to visit us one day, what would they look like? "Hey, guys, just a moment before we continue...
BE sure to join the Insane Curiosity Channel... Click on the bell, you will help us to
make products of ever-higher quality!" First, a true alien would have
to be bilaterally symmetrical, endothermic with excellent manipulative
abilities, and have a rigid brain case. If we consider the living beings present
on Earth, we realize that all the more complex ones, particularly on land,
exhibit bilateral symmetry, that is, it is possible to divide the body with a sagittal
plane separating two mutually specular halves. This is true for a human, as well as for
a bear or a mosquito, but it is not the only possible configuration; for example, an
octopus or a starfish exhibits radial symmetry. On Earth, all the most complex
organisms exhibit bilateral symmetry. The appearance of bilateral symmetry
has been of enormous importance in the evolution of living things on Earth.
The earliest organisms with bilateral symmetry moved in a single direction, and
this caused one end of the body to come into contact with its surroundings
before the rest of the organism did. The sensory organs therefore developed
on this end, because this enabled them to detect the characteristics of the
environment in which one was moving sooner, and for the data from these
organs to be processed more quickly, the nerve centers that performed this
function needed to be as close as possible. Over time, millions and millions of years,
this led to the development of a head, which contains the brain and has the sense
organs of sight, taste, smell, and hearing. Most scholars believe, based on what
can be observed in the millions of animal species on our planet, that even a
hypothetical technologically evolved alien would exhibit some symmetry, probably bilateral.
However, we must not forget," someone objects, "that our evolutionary path is the result
of a very long sequence of random events, so much so that if it were possible to restart
from the same initial conditions thousands and thousands of times, not a single other
time would this path be identical, and all the more reason that this would be
valid in an environment, albeit similar, that is nonetheless alien to our own.
A being developed in a distant world might therefore not have bilateral or even
radial symmetry, which would obviously make it appear quite different from us. Such
a creature would appear very strange and ungainly to our eyes because of how our
brains evolved, but it is still possible. But this is an objection that could be answered
by saying that the diversity they are talking about could only occur among lower animals
and not in a species that has been able to evolve to the point of technology. The
reason is intuitable...Octopuses get by in their environment, but they certainly
could not exist as land creatures. Moreover, no matter how pliant their tentacles are,
they cannot manipulate objects in fine ways. An ability they did not develop because it
was not necessary for the marine environment. Deciding factors would be the
specific characteristics of the host planets. Earth's fauna, including humans, is absolutely determined by the history of the
planet through millions of years of evolution, climate change, habitat adaptation, and
mass extinctions due to external events, for example, the impact of large meteorites
or the explosion of nearby supernovae. This means that even under exactly the same geographic
and environmental conditions, i.e., in the case of a planet identical to Earth, with the same
astronomical and astrophysical characteristics, evolution could certainly have taken totally
unpredictable paths and thus capable of producing differences far greater than a pair of pointy
ears, but always within well-defined limits. However, there are two environmental
components capable of directing the evolution of an ecosystem.
Gravity is a key factor influencing the development of all organisms. In addition to
limiting the size of land animals, gravity also imposes several very specific adaptations.
We can see evidence of this right here on Earth. Organisms that made the transition from
water to land had to develop complex limbs and skeletons because they no longer had the buoyancy
of water to compensate for the force of gravity. Of course, on Earth, with the same gravity, there
are complex living things of all sizes, from the mosquito to the blue whale (the latter probably
approaching the maximum size attainable by any animal on our planet), but there are probably
limits for a technologically evolved species. To be able to process metals, for example, a
creature could hardly be as small as a squirrel, just as, on the other hand, a being the size
of a brontosaurus would have serious difficulty developing aircraft capable of supporting
its own weight, let alone space capsules. These are just two examples of how dimensions too
small or too large would constitute, if not an insurmountable limitation, at least a major brake
on the technological development of the species. Imagine a hypothetical situation in which
Earth's gravity is doubled. Although this would not necessarily force all complex life on
earth to resemble a squat turtle-like creature, the likelihood of being bipedal humans
would decrease dramatically. Even if we could maintain our two-legged method
of movement, we would certainly be much shorter and have larger bones to accommodate
the stronger force of gravity. Meanwhile, an Earth with half the gravity would
probably have the opposite effect. Earth animals would require less muscle and
weaker skeletons to cope with gravity, and life, in general, would be taller and larger. While we
can theorize about the general characteristics of life in high or low gravity, there is no way to
predict more subtle adaptations. Such adaptations would further alter the appearance of alien life.
The atmosphere obviously also plays a role in directing evolution... For example,
arthropods that lived during Earth's Carboniferous period some 300 million years
ago were significantly larger than their modern counterparts (dragonflies nearly a meter long
were running around in those days!). And this was due to higher atmospheric oxygen content:
up to 35 percent versus 21 percent today. And OK... all this means that similar
environments develop similar forms regardless of their evolutionary history.
On Earth, this is called parallel evolution. The canonical example is the mouth shape of the
flamingo, which begins an almost exact replica on a small scale of the mouth shape of whales even
though they share no common history. Why? Because they are both filter feeders of microscopic
organisms in the water and both have hinged jaws. That shape is the most efficient to do that
job. And at that point, it's physics, not biology. If we went to another planet and found an animal
with articulated jaws that eat the same way, it would have the same mouth shape.
The same is true for all evolution in similar environments. The fact that the
environment is on another planet is irrelevant. Final considerations
In short, for me, it goes like this: If the alien has come down to us, or at any rate possesses
the means to make contact with our species, it will be of an intelligence at least equal to
ours, and therefore from an evolutionary point of view can only be structurally very similar to us.
It may be strange in all the ways you can imagine: scaly skin, number of fingers varying from three
to five, ridges on the head, bat ears, etc...but it will definitely be similar to us in size,
symmetry, and number of limbs, upright posture, head with eyes and openings for mouth and nose.
Of course, coming down the ladder of the spaceship will not be a gelatinous, telepathic blob
incapable even of using a screwdriver or tapping on computer keys...nor will it be a
cephalopod like the ones in the movie Arrivals, who took over the science of space travel
by talking to each other in ink splatters... So what about you guys, whose side are you on? The Divergent Evolution: Do
Aliens Have To Look Like Us? One of the main lines of thought in contemporary
astrobiology is that extraterrestrial intelligent life will be very similar to us in many ways,
especially when higher levels of organization are considered. This is because the processes
that give rise to life are assumed to operate independently of the environment, and also
because physics severely limits the variety of feasible options. Therefore, the probability
that intelligent life is evolving elsewhere in the cosmos may be very high, although the probability
that it is humanoid in type may be very low. The truth is that we know very little. For example,
we do not know if Earth was the lucky winner of a cosmic lottery or if life has appeared
elsewhere in the universe. We do not even know if a hypothetical ET would follow the same laws
that we know on the only planet that definitely supports life. There is little doubt that physical
and chemical laws apply throughout the universe, but there is not the same certainty about
biological laws. Frankly, if we were to encounter life on another planet, we might not even be able
to recognize it. In short, our first contact with advanced aliens might be a blow to our self-esteem
as we face, who knows, a more intelligent species than our own. But if that first contact ever
happens, we should be prepared for another twist: the aliens may be so different from us that
they won't even realize that we are sentient. At first, glance, trying to imagine the
physical appearance of a technologically advanced extraterrestrial seems like an absurdly
impossible task. For one thing, we don't even know if intelligent beings of extraterrestrial
origin exist, let alone what their home planet might be or what their morphology might be.
Our challenge is therefore limited to using what we know about the evolution of
intelligent life on Earth, considering possible extraterrestrial planetary environments,
and making a series of reasonable assumptions. THE TWO VIEWS
The disagreement about the likely appearance of intelligent extraterrestrials is divided into
two opposing positions. On the one hand, some take a more anthropomorphic view of ET and believe that
it would be essentially humanoid, with two arms, two legs, a head on top of the body, and the
major sensory organs located on the head. On the other hand, some exobiologists believe that
intelligent ET could appear in a completely exotic form because the creature would inevitably have
followed an evolutionary path completely different from that of humans and would have arisen in a
planetary environment very different from Earth. THE ANTHROPOMORPHIC VIEW
Many scientists believe that aliens may not be so different from us, and to support
this hypothesis they usually cite a well-known phenomenon called convergent evolution.
Essentially, species exposed to the same environmental pressures tend to develop similar
anatomical features through natural selection. This is why a mammal like a dolphin looks
similar to a cartilaginous fish like a shark. Even a creature that evolved and lived in
the liquid ocean of a distant exoplanet would probably evolve a fish-like
form to move quickly. After all, the laws of physics are the same here as
they are on the other side of the galaxy. According to proponents of the humanoid form, speculation about ET morphology must take
into account what happened here on Earth, where it was in an aquatic environment that
organic matter based on carbon compounds began to develop. And it was the first marine creatures
that developed the characteristic that would determine the future form of Earth's inhabitants
- that of bilateral symmetry in body shape. This form, in addition to being the most
"economical" in terms of genetic programming, minimized resistance and turbulence in
the water and became the characteristic of all marine creatures, then
transferred to terrestrial animals. ----
Proponents of the humanoid form also argue that intelligence - as it is tied to the use of tools
- can only arise as an evolutionary adaptation among terrestrial species. The use of tools by
marine creatures would be extremely difficult due to the density and viscosity of water.
Moreover, it seems likely that intelligence can only arise in predators. What about birds?
Well, there are some really intelligent ones, like the crow, for example... But for an alien
in the form of a bird to build tools and develop a technological type of intelligence, it would
need appendages similar to ours... arms, hands, and fingers. Which is obviously incompatible
with an animal that uses its upper limbs to fly. Yes, because the development of legs, arms, and
grasping appendages is a delicate evolutionary step on the ET's path to intelligence.
Primitive technology requires the ability to hold and manipulate basic tools and
weapons with a certain degree of sensitivity, such as clubs, spears, knives, and tendons
for bows. And the ability to combine all of this with movement speed, otherwise our E.T.
runs the risk of remaining forever confined to their comfort zone and never being able to
travel around the galaxy aboard a spacecraft. The issue of the number of limbs is one of the
most controversial when discussing the morphology of intelligent ET. The four limbs we have are the
product of genetic inheritance from our earliest mammalian ancestors, which was in turn inherited
from fish with four fins that ventured onto land. But is there an optimal number of limbs?
Well… a single limb would obviously be completely useless… For strictly mechanical reasons,
three points are geometrically required to define a surface plane: two points form only
a line. ETs trying to stand on only one or two levers will promptly fall face down. We humans
can remain upright only because our large feet provide additional points of contact with
the ground, but without fingers or toes, at least three legs are necessary.
Are tripod aliens possible? Traditional biologists say no. A three-legged creature
must lift at least one limb off the ground, at which point it loses its planar support
base, a statically unstable and dynamically precarious situation. Four-legged creatures seem
better from an engineering standpoint since the creature can remain balanced when one leg is in
motion. Ancestral fish only had fins in pairs, so should all limbs evolve in pairs as well?
Xenobiologists are not convinced. Most running bipeds and quadrupeds keep two or fewer limbs
on the ground during locomotion, so dynamic three-point stability is probably unnecessary.
Terrestrial life need not always evolve from fish with even fins: the descendants of a starfish,
for example, might have an odd number of legs. More than four legs are also plausible for
massive, intelligent animals. Odd appendages are often used for highly specialized purposes,
such as the prehensile tail of monkeys and the flexible trunk of elephants. The key to superior
multipedia is neural control. The neural circuitry for an extra limb is far less than that needed to
add, say, another eye. Muscles require thousands of new neurons, but eyeballs require millions.
About one-third of the mammalian brain is devoted to sensory functions, while only a small fraction
is devoted to motor control; aliens are much more likely to have extra arms than extra eyes or ears.
As we mentioned earlier, vertebrates on Earth have four limbs only because they share a
common ancestry with four-finned fish. Had we instead evolved from Euthacanthus, a Devonian
fish with no less than seven pairs of fins, we might be hexapods or even heptapods today!
After all, there are many advantages to living on six legs. On high-gravity worlds, hexapods
are a good way to distribute mechanical stress and reduce the risk of bone fractures. Injury
or loss of a limb is more catastrophic for four-legged humans than for six-legged humans
(who have "spare" limbs). Hexapods also have better balance because, unlike quadrupeds, they
can maintain a stable supporting tripod on the ground even at high speeds.
And it should not be too difficult to coordinate all those legs...
In short, according to mainstream biologists, there seems to be no escape from the dictatorship
of the principle of evolutionary convergence... The exotic forms invoked by some astrobiologists
may exist, yes, and maybe the most common, but the point is that they are not evolutionarily
plausible, nor can they explain how they could have achieved technological intelligence in the
absence of the most basic manipulative skills. EXOTIC BIOLOGY
However, many scientists argue that there is very little chance that
an alien life form could remotely resemble us, given all the possible variables that could make a
difference...such as environmental ones, referring to the particular geological, atmospheric,
and meteorological conformation of the planet; physical ones (first and foremost, the force
of gravity, which is different from Earth's); planetary ones (e.g., a habitable planet could
have a more complex orbit and spatial geography than Earth's); and biological ones, related to
the evolutionary process and natural selection. One possibility often suggested by more radical
exobiologists is that extraterrestrial life may depend on the chemistry that does not require
the carbon atom. Although carbon chemistry is undeniably the optimal chemistry on planet Earth,
one can hypothesize that biological life is built around silicon, which is very common in the sand
and used specifically to make electronic circuits. This could affect the body structure
of extraterrestrials, as they may require different types of proteins, enzymes,
nucleic acids, and other cellular components. For example, an alien might have very thick,
tough skin to protect itself from radiation, or a completely different vascular system to transport
nutrients and oxygen throughout the body. In the 1960s and 1970s, the famous astronomer Carl
Sagan was convinced that technologically advanced species could evolve indifferently on land, sea,
or air, with unimaginable chemistry, shapes, sizes, colors, appendages, and opinions.
This, in turn, influenced the science fiction of those years, which began to imagine
non-humanoid or even decidedly exotic life forms, such as sentient clouds of interstellar
gas thousands of kilometers in diameter, or bizarre "balloon"-shaped creatures
floating in Jupiter's gas belts, using a hydrogen-based metabolism instead of oxygen.
Some suggest that extraterrestrial intelligent beings might resemble insects, birds, fish, or
even plants. They could have a spherical shape, be sticky jellyfish-like creatures (like the
heptapod beings proposed in the movie Arrivals), or even exist as a planet-sized ocean intelligence
like the one in Stanislaw Lem's novel Solaris. But old-school biologists
shake their heads and ask, "How could these creatures have become
intelligent? They seem to us to be only fanciful hypotheses, without the slightest
relation to physical and biological laws! And to this, the exobiologists reply that if the
goal is to scientifically hypothesize intelligent alien life forms, then it is necessary to
abandon all anthropocentric preconceptions and accept creatures that differ from us in
every parameter. The principle of convergence is a popular one in the field of evolution,
it is true, but it would hardly be able to reproduce all the physical characteristics
familiar to us in creatures scattered over billions of other distant worlds...
True, we won't have an answer for many years, decades, or even centuries, but it's still nice
to talk about it and to anticipate issues that will affect our great-grandchildren
very closely in the near future... P.S. And you, what do you think? Are you on the side of the old biologists or on
the side of the exobiologists? These are some considerations in defense
of the "aliens other than us" hypothesis.
