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Hobbes's avatar

If we ever encounter life on other planets, how likely do you think it is that it will resemble life on Earth?

Asked by Hobbes (7368points) July 21st, 2011

Would DNA form the same way on planets with similar conditions? It seems to me that there must be at least some planets out there which resemble Earth. Is it reasonable to expect that even if life evolved differently, it would follow the same basic biochemical processes?

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26 Answers

Blackberry's avatar

I’m not an evolutionary biologist, so I’m sorry if anything I say sounds asinine. But I was thinking that our universe does seem to follow a of set laws. For example, the way we came about on our planet. Then we have the way our planet formed, so if we went in reverse from now. We would see the process of evolution going backwards, then the evolution of the planet.

Now if we apply a similar process to a similar planet somewhere else (which can’t be that different, because we need certain conditions to live, right?), yes there are millions of ways we could turn out different, but I guess it would ultimately be up to whatever evolutionary process took place by chance. Millions of different things could happen in the course of billions of years, but I think the system would be the same as far as a way for this species to breed, breathe and take dumps? Lol.

I’ll apologize ahead of time lol.

marinelife's avatar

Not at all likely. It may not be carbon-based life. It could be sulfur-based.

the100thmonkey's avatar

I suspect ET would be CHON-based (carbon, hydrogen, oxygen, nitrogen) as they are the most abundant elements in the universe. However, it would be crazy to assume that a molecule like DNA is the only way that data can be biologically encoded and transmitted.

FutureMemory's avatar

As @marinelife said, it might not even be carbon-based. It could be vastly different than what evolved on Earth.

trickface's avatar

How many undiscovered elements exist?

Who is to say an undiscovered element could not be a framework for undiscovered life?

josie's avatar

I am with @the100thmonkey . If life is found elsewhere, I bet it will be loaded up with carbon and oxygen. Not certain that DNA will be the universal formula for the Unviverse to figure out a way to know itself.

the100thmonkey's avatar

@trickface – the elements at the heavy end of the periodic table tend to be unstable and radioactive. Hypothetical elements in the Island of Stability might be stable enough to bond with other elements and form molecules, but they’re theoretical, and lack the experimental support of having been observed.

If we apply the Copernican Principle – that the earth is not in any way special in the universe, it suggests that life made from exotic forms of matter, while not impossible, is unlikely.

Zaku's avatar

Um, I think there would likely be many chemical similarities, and mechanical ones, but there would tend to be many striking differences, especially at a basic level. Like how almost all animals are digestive tubes with a lot of bilateral symmetry, a head where the usual organs are, etc. There could be some major differences at early stages which would be shared by many or most species on that planet, which could be quite different from what we have here.

redfeather's avatar

I doubt it, but I hope they’re sexy.

PhiNotPi's avatar

I think that they would be carbon-based. Carbon has some of the best chemical properties for life. The next element that has these same properties is silicon, but waste management would be complicated. CO2 is a gas, but SiO2 is a solid (quartz).

They may or may not be left chiral (all Earth life is left chiral, aliens might be right chiral).

If they have eyes, they will probably have eyes similar to eyes on earth (eyes evolved in vertibrates and cephalopods seperately, and are very similar except for where the nerves are, so this may be a very ideal shape).

They probably do not have DNA/RNA, but they probably have an equivelent to it. The existance of DNA helped evolution go much faster on Earth.

They will probably resemble bacteria on earth, but may have different ways to produce energy. There are bacteria on earth that have very strange ways to produce energy.

filmfann's avatar

As I recall, there are 3 distinct kinds of DNA on our planet. Human DNA closely resembles other animals, as well as flowers, trees, and such. There are also Jelly Fish, which have a completely different strain of DNA. (I can’t recall the third kind)
If these DNA strains can form here, they can form anywhere, and they may be as vastly different as we are from jelly fish. (no offense, Dr. J)

gondwanalon's avatar

Life composed of DNA and RNA on this little known planet is presented in so many forms that it is mind boggling. Many life forms on Earth are so strange and alien looking that space exploration may someday discover something like them out there on some other little known planet/moon. I don’t know if DNA can be altered and still support life, but if it can, then even more weird beasts are likely to evolve.

mrrich724's avatar

We look the way we do because we evolved and adapted to live in our conditions. While I’m not convinced that the ET’s out there look like what the movies makes them look like, I could see aliens looking just like us if the planet was oxygen rich like ours, or looking wildly different, say, with slimy green skin b/c their “air” is composed of other elements which cause them to absorb it in a different manner. Or with gigantic eyes because their planet doesn’t get the same sun we do.

Look how drastically different the fish do that live so far down. They look like aliens!!!! And that’s here on earth, but the pressure and lack of light have caused them to look different.

RealEyesRealizeRealLies's avatar

DNA is our information storage and processing mechanism. It is not necessary that all life forms use it. But they would require some type of information processing. I can envision entities based completely on soundwave vibrations, or photons, or even magnetism.

We would not be capable of directly grokking those life forms. They wouldn’t even have bodies.

ETpro's avatar

The idea that we need earthly coditions for life doesn’t even make sense when you begin to study the diversity of life on Earth and the range of environments in which it flourishes. We find life thriving in the incredible frozen waste of Antarctica and again in boiling sulfuric acid under incredible pressure around volcanic fumaroles in the extreme depths of the oceans.

It is an incredible outgrowth of human hubris to expect that life anywhere else would have to be pretty much like life here. We have no reason to rule out life based on completely different chemistry. We can’t even rule out the possibility of a collection of atoms and molecules that is an intelligent life form and dwells in the center of the Sun or some other star. I completely agree with @RealEyesRealizeRealLies. For all we know, extra terrestrials are here walking among us, perhaps even through us, and we don’t recognize them as life or even notice their existence.

When this topic comes up I am reminded of a Terence McKenna lecture in which he observed, “The search for a radio signal from an extraterrestrial source is probably as culture-bound an assumption as to search the galaxy for a good Italian restaurant.”

atlantis's avatar

If it’s in this universe, it will follow the same patterns of evolution and DNA as on earth. The laws of physics, before anything else, determine the chemical structure of DNA which, in turn, contains the instructions for biological structure.

That being said they may end up looking different than earth life, due to environmental factors which force evolution down a specific path. But wherever life is, it most probably started the same way as it did on earth. Of which our knowledge is still conjectural.

For more information read The Greatest Show on Earth by Richard Dawkins. Very entertaining.


I don’t think it would resemble anything like on earth. Life on other planets probably exist on a different dimension and structure, perhaps not even made of molecules or in need of oxygen. It would be really weird, almost unfathomable, something we’ve never seen before, if we can see it that is.

ragingloli's avatar

Let us see…
– Carbon is one of the most abundant element in the universe, so alien life would very likely be carbon based.
– Since life starts out simple and in simple units, it would most likely be cellular and multicellular life. Internal structure would be quite different, but still based on the same necessities.
– Evolution requires the transmission of some sort of genetic materials, so alien life would most likely be centered around something analogous to DNA.
– Life moves around in the environment, to search for food, or avoid getting eaten. It therefore needs some form of locomotion. Fins have proven to be the most successful option for marine animals, so it is likely that alien marine life has fins, too.
On land, the most successful option has been proven to be legs. Sure there may be worms and other legless critters, but the biggest alien life forms would most likely have multi joint legs (multijoint because of greater speed and flexibility). How many legs? If you just have two legs you require a lot of processing power to be stable, too many legs and you increase the energy requirements and the required processing power to control that many legs.
I think that 4 legs is the perfect tradeoff in terms of processing power and energy requirements. Certain niche animals will have more or less legs, but it is unlikely they will be in a dominant position.
– Further down the path of evolution, when intelligence emerges, it will become necessary to interact with your environment. The most likely event is the retooling of one set of limbs from locomotion to interaction. That means one set of legs would most likely transform into a set of hand analogues to grab things (number of fingers, your guess is as good as mine).
Which set of legs? Well, the most advantagous option is one of the front sets, because walking upright increases situational awareness and survivability, so the hind legs would stay locomotive. Increased intelligence would cover the increased computational requirements. So we have an upright walking creature with arms and legs.
– Control of limbs and behaviour requires computational ability and the ability to send control signals to various parts of the body so the creature would most likely have something analogous to a brain, a centralised processing core. Sure you can have it decentralised, but that increases the distances signals would have to travel, increase the time a creature needs to react to outside events, thus decrease survivability. So you end up with a brain as the most likely option.
– A creature needs to sense its environment, the more ways it can, the greater its survivability. You can sense heat, touch, chemicals in the atmosphere, sound and light. Heat and touch would most likely be sensed all over the body, to ensure the creature knows when something is knibbing at its legs. The creature would also most likely have some sort of light sensing organ (eyes), sound sensing organ (ears) and something to smell with (nose). Where would you find those eyes, ears and nose? Most likely at the front, the direction in which the creature usually moves to find food so that it knows where it is going. To minimise travelling time of signals from the sensors to the brain, and thus increase survivability, the brain would therefore be most likely located right next to the primary sensors. It is also advantagous for survival to have the sensor pod be movable independently from the rest of the body. The creature would therefore most likely have a head with a brain, eyes, ears and nose. It would also likely have at least two of each eyes and ears, to have some sort of triangulation to increase situational awareness and thus survivability.
– The creature needs to eat, so it most likely would have some sort of internal digestive system, with the entry point close to the front, to decrease the effort and time needed to feed, which increases survivability. It would also be close to the primary sensors, to make feeding easier and faster.

Combine all that into one, you end up with a carbon based, multicellular creature that has 2 or more arms, hands, 2 or more legs, walks most likely upright to increase situational awareness, with a (necessarily) head at the top that has eyes, ears, nose(s) and a mouth.

Voilá, a roughly humanoid alien, derived from environmental factors and evolutionary advantages.

LostInParadise's avatar

Convergent evolution gives an idea of how the same challenges lead to very similar solutions. The eye has separately evolved, I believe, three times. Insect wings, bat wings and bird wings all evolved separately. Whale and dolphin fins evolved separately from fish fins.

LuckyGuy's avatar

I’ll throw out another possibility based upon element abundance.
Hydrogen is one obvious choice. Helium and neon? Useless – as they don’t react easily. Oxygen: great choice But let’s skip carbon for no other reason that to be different and present an alternate.
Iron is abundant -roughly ¼ that of carbon. So let’s use that.
OK , start with iron. Iron with traces of carbon combine in many ways. As do oxygen and hydrogen. This life form might develop on a star that is radiating a lot of electromagnetic energy. It can use the E field to feed itself and the M field for locomotion. To harvest this energy it will need a ferrite tail (Fe2O3) that it can move and direct toward or perpendicular to the field. The body would be comprised of a conductive exoskeleton that the being could thin out or thicken to regulate high current to avoid overheating. The longer the tail the more energy it can harvest and the faster it could move to another location to obtain trace nutrients. Communication wold likely be acoustic as there would be too much electromagnetic interference.
It would not take long before they were standing on the street corner checking out each other’s ferrite wing-dings.

ETpro's avatar

@worriedguy Worthy of exploration in a piece of science fiction. Silicon is also a tetravalent metaloid capable of bonding much the same as carbon does in multiple ways to itself and other elements to form long chain compounds. SiH4, for instance is a pyrophoric gas with a similar tetrahedral structure to methane, CH4.. It is a widely available metaloid and thus might well be a basis for life quite different from what evolved here on Earth.

LuckyGuy's avatar

@ETpro I like your the Si concept. Their energy process could involve the metabolizing of SiH4 to CH4 and vice versa with the addition of an external energy source be it solar, electromagnetic, or high temperature. If the source was intermittent due to planetary rotation for example, the Si may be stored in cone shaped mounds, not unlike bactrian camel humps, for processing when the source was available. Those with the largest storage capacity would best be able to weather the regular energy droughts.
I will modify my previous post’s last sentence.
It would not take long before they were standing on the street corner checking out each other’s ‘silicones’.

ETpro's avatar

@worriedguy Yikes. If they like CH4 they’d love present day Earth. :-)

dalepetrie's avatar

I’m no expert, I’m not going to pretend my answer is based on anything, just what I think is logical based on a lifelong interest in science and astronomy.

Life on earth comes in many, many forms besides humans. I believe there is definitely life out there, the universe is so incredibly vast that I’d expect that just about anything that can exist based on the laws of science (which we don’t fully understand yet) probably does somewhere, just the law of averages, and in fact it seems very likely to me that any form of life that has evolved on any planet probably has evolved in a similar fashion on at least one, and probably multiple other planets.

My belief is that the universe is so incredibly vast, probably not exactly “infinite”, but if not, there is probably something beyond what we think of as the universe…infinity does exist and something has to fill it, that’s the way I conceptualize it anyway, and of course I could be wrong.

Anyway, there are certainly billions and probably trillions of planets (or more) that could support life…I base this simply on the vastness of the universe and the concept of “infinity”. Life is tenacious, where life can exist, I suspect it most likely will take hold at some point. But the vastness of the universe and the laws of physics (at least to the extent to which we currently understand them), make it, shall we say, at minimum a very “advanced” proposition to traverse the distances between any two planets with “intelligent” life on them. Because of this, I’d say a society advanced enough to figure out a practical way to visit our planet (which is the only way any of us would be likely to encounter intelligent life on another planet in our lifetimes) would need to possess certain characteristics we’d associate with intelligent life on our planet.

What I mean is, if you look at the vast number of life forms we have on our planet, there are many characteristics that seem nearly universal, and when you look at more “advanced” forms of life, those who possess characteristics we perceive as “intelligent”, all such creatures including man share even more characteristics than we do with all these other life forms. Consider that there are insects with eyes and ears, but we don’t see them as nearly as similar to us as we do apes.

So, where does this leave me? My thoughts are that if on one planet, so many forms of life, millions of them just on this one planet, can have such variation, the vastness of the universe means that there are likely millions, maybe billions of intelligent species that have a number of things in common with man, but there are probably trillions to a factor of trillions of life forms that bear little or no resemblance to man, and surely an unimaginably vast number of these life forms have evolved to the point where we’d consider them to be intelligent. Of the pool of all intelligent life forms, that fall within a spectrum of no resemblance to man to almost identical to man, some random number may have evolved to the point where they could have mastered interstellar travel. Some even smaller subset of that might have figured out how to get as far as Earth…when you keep filtering into smaller and smaller subsets, that pool of potential visitors to Earth gets dwindled down to the point where it’s unlikely we will be or have been visited by intelligent life.

Any willing to try would certainly want to be aware of our capabilities and our limitations, they would want to observe us to learn how to approach us (assuming their intentions were to make peaceful contact with us…not a foregone conclusion…in fact a very large subset of those able to visit us might have malevolent intent). Based on all this, I would conclude:

1) We are very unlikely to encounter “intelligent” life in any of our lifetimes.
2) If we do, it’s a crap shoot to know if we’ll ever even get to see them before they annihilate us.
3) On the off chance that we are visited by intelligent life who reveal themselves to us in order to communicate with us, there will probably be at least “some” recognizable features, but given the vast diversity of life, the law of averages says there will be far more unrecognizable features.

So my conclusion is this….what might visit us at first would probably bear at least “some” resemblance to human life, and “some” to other forms of life on our planet. If we were to get to see the full spectrum of life on their home planet somehow, I’d say we’d find even more similarities. But in the end, I think what we’d end up seeing is something that would be shockingly different than what we know of life. So the answer is yes and no, and it will depend greatly on your perspective…but I’d say the layman’s conclusion would be “no”, but the scientists conclusion would probably be “yes”.

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