Why are we the size we are? What about extra-terrestrials?

I got to thinking the other day about how different aliens from other planets/galaxies might be from us… One thing in particular I wondered was how they would compare to us in size. You usually see them in pop culture as about the same size as humans, but I began to think that perhaps they would be microscopic in size… Or perhaps they would be giants in comparison.

Following this line of thought, I then began to wonder why humans, animals, and other organisms here on earth are the size we/they are. Why are we not much larger or smaller? Why are humans and animals not 100 feet tall? Is our size simply a product of our environment, or is there a biological/physical/chemical/scientific reason behind it all? Would we be drastically different if earth was drastically different?

An incredibly complex topic with no simple answer.

To start with you need to understand the pure engineering constraints placed on animals. As an animal grows larger it needs a more massive support structure, which means a massively thick carapace or huge bones. However bones and carapaces themselves add weight to the animal, requiring larger muscles to move them, which in turn requires larger support structures and so ad infinitum. What that means is that any animal that wishes to move very rapidly approaches an absolute maximum size.

If you shrink an animal you run into problems of surface area. As an animal shrinks its surface area decreases as a square function because it is, well, area. However its volume decreases to a cube function. What that means is that for every quartering of volume an animal loses only half its surface area. A small child or a dog is only one quarter your weight, but has fully half the area of skin. That becomes a problem because dealing with heat loss from that skin area, as well as pathogens, water loss and so forth is a task relegated to the internal organs: organs that fill volume. As a result a child’s muscles, livers kidneys and so forth all have to run twice as fast as yours just to maintain normal function.

As organisms get ever smaller it becomes harder and harder for their skin to contain all the organs needed to keep them alive. As a result smaller organisms are forced to become ever simpler to shed excess internal material, or are forced to live in an ideal environment where they don’t have to work hard to survive. What that means is that small organisms have a hard time being either adaptable or intelligent. That places a very real lower limitation on intelligent life.

Intelligent life also needs to be able to harness some external power source to create civilization. Fire is good for organisms our size. However of we were much smaller we would have a hard time controlling fire. Imagine even a cat-sized organism trying to gather together enough wood to keep a fire burning for more than an hour or so? Certainly by the time we reach rate size fire is simply not an option. But if we get much larger our surface area become so small relatively that we can’t effectively warm ourselves by using fire. So this places a limit on alien civilizations somewhere between cat sized and horse sized.

You can then go on and speculate endlessly how other life-forms might have evolved and what that could achieve. Other lifeforms wouldn’t have muscles like our own and might find it easier to grow to immense sizes by use of hydraulics or metallic motors. But they will still have some sort of upper limit imposed by engineering constraints although that might be far larger than the largest whales. If we then factor in organisms that evolved on planets with different gravity or far more available energy and so forth then we could produce endless wild theories.

Reducing size is far more of a problem. The issues of relative surface area apply no matter what an organism is built from and the issues of fire control are the same.

As Blake said, the answer is bound to be complex, as well as filled with opinions and caveats, and incomplete. Which isn’t to say it’s a bad question at all; quite the opposite. The best questions have no single right answer.

Blake brought up what’s probably the most important engineering constraint, which is surface-to-volume ratio. That limits upward size because of the need for weight support (already mentioned) and also food consumption. Nutrients are absorbed through the walls of your gut, which is a surface. Larger size, less gut surface per animal volume.

Then there is differing gravity & atmospheres.

Higher gravity worlds would demand structure to hold up under the increased weight. So, no giants there.

Lower gravity might allow a broader range of sizes.

And the oxygen content of an atmosphere could vary wildly (Earth’s has, over many millions of years). How easy it is to retrieve oxygen could limit your size.

Highly advanced civilisations become beings of pure energy.

(I know this because I saw it on Star Trek. :slight_smile: )

Size doesn’t matter.
someone had to say it, right?


On one of the Trek animated eps they had a miniature civilization which they saved. I wonder
how small alien lifeforms could get and still develop a civilization-too much at the mercy of
weather/volcanic/etc. events? [In the ep they were threatened by a volcano IIRC]

The classic essay on the subject is On Being the Right Size by J. B. S. Haldane.

Please reboot your humor unit and install Joke Detector upgrade number W stroke H dash zero zero five, rev. H. Thank you.

Whoosh yourself.

It could be possible (at least, it’s a common-enough SF trope) to have a distributed intelligence, or “Hive Mind”, if you will, where individual units are smaller and non-sentient, but share in a larger sentient extelligence. That’s one possible way to overcome size limits, especially if you think of say, communal organo-metallic blimp-beings with radiowave “telepathy” living in gas giants, or something else equally as far removed from what we know as life. 'cause, you could argue that the *sentient unit *is the entire hive creature, and we’re back to square one.

Heh. Clever. Very clever.


No factual answer possible.

Moved to IMHO. It’s a whole different civilization over there.


But as we got larger, external heat sources would also become less necessary. Even so-called cold-blooded creatures generate some heat internally (i.e., according to their volume), but the rate of heat loss depends on surface area. So a very large creature might be able to maintain its body temperature year-round without need for fire. Fire would, however, still be useful for other purposes (light, scaring away predators, clearing large areas, cooking food, etc.)

I would expect the major constraint on large organisms would be falls. Most animals seem to have their heads (containing the all-important brain) near the front and/or top of their bodies, and a tall creature will occasionally fall over. If your head is too far from the ground to begin with, it’ll fall a long way, and is likely to break (there being limits on how strong bone or the equivalent can be).

Very interesting question and discussion. I have not much to add except to mention Pando, the largest and likely oldest organism on Earth. Obviously not intelligent, but I wouldn’t be surprised to find something like that on another planet. Perhaps even a planet sized Pando-like organism, given the right conditions.

And I also came by to drop this Futurama quote:

The problem is that if fire has no value for warmth it is hard to imagine why any creature woudl learn to harness it in the first place. Cooking food, clearingland and so forth all had to come after we had alreadylearnedot master fire, and it’s hard to imagine why we would bother to learn to master it if it has no immediate use.

This is like a lot of inventions such as writing, kites. metalwork and sails that would have been of immense value to hunter gatherers but were never developed because the first step simply never existed. If a person can’t pick somehting up and immediately know how to apply it then they will just drop it again. In the case of fire warmth was the only immediate use.

Oh, I’d say cooking food ranks right up there in the utility of fire. Because heat can soften and denature such a huge variety of plant and animal life, it probably gave primitive humans hundreds of new food sources. It may not be as intuitive as “fire makes me warm” but it seems to me it would only take a couple of happy accidents (e.g. previously inedible, now well-cooked roots discovered in the ashes of a found fire) for the usefulness of fire to be learned and adapted.

We think people. On this planet we have had dinos over 125 feet long. We have fleas and miniscule insects. Viruses and germs are incredibly small. Elephants and whales which are alive today are huge. So why pick peeps for your standard.

People keep trying to say size is related to necessity. The example in yesterday’s news was the explanation that the small human remains nicknamed Hobbits found in Indonesia recently were small for a reason: No larger people around to compete with them, less need for food, yadda yadda.
But all those explanations are bogus. Look at ants. Some are tiny and some are huge and all of them are alive and doing well and have exactly as long family trees as everything else on the planet.
The largest birds, the Moas, existed up until man in nearly identical habitats as their tiny kiwi cousins. Food supply and competition didn’t matter.
The rule is not survival of the fittest traits, but survival of any non-fatal traits.