Why is there such similarity in all Earth life forms?

If I were asked so summarize life on Earth, the phrase "variation on a theme" seems appropriate. So many very different species share so much in common. For example almost all multicellurlar organisms have teeth, stomach, intestines, hearts, livers, brains, arteries, veins, blood, skin, eyes, lungs, nostrils, mouth, etc.

Why is there so much consistency...so much sameness...various variations on a theme? Thre is a lot of similar architecture. Is it likely that if we were to find another planet similar to Earth and life, if it existed there, would be very similar to what we find on Earth? Is it likely we will find the same architecture for life elsewhere in the Universe, lungs, heart, liver, etc? Why is there no radically different kind of architecture or is there something intresically critical to the basic design we see here on Earth...given the environmental circumstances present on Earth?

 

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Because of their common ancestry.

Ok, so that's not the full answer. It's a lot of it, but the obvious (to me, anyways) next question is why features that are provably not due to common ancestry (such as, for instance, the eye, which has developed multiple times way past the branches in the genetic tree) are still shared across species. The answer to that is (again, simplified) due to common evolutionary pressures: The eye is a good survival feature to have on this Earth; there's light, being able to use it to navigate is a good thing.

Another big part of the answer is that at any given moment time you're only going to be looking at a tiny part of the biodiversity that the Earth has had. Also, the "designs" that you will commonly see will share features for the exact reason that they survived to pass on those features to their offspring. This reinforces the somewhat false image that certain features may be intrinsic to life on Earth, when, in fact, they arose to suppress the biodiversity through evolutionary fitness, luck, and randomness.

 

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The function of sight, sensing an individuals surroundings, seems to be one function inwhich there is a lot of function divesity. Seeing is an act of using passive radiation reflected off of objects. But there are organisms that use active radiation (sonar, sound) to detect objects in the environment...bats, whales use this kind of active radiation. Still others use movement of water (gators) and some use chemical sensors, dogs, sharks, etc. to sense their environment.

 

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Funny enough the examples you give are only true for a group of rather closely related organisms within the kingdom of animals.
Plants and fungi obviously do not possess any of these criteria, and many animals only share few of the listed traits....

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Plants are very different, and I did intend to exclude them and limit this discussion to animals. There are variations in the animal kingdom as well...jelly fish do not have hearts. But the do have stomachs. The point is that most animal life forms tend to follow the same general architecture. There is a lot of similarity in single cell life forms as well.

 

Similarities exist in modern animals, because 99% of the species ever to have existed are now extinct. It's hardly a surprise the remaining 1% have some similarities often.

 

http://en.wikipedia.org/wiki/Parallel_evolution

http://en.wikipedia.org/wiki/Convergent_evolution

 

So given that parallel evolution has favored these traits on Earth, is it reasonable to conclude that animals with these traits should be found on other Earth like planets in the Universe or is this evolution purely random?

 

If you limit this discussion to multicellular animals, then you are limiting it very narrowly. Remember most of the history of life on Earth there were no multicellular animals. Complex animals only existed in the most recent eras of life on Earth (starting in the Cambrian era, only 550 million years ago). Before that it was the Proterozoic period...

Limiting things to only animals, and only some of those, means you are looking at only a fraction of the species that developed since the Cambrian Explosion. This is why the common ancestry argument is so powerful, because you really are looking at species closely related to one another, in the grand scheme of things.

Most of the animals we tend to think of, due our own biases, are also surface-dwelling land animals. Animals like the sea urchin, sea cucumbers, clams and oysters, worms, etc. Humans, sea cucumbers, horseshoe crabs, flatworms and ferns had a common ancestor only a few hundred million years ago. It make me think of just how far we've diverged in about 600 million years.

Even at the most basic level life on earth is largely comprised of the 20 amino acids, particularly the "left-handed" versions of them. Are there no other similar chemicals (like, say the "right-handed" versions?) that could have become the basis of proteins? There really are, the reason we have only 20 (and give them a special designation) is likely just that our initial ancestor common to all life on Earth happened to use those 20 (or 19 or 21) and that basic plan was passed to all of its descendants, including us.

Restart the clock on a different planet with a slightly different chemical makeup, and you'd likely get a different mix of chemicals. Probably some of the same amino acids, possibly using the right handed versions, perhaps a few other complex organic molecules that we don't have a special name for.

There might well be some commonalities on other worlds, but it's not even clear that the development of multicellular life was a foregone conclusion. On Earth, it seems that multicellular life required the assimilation of the mitochondrion (believed to have once been a separate organism) into eukaryotic cells on a genetic level. That may well have been a fluke rather than an inevitability for all we know. As such, without being sure what the cellular chemistry might be of alien organisms, I'd be reluctant to guess at what the similarities might be (if any).

 

Yes, it's reasonable to assume that in similar conditions similar lifeforms arise.

 

joe:

Because all life is a product of descent via modification, which inevitably produces a nested hierachy.

http://www.talkorigins.org/faqs/comdesc/section1.html#nested_hierarchy

 

I recall on the first day of my Microbiology course the instructor pointed out that we are 99% genetically similar to chimps (some say 98%).

He then told us we are 85% identical to mushrooms.

 

Given that other planets will have similar composition, simply due to the fact that Iron, Carbon, Hydrogen and Oxygen are quite prevalent, and similar environmental pressures will be present, I guess yes. The interesting question is whether these planets will have experienced similar events, meteor strikes, large volcanoes, and whether without such events intelligent life would be able to evolve, or whether size and might would prevail always, and the age of the dinosaurs never end.

Of course, on Earth we also have animals with copper based blood, it's an interesting thought experiment to wonder if Copper were more prevalent on another planet, that we might see land based animals with copper blood. Maybe a copper alloy meteorite depositing a a large amount into the mantle early on in the planet's development could do this?

 

Mushrooms are more closely related to animals than they are to plants.

 

There are branches within the animal kingdom where you can see distinct differences in the evolution of, say, the nervous system, digestive tract, and ways of breathing (lungs come up pretty late). Also it depends on how broad you define a certain organ. A heart, for instance is essentially a muscle that moves a fluid around. Animals got that pretty early. But insects got nothing as sophisticated as in, say, mammals. Also teeth arose several times during evolution, as were eyes. One of the most interesting examples of parallel evolution is probably the comparison of cephalopod and vertebrate eyes.

 

I say carbon

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The most common multicellular animals on land are insects, which lack some of those features and feature radically different (from mammals) structures in the ones they do have.

 

I think insects have extreme variations of things one finds in mamals and reptiles...for example they all have tracheas but instead of just one in mamals and reptiles insects have them on each of their legs.

 

silicon life haha.

joepistole you are some what right.

much of animal life on earth is similar. sure insects dont have teeth but they have something similar to teeth, that act as teeth. they dont have lungs but have something that behave as lungs. could keep going but, im sure its not necessary. for sure though they are multicellular eukarotic heterotrophs. that is the interesting part, it suggests a single ancestor.

but the most interesting is how diverse life has turned out to be on this planet. it seams that each day some new life form is discovered that breaks some rule. so while it could be reasoned that life on another planet may have evolved many of the same characteristics found in organisms here on earth. it also could be that life on another planet may be so different that we may not be able to recognize it as life at all. we could be the oddballs.

now here is an interesting thought.
what if, as on earth, there was a universal evolution. in other words the "laws" that formed life here on earth, the way life is/was created on earth. what if it is not unique to the earth but, unique to our universe? the earth could be the universal equivalent of McDonalds.