Parallel/convergent evolution between stellar systems
There are two main schools of thought about the forms life, especially technologically sophisticated life, would have on various worlds, especially those belonging to different stars. The first is that you'd find many forms similar to one another regardless of what worlds they came from, as long as certain factors such as oxygenated atmospheres and presence of certain minerals held constant. The second is that you wouldn't, that the directions taken by evolution are essentially random in nature, dictated by chance occurrences ranging from the timing of asteroid and comet impacts to the chemistry of a world's early oceans, extremely unlikely to resemble evolutionary pathways taken anywhere -- or anywhen -- else. My own feeling about this is that biological reality anywhere is a result of both tendencies, but with the first predominating in the mix.
My reasons for this are twofold.
First, certain forms have a far greater selective advantage than do others. Think about the difference in respiratory systems between invertebrates and vertebrates. All other things being equal, the bigger you are, the greater advantage you have over both those who prey on you and those you prey on -- size does count. Think of a toy poodle chasing a mastodon -- who wins? But the bigger you are, the more oxygen you need to take in to stoke the fires of metabolism, and the harder it is to get it into your system. Today's atmosphere contains 21% oxygen. The largest terrestrial invertebrates are only a few inches in size, whereas the largest terrestrial vertebrates can be huge, such as elephants, which can weigh over ten tons and stand 14 feet or so at the shoulder. Why aren't there any insects, arachnids, or other land invertebrates that size? Because the respiratory systems of invertebrates are passive systems, admitting air in through openings in the body, but depending on gas diffusion across membranes to enable it to reach all parts of the body. If an invertebrate's body is too big, oxygen from the atmosphere won't reach many parts of it, which means that those parts, starved of oxygen, will become gangrenous or simply shut down for lack of what they need to do their job, and the invertebrate in question will die. Creature feature movies of the 1950s such as Them! to the contrary, oxygen levels in our modern atmosphere won't permit such organisms to live.
Vertebrates, on the other hand, have active respiratory and circulatory systems, pulling air into the body via the mouth and/or nasal organ, from whence it goes to the lungs, sacs full of heavily folded and intricately laced tissues which together have a vast area. The gases thus inhaled are easily picked up by those membranes, which transfer them to the circulatory system, which carries them to every part of the body. Hence the combination of lungs and a closed circulatory system enable vertebrates to grow to enormous sizes.
Here is just one of many examples showing why certain forms confer real biological advantages on those creatures that possess them that other forms don't give their owners. Other examples are the head-to-tail bilateral symmetry exhibited by everything from worms to humans, which gives advantages over animals possessing only radial or n-fold symmetry (where n is a prime number greater than 2), allowing for the formation of a head at one end and organs of elimination at the other, which provides sanitary benefits as well as others; stereo vision of the sort that we share with creatures such as the cat family, bears, canids and wolves, and other predatory vertebrates, which allows one to focus one's full attention on prey and thereby up one's chances of a successful hunt; and plantigrade bipedalism, i.e., an upright stance, standing firmly on two feet the soles of which touch the ground from heels to toes, enabling one to see much farther over variable terrain with lots of ground cover than if one were on all fours. So certain forms have selective advantages over others, and are likely to appear among the biota of a world because evolution will push for them over less advantageous bodily architectures.
The second reason is more esoteric, but, I think, may prove to have predictive value for biological systems. It has to do with chaos, in the scientific sense, especially as applied to biology. All biological phenomena are chaotic in nature. That is, everything from heartbeats to brain function, to the distribution of leaves on trees and trees in a forest, to the behavior of animals in a pride or pack, has chaotic aspects, and retains them until the organism or biological system in question comes to an end. Oddly, chaos equals life and total order equals death, the opposite of what we used to expect.
Chaos is a fractal phenomenon, and like all fractal phenomena, it displays the property of self-similarity at all levels. No matter how much you magnify a chaotic entity, you discover the same patterns again and again at every level of magnification. Life is one such phenomenon, and therefore the same general patterns of form and function should arise wherever there is life. Note that I am not saying you will get exactly the same types of organisms on all worlds in the universe. You will, however, find a great deal of similarity of form and function in a general sense from ecosystem to ecosystem and world to world. Minor details such as coloration, the exact shapes of scales or their derivatives such as hair and feathers, whether an organism has three toes or four, and so on and on will differ greatly. But when it comes to essentials, what the classical Greeks would have called the Ideals of biology, there should be a great deal of similarity between system and system, due to the self-similarity of biological reality everywhere.
These two reasons taken together -- the advantages conferred by one style of biological architecture over another and the self-similarity of chaotic systems -- almost certainly guarantees that wherever life of any kind is found anywhere in the universe, numberous similarities between it and life found anywhere else, at least of a general kind, should be he rule.
My reasons for this are twofold.
First, certain forms have a far greater selective advantage than do others. Think about the difference in respiratory systems between invertebrates and vertebrates. All other things being equal, the bigger you are, the greater advantage you have over both those who prey on you and those you prey on -- size does count. Think of a toy poodle chasing a mastodon -- who wins? But the bigger you are, the more oxygen you need to take in to stoke the fires of metabolism, and the harder it is to get it into your system. Today's atmosphere contains 21% oxygen. The largest terrestrial invertebrates are only a few inches in size, whereas the largest terrestrial vertebrates can be huge, such as elephants, which can weigh over ten tons and stand 14 feet or so at the shoulder. Why aren't there any insects, arachnids, or other land invertebrates that size? Because the respiratory systems of invertebrates are passive systems, admitting air in through openings in the body, but depending on gas diffusion across membranes to enable it to reach all parts of the body. If an invertebrate's body is too big, oxygen from the atmosphere won't reach many parts of it, which means that those parts, starved of oxygen, will become gangrenous or simply shut down for lack of what they need to do their job, and the invertebrate in question will die. Creature feature movies of the 1950s such as Them! to the contrary, oxygen levels in our modern atmosphere won't permit such organisms to live.
Vertebrates, on the other hand, have active respiratory and circulatory systems, pulling air into the body via the mouth and/or nasal organ, from whence it goes to the lungs, sacs full of heavily folded and intricately laced tissues which together have a vast area. The gases thus inhaled are easily picked up by those membranes, which transfer them to the circulatory system, which carries them to every part of the body. Hence the combination of lungs and a closed circulatory system enable vertebrates to grow to enormous sizes.
Here is just one of many examples showing why certain forms confer real biological advantages on those creatures that possess them that other forms don't give their owners. Other examples are the head-to-tail bilateral symmetry exhibited by everything from worms to humans, which gives advantages over animals possessing only radial or n-fold symmetry (where n is a prime number greater than 2), allowing for the formation of a head at one end and organs of elimination at the other, which provides sanitary benefits as well as others; stereo vision of the sort that we share with creatures such as the cat family, bears, canids and wolves, and other predatory vertebrates, which allows one to focus one's full attention on prey and thereby up one's chances of a successful hunt; and plantigrade bipedalism, i.e., an upright stance, standing firmly on two feet the soles of which touch the ground from heels to toes, enabling one to see much farther over variable terrain with lots of ground cover than if one were on all fours. So certain forms have selective advantages over others, and are likely to appear among the biota of a world because evolution will push for them over less advantageous bodily architectures.
The second reason is more esoteric, but, I think, may prove to have predictive value for biological systems. It has to do with chaos, in the scientific sense, especially as applied to biology. All biological phenomena are chaotic in nature. That is, everything from heartbeats to brain function, to the distribution of leaves on trees and trees in a forest, to the behavior of animals in a pride or pack, has chaotic aspects, and retains them until the organism or biological system in question comes to an end. Oddly, chaos equals life and total order equals death, the opposite of what we used to expect.
Chaos is a fractal phenomenon, and like all fractal phenomena, it displays the property of self-similarity at all levels. No matter how much you magnify a chaotic entity, you discover the same patterns again and again at every level of magnification. Life is one such phenomenon, and therefore the same general patterns of form and function should arise wherever there is life. Note that I am not saying you will get exactly the same types of organisms on all worlds in the universe. You will, however, find a great deal of similarity of form and function in a general sense from ecosystem to ecosystem and world to world. Minor details such as coloration, the exact shapes of scales or their derivatives such as hair and feathers, whether an organism has three toes or four, and so on and on will differ greatly. But when it comes to essentials, what the classical Greeks would have called the Ideals of biology, there should be a great deal of similarity between system and system, due to the self-similarity of biological reality everywhere.
These two reasons taken together -- the advantages conferred by one style of biological architecture over another and the self-similarity of chaotic systems -- almost certainly guarantees that wherever life of any kind is found anywhere in the universe, numberous similarities between it and life found anywhere else, at least of a general kind, should be he rule.