Parallel/convergent evolution between stellar systems

[polaris93]

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

Comments

[jordan179]

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.

Interestingly, it's now believed that the reason it was possible to grow (relatively) giant land arthopods in the Carboniferous is that the Earth's atmosphere was enjoying a temporary up-spike in oxygen levels. Another peak of oxygen levels occurred in the Mesozoic, but there were too many small to medium-sized vertebrate predators by then for gigantic insects to flourish. Instead, high oxygen levels may have made it easier for dinosaurs to grow to be the largest land vertebrates of all time -- which, since they had lungs not merely like our own but like the superior ones of avians, was very large indeed!

[polaris93]

Yes. There's a paleobiologist/astrobiologist named Peter Ward at UW who believes that the level of oxygen in the atmosphere and the oceans is the primary driver of the evolution of animal life. When it's high, you get these fantastic creatures such as the giant arthropods of the Carboniferous and the dinosaurs of the Mesozoic. When oxygen levels drop, you get mass extinctions, but also evolutionary radiation among animals as they strive to adapt to the harsher conditions. All animal life requires oxygen (so do fungi), and they require it right now, all the time, or they die quickly. When there's lots of it, you get the giants; when there's much less of it, you get new creatures. You're right that increased oxygen gave the giant dinosaurs their chance, but dinosaurs first came into existence in the Triassic, when oxygen levels were still very low after the End-Permian Event. They had an edge over what was left of the Permian fauna, especially the protomammals, because they were built for low oxygen conditions, with lots of

[jordan179]

Your arguments seem valid, but they are hard ones to test against reality until we encounter another planetary ecosystem which has evolved large complex organisms like our own. You're right though in that any such organisms will meet similar evolutionary challenges, and will have to find some workable and competitive solutions for them.

[polaris93]

Concerning the evolutionary trends of bodily architecture, Peter Ward goes into that in many of his books. Available oxygen is just one driver of evolution, dictating the forms respiratory and circulatory systems take and thus the bodily structures accommodating those structures. There are plenty of other drivers, such as need for food, need for water, the ability to get rid of accumulated toxins, need for self-defense, and so on. But there's a limit to the type of forms life will take, especially complex animal life. For example, we have never found the remains of one single wheeled organism, i.e., a creature with wheels as a natural part of its body. Wheels are just too hard for nature to do. Also, in a given habitat, there's a limited range of forms that confer strong advantages to various kinds of organisms, and you'll find the majority of organisms in that habitat exhibit variations on those forms. Stephen Jay Gould theorized that if you went back to the beginning of life on Earth and started it all over again, you'd get a

[polaris93]

As for self-similarity of evolutionary trends over all worlds with life, we now define life in its broadest terms thermodynamically, and the thermodynamics of life turn out to be chaotic wherever they are. Living organisms maintain their internal states through homeostatic mechanisms which are inherently chaotic, in the scientific sense. Their distributions and the interrelationships they have with one another and their environment have chaotic characteristics. And so on. As defined thermodynamically, life will thus display chaotic properties no matter where we find it, and since chaos exhibits self-similtarity at all levels, that same self-similarity will be evident across the biota of numerous living worlds. That, too, can be tested on very small organisms, and modeled in computers -- some grad student could do a very nice sort of project about that for a Master's or doctorate degree in biology. :-)