Is the Earth ideal for complex life?

[Moontanman]

Is the Earth really ideal for complex life or has complex life evolved to fit a barely habitable Earth?

 

Vast areas of the Earth are extremely desolate, very little of the Earth is really suitable for highly diverse ecosystems like the rain forests or coral reefs. CO2 is highly limited and this affects the growth of plants and the Earth's Temperature depends on CO2 being a very tiny part of the Earths atmosphere. Can we postulate a planet that would be more welcoming to complex life as we know it?

 

The movie Avatar gave some hints of this idea but I think it can be taken further. A planet less dense than the Earth but larger in diameter would be able to hold a denser more extensive atmosphere. If you look at Saturn's moon Titan and it's huge atmosphere you can see how density of the planet can affect atmosphere. Titan is bigger than Mercury but much less dense, it's surface gravity is lower than Mercury's, this gravity gradient allows for a very deep atmosphere.

 

On a planet of Earth mass but larger diameter the gravity gradient could allow for a much more extensive atmosphere. Even if the relative proportions of gases were the same the would be far more of everything available for life, CO2, Oxygen, even nitrogen fixing is chemically easier under high pressure than low. a 10,000 mile diameter planet, with the same gravity as the earth could allow for a 150psi atmosphere, more water vapor, easier dispersal of seeds and spores.

 

Such a planet looks far more friendly to complex life than our own thin atmosphere, barely habitable planet. A thick atmosphere would allow the planet to be further from the a star or a dimmer star. A thick atmosphere might allow life to crawl out onto land sooner. Possibly more rain over more of the planet even with smaller oceans. Would such a planet be more user friendly?

 

In avatar the planet was a moon of a giant Jupiter like planet, this could have different effects but would they be positive or negative?

[JMJones0424]

I hate it when I remember something I read a few years ago from a library book and can't find an online source to quote it. I think it was Amazon.com: The Quark and the Jaguar: Adventures in the Simple and the Complex (9780716727255): Murray Gell-Mann: Books http://www.amazon.com/Quark-Jaguar-Adventures-Simple-Complex/dp/0716727250

 

One of my favorite books, I need to read it again. Gell-Mann talks about the need for some adversity in order to "prompt" change. Theory being, if everything is perfectly suited to whatever form of life inhabits that environment, than natural selection will tend to make the evolution of a species stagnate. But if things are just adverse enough, new and better ways of accomplishing things are given a chance to prevail. Of course, if things are too adverse, then nothing succeeds. Hopefully someone will be able to correct my foggy memory.

[modest]

If Punctuated Equilibrium is close to being correct then a planet beneficial to the evolution of complex species would perhaps have continents that quickly break apart and rejoin—constantly separating small groups of populations from the larger and changing their environment before reintroducing them to different larger populations.

 

In that case... I guess the mantle would be less viscous and flow quicker with continental crust that breaks up easier.

 

~modest

[sman]

If Punctuated Equilibrium is close to being correct then a planet beneficial to the evolution of complex species would perhaps have continents that quickly break apart and rejoin—constantly separating small groups of populations from the larger and changing their environment before reintroducing them to different larger populations.

 

In that case... I guess the mantle would be less viscous and flow quicker with continental crust that breaks up easier.

 

~modest

 

I submit Australia as a planet beneficial to the evolution of complex species, given the Marsupial fauna there...no need to break it up.

 

BTW, I've never understood the difference between Punctuated Equilibrium and good old fashion Allopatric speciation. I have a feeling there isn't one.

 

When I think of complex life, as a category, I think of eukaryotic life, as opposed to prokaryotic, or bacterial life. Maybe what this thread has in mind is more like Mamalia or even human life. I do not believe that human intelligence (and conceit) is an inevitability of eukaryotic life. But I may be off-base here. Ideal conditions for eukaryotic life, so far as I can tell, is nothing more than a planet that's been crawling with bacteria for a billion years. Continents may dance the whole tango in a billion years.

[Moontanman]

Another consideration is that a larger planet would have more active plate tectonics, carbon and possibly other elements would be recycled faster through subduction and subsequent volcanism.

[Boerseun]

I kinda have to agree with JMJones's reply.

 

If you look at sharks, the Earth have been the ideal environment for them for more than 400 million years, now. The same stasis should apply for humans (and every other intelligent species in the same circumstances) who have effectively taken control of their environment, and change the environment to suit them, not the other way around that evolution would depend on.

 

A much thicker atmosphere might be a good idea for accelerated evolution, but it might also prevent an ocean by absorbing all the water. I think that a sizable moon might be more important than a slightly thicker or thinner atmosphere, in that it will raise tides which might cause some animals and plants to adapt to live out of of water for longer and longer periods, eventually giving rise to permanently land-based forms.

 

I think there are many paths evolution can take that we aren't aware of and probably can't even imagine, like the globally share "USB plugs" of the Avatar environment. Probably not as outlandish as that, but you get the picture. We're pretty bloody amazing as it is, if you think about it. We've evolved organs that can detect nuclear fusion from hundreds of light years away. That's pretty cool in anybody's book, I'd say.

 

So yes - I think that some environmental dynamism is essential. Not necessarily of the fire-and-brimstone volcanic or impactor kind, but any mechanism that can regularly divide and isolate subgroups from species should do. And that will only last up to the point when intelligence is achieved, because then the species will take control of their environment and break down the barriers that caused the genetic isolation in the first place. And then they will discover electricity, invent computers, criss-cross their planet with a tangled web of wires and call it the "Internet", invent forums, and then ask the question "Is Gobblefetzer 6 ideal for complex life?"

[Eclogite]

BTW, I've never understood the difference between Punctuated Equilibrium and good old fashion Allopatric speciation. I have a feeling there isn't one.

They seem to me entirely different. punctuated equlibrium is an observation of the pattern of speciation events over time. Allopatric speciation is one possible source of such a pattern.

 

Back on topic. Plate tectonics, as others have observed, has likley played a major role in the evolution of life on Earth. We do not observed plate tectonics on any other planet. (Although there is some evidence for very early stagnant lid tectonics on Mars.) It might be difficult to obtain the combination of composition, thermal history, crustal rigidity etc on a less dense planet and deliver effective plate tectonics. I'll give it some thought.

[Moontanman]

I think the idea of a larger planet being more suited to life has merit, more plate tectonics, more atmosphere, thicker atmosphere would not soak up the oceans but it would allow for a higher concentration of water vapor, less deserts maybe no deserts at all, lots of rain and a bigger planet can have a big moon as well. The process that is thought to have made our moon is now thought to be more likely that was at first thought.

 

So higher pressure air, more of everything at those higher pressures. More plate movement, faster recycling of elements. a larger planet might at least be as good as the if not better than the earth.

 

Lower density would allow a deeper atmosphere but depending on how big the core was it might impinge the magnetic Field. if the planet had a large iron core it would have to be covered in deep layers of silicates to offset the gravity of the large core but i don't see this as a handicap.

[Essay]

Thanks for the interesting thread.

 

I see co-evolution (between life and geology) as a dominant feature of planetary systems and so a wider range of "habitable" planets would work for maintaining life, but via the anthropic principle we must be on a "goldilocks" planet with enough stability, diversity, and chaos to favor wide and deep evolution. But that could just be from our prejudiced perspective.

 

However I'd like to point out something very telling, imho. In deep space there are a few tens or up to 100 minerals that form naturally from the elements. On most planets a few hundred or up to less than 1000 may form under various geological conditions.

But on Earth there are 4 to 5 thousand minerals, many more than would form from simple geologic evolution. Life itself is responsible for most of the unusual minerals found here on our planet. Lucky us! Presumedly then, finding unusual minerals (or their spectra) on exo-planets would be a good sign of life.

 

Evolution of Minerals: Scientific American

The March 2010 issue had a neat 8 page article on this [but this link offers only a preview].

 

Perhaps the abundance of minerals on a planet is an indicator (after the fact) for the conduciveness to evolution and of evolution's success.

 

;)