Soul Invictus

I've no strong science background, however I thought this up earlier today.

According to modern science, how long has Earth been habitable for man?

From our knowledge, what is the potential for humans to survive/flourish on other planets? (If there's a known reference, I'd like to know). I wish I knew more about which planets were well too hot and well too cold, etc.

This would lead to my main question. Has anyone ever theorized how Earth is the sole planet that man can survive on? (Assuming this is true)

Thanks

PS - I did not intentionally try to post slut by having this in this forum as well as the science/skepticism forum. I wasn't sure which would be the proper forum is all.

Thanks again

Roller

Since the fossil record of animals starts roughly some 600 mil. years ago, I would say at least that long. Of course, than we would have to survive all those nasty global extinctions. Although I think atmosphere did change through this 600 mil years so it is possible that it was less hospitable than today.

Oxygen would be essential. Another thing is atmosphere pressure. For example, Venus has atmospheric pressure around 90 times the one on Earth. That would be nasty. Also average temperature on Venus is somewhat greater than 800F. Average temp on Mercury is greater than 300F, although the night side gets pretty cool at -300F. Mars is somewhat better, but still a bit cold with average temperature around -63F. The rest of the planets are gas giants (Jupiter, Saturn, Uranus, Neptune) with no discernible solid structure. Pluto is, again, too cold. And, of course, atmosphere on all of these planets is very inhospitable for humans.


There is a book from Carl Sagan called "Cosmos". Some parts of it may be little outdated now, since it was written in early eighties, but, nevertheless, it is an excellent read and I believe it covers all of your questions.

lpetrich

It depends on how much oxygen is in the air and what one is willing to eat.

For much of its history, the Earth's atmosphere had much less oxygen than it now has, though precisely how much is very uncertain.

A hint as to how little oxygen one can subsist on can be derived from the experience of mountain climbers and aviators in unpressurized airplanes. At 18000 ft / 5500 m, one has a serious risk of "mountain sickness" or "altitude sickness" -- and the oxygen concentration is only 50% that of sea level.

The Archean had less than 1% of present-day oxygen, meaning that it was inhospitable.

The Proterozoic started with the emergence of oxygen-releasing photosynthesis; however, its oxygen level was likely lower than present-day levels, perhaps 5-15% in the early Proterozoic.

The oxygen level goes up to close to present-day levels in the Cambrian and afterwards.

But I'll have to hunt down good estimates of all of these oxygen levels.

So we'd be safe after the Cambrian and perhaps a bit before it.

Now to the question of what to eat.

Multicellular algae started appearing late in the Proterozoic, about a billion years ago; we could have eaten seaweed. There may also have been bacteria and algae inhabiting the soil, but that would not be much of a diet.

Just before the Cambrian (525 myr) was the Ediacaran fauna, some of which was like present-day sea pens and some of which was just plain odd. I'm not sure if anyone has ever tried eating sea pen; I suspect that it will largely be connective tissue -- gristle.

After the Cambrian began, we find lots of marine worms and shellfish and arthropods and such; Anomalocaris was half a meter / 18 in long, and would have been as satisfyingly meaty as a lobster.

Fish proliferated in the Devonian, some of them getting very big; fish have been in the Earth's bodies of water ever since. So if you like fish, you can go back as far as the Devonian.

The Devonian was also the time of the invasion of the land.

Plants had made it before the Devonian (417-354 myr), with the biggest ones growing only a meter tall at the beginning of that period. However, by the end of the Devonian, plants could become big enough to be the first trees, and they learned how to produce seeds.

Arthropods made it early in the Devonian, but the really nice thing was the emergence of the first amphibians toward the end of that period. One could now eat some "real" flesh instead of bugs!

So the Earth has been a nice place to live ever since the Devonian.

Data

But this only means that we were developed specifically for the environment, not the environment specifically created for us.

Soul Invictus

Yes, I was hoping to find out what the backdrop was for our being able to survive the environment here. I do not know the history of the origin of man from the scientific standpoint.

lpetrich

Soul Invictus, as to why we can survive on the Earth, I suggest that it's because the Earth is the only place in the Solar System that can allow us to come into existence.

As to how humanity originated, that's an extremely long story, at least if you wish to push it all the way back to the origin of life. I'm not sure what you want; I suggest that you get yourself over to the University of California Museum of Paleontology site and http://www.talkorigins.org If you wish, I'll provide a simplified thumbnail sketch.

Yangja Isuko

well, both mars and venus are thought to have had large bodies of water in their past (the opinions about how long ago seem to vary somewhat) and it is generally agreed that water is a prerequisite for earthlike life to form.

then there's titan, one of saturns moons, thought to have oceans of liquid methane and being in every way a kind of 'primordial' earth which has all the ingredients for life except for the fact that it's too far from the sun to kickstart the process.

europa is another contender for life with it's ice covered surface, as it's believed underneath this ice may lie a massive ocean. some have even hypothesized that the atmosphere of jupiter may contain life of some sort.

ofcourse, none of these worlds are currently habitable to man, though mars and venus could most likely be terraformed to be more like earth and thus habitable for us. (would take a loooong time though)

as for planets around other stars, �t seems extremely unlikely that there *wouldn't* be at least some planets in the universe we could live on, what with the vast amount of stars out there. and we know that planets are fairly common, we've allready verified the existence of more than 100 extrasolar planets. while these are all gas giants due to us not having sensitive enough equipment to detect smaller rocky planets, surely they must be there.

contracycle

Well, its more that the accidental convergance of conditions - like liquid water in such quantity - allowed a space in which life could develop. After it had developed, we are just one of its many and varied manifestations.

I don't think it is likely that this is the only existing planet with these or similar conditions. Therefore I donn't think it is *likely* that this is the only life-bearing planet, but that is only speculation.