How far back in time was the earth habitable by humans?

Suppose I have some excellent camping gear, a time machine, and a desire to really get away from it all. When would be the earliest time period I could visit on earth? I guess this is three seperate questions.

When is the first period of time that I could survive (i.e. - breathable air, drinkable water, etc).
When would things be green enough on land for my comfort? (Grass, trees)
When, pre-dinosaurs, would I have to worry about other land creatures large enough or dangerous enough to really endanger me?

Just wondering.

The main problem is that global oxygen and carbon dioxide levels aren’t stable over hundreds of millions of years. The Carboniferous nd the Mesozoic had high CO2 levels, possibly 8 times modern levels. Oxygen levels could by higher as well. The Permian extinction is now theorized to have been caused by global low oxygen levels, during the mass extinction oxygen at sea level would be equivalent to that at mountaintops today. And you could get odd CO2 or O2 levels if you showed up at the exact wrong time. Probably not enough to kill you right away, and probably during most geologic periods you’d be fine, but it’s easily possible to find yourself getting altitude sickness and shortness of breath at sea level if you hit things wrong.

The Silurian saw the first land plants, but you don’t get much beyond moss until the Devonian. The big problem is that our familiar plants don’t show up until very late in the game. Grass doesn’t show up until 50 million years ago. And angiosperms only show up in the Cretaceous. Go back before the Cenozoic and you’re going to find very few plants that will be edible to humans or other modern mammals.

The Permian was full of large critters that could be dangerous to humans. Prior to that and most animals are going to be smaller than humans. Back in the Carboniferous there were very few large land animals, biggest were about 2 meters. Probably not much danger from them, except the “ick” factor for giant bugs and invertebrates. But small animals can be dangerous, and there were probably lots and lots of creatures in the past that were poisonous and/or toxic, it’s just that we have no way to know.

Also, remember not to step on any butterflies.


Wasn’t a lot of the atmospheric instability also due to a higher level of volcanic activity in those days?

Some description of plants during various eras:

Maybe the Devonian / late Silurian. How do you like ferns? As noted, fluctuations in oxygen and CO2 levels make things problematic. If you are going to attempt to pick a period which had an appropriate breathable balance, an absolute limit to your search would be the time frame during which the Earth went from a reducing atmosphere to an oxygenated atmosphere. This happened during the Proterozoic Era, 2500 - 500 million years ago. You wouldn’t have anything approaching the current oxygen levels until the tail end of that time:

That claims that the ozone layer, which you might wish to consider a necessity, was in place about 600 MYA.

As noted upthread, your choices of humanly edible plants would have been very limited before the Cretaceous period. I once read a comic where a vegetarian time traveler stranded in the Jurassic had to get by on mushrooms and pine nuts.

There were some pretty large amphibian predators back in the Carboniferous period that might have been about as dangerous as crocodiles.

If you wanted to try to survive on nothing but marine life (animal and edible seaweed) you might be able to survive in the Ordovician.

How about single celled dangers?

Well, there’s no reason to suspect that bacteria or viruses adapted to infect ancient species would be particularly adept at infecting humans. Most human diseases are passed from human to human. New infections from animals to humans typically come from domesticated animals, because those are the only animals that come into intimate contact with humans. But the typical pathway is that millions of domesticated animals get dozens of different diseases every year, and one maybe one flu strain jumps from one population to one farmer in China, and he passes it on to his family, and it spreads across the globe.

If you get adopted into a herd of Parasauralophus, and they come down with Parasauralophus pox, there’s a slight chance Parasauralophus pox might jump species to humans. But that’s pretty unlikely. Hundreds of millions of humans cohabitating with hundreds of millions of Parasauralophus might be a different story.

Of course, you’d have to worry about the typical wound infections and such. But I’d guess the likelyhood of catching the Andromeda Strain (and vice versa) are pretty low.

What’s interesting to me is noting that our Earth was once completely inhospitable to humans, but now is hospitable. This obviously plays into evolution, but what’s to say that natural events on the Earth don’t relinquish those human-hospitable circumstances?

Were these critters particularly agile? I suspect that many of them had pretty primative nervous syetms, and probably moved pretty slowly. Those foot long dragobnflies were cool, though!

There was nothing primitive about them; they were every bit as biologically sophisticated as any creatures alive today. You betcha some of them were extremely fast and agile.

Speculation/emotional response ahead. The Administrator General has determined that specualtion may be hazardous to the health of the GQ forum. Use with caution.

I’ll grant you that nothing had specifically evolved to infect a species that didn’t exist yet (i.e. Humans), but I don’t think that life on Earth 200 to 65 million years ago was “less advanced” or “too primitive” compared to the human, or his/her immune system.

Q.E.D. said “There was nothing primitive about them; they were every bit as biologically sophisticated as any creatures alive today.”, and I agree, and I would think that it would extend down into the bacterial and virual type of critters, too.

So what may be a minor inconvenience for the mammal-type ancestors of the day may be more serious to the human, even though that pathogen did not evolve specifically to target humans as it’s living enviornment.

I’ll grant you it would be difficult to gauge the health risk (to humans) of an extinct single celled critter from a 65 million old fossil… but isn’t it worth packing some antibiotic?

Keep in mind that you’d be bringing your OWN bacteria with you - we all have various potentially hazardous samples of bacteria, viruses, and fungi hiding in nooks and crannies on our bodes. Yeah, bring some Neosporin or whatever.

That said “living fossils”, that is, types of creatures that have been around a long time, usually aren’t good to eat. Brachiopods, for example, will induce vomiting when consumed. Cycads (a type of tree-like plant) are “edible” IF you know how to process them, and even then may be associated with long-term neurological damage. Ginko seeds are edible but, again, require processing. Most likely you can survive on crocodiles and sharks - both quite edible - if they don’t eat you first, and other animals, but prior to the arrival of of anigiosperms on the scene eating vegetarian would be really, really difficult at best. Keep in mind that sticky simply to animals still isn’t a guarantee - puffer fish are famously toxic, as just one example.

There’s a certain amount of homeostatis between the physical Earth & the biosphere. But there’s nothing to say that the physical situation couldn’t change enough to thow the earth into an untenable state for current biology. Heck, ice ages aren’t exactly friendly to our current way of life.

And then there are accidents such as meteor collisions.

So yes, the Earth could throw us off for good. But, barring that meteor, it will probably be a slow process and we are pretty darn adaptable now. So we could find a situation in, say, 100 million years where you or I could not survive out in Nature, but the then-current humans are doing just fine with it.

See & for more.

It also depends on how the time machine works…if it works like a classic time machine in that you get transported to the exact same spot on Earth in the past, there is a decent chance that “the spot” is either underwater, deep underground or in the air.

never mind the plants, how about going for some meat? Dinosaur eggs? V-raptors or a trakadon roasted over a spit?

More likely it would be in space, since the Earth would have moved /pedant.

Zombie thread. :eek:

::aims shotgun::