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Say you were to drop a (bunch of) human(s) at some past era of our Earth. I'm assuming they might encounter a few problems such as the composition of the air not being breathable, the water being too rich or too poor in some chemicals, some diseases being totally new to their immune system... Maybe they themselves bring some diseases from the present and eradicate all source of food...
My question is: how early could they be dropped and still have a chance of survival?
I love this one. I really do.
Quick Answer:
I'd put the date at roughly 2.3 billion years ago, give or take. This is the date of the Great Oxygenation Event. It's when organisms (bacteria) began putting oxygen into the atmosphere in large quantities as a waste product of photosynthesis. The atmosphere before that has a lot of carbon dioxide, which wouldn't have very good for our intrepid explorers. But this new one sort of resembled the one we have today, although the nitrogen from the earlier atmosphere is still here. This graphic gives you a sense of how high oxygen levels were:
Image courtesy of Wikipedia user Loudubewe under the Creative Commons Attribution-Share Alike 3.0 Unported license.
The red and green lines represent the high and low estimates of oxygen levels. Note that this atmosphere would only have been about 5% oxygen, as opposed to the 20% we have today. To reach that, you'd have to go back less than a billion years from the present date.
Long Answer:
This is really just a bunch of additional details that aren't quite as important (okay, I guess they are).
As Neil mentioned, humans need food. And 2.3 billion years ago, there wasn't much of that around. In fact, there wasn't much life around. To make this an even more graphics-heavy answer, I have this chart, which shows you just how bacteria dominated our planet:
Image in the public domain.
It took multicellular life another 750 million years or so to arrive after the Great Oxygenation Event. So the day's meals would be bacteria. But human's can't really live on bacteria, so our intrepid explorers would have to wait until the Cambrian Explosion 542 million years ago, when land plants formed. Aside from that, they could try to catch some aquatic creatures (trilobite stew, anyone), but it would be hard.
Another problem with dropping our explorers off 2.3 billion years ago would be that the Earth may have been undergoing a repeating phase in its life known as the Snowball Earth phase (in this case, the Huronian Glaciation). Large portions of the planet were covered in ice, making the planet pretty inhospitable.
Mark Adler pointed out that carbon dioxide levels are also important, as levels above 5% are toxic to humans. Currently, it's at roughly 0.0397%, or 397 parts per million. In years prior, that number was a lot higher. This graphic indicates that carbon dioxide levels would not have been favorable until a few hundred million years ago:
Image courtesy of Wikipedia user Merikanto~commonswiki under the Creative Commons Attribution-Share Alike 3.0 Unported license.
So 300 million years ago, there was a favorable "island", but that soon went away. You'd have to wait a while longer before levels returned to what they are today.
As a few users have pointed out (all correctly), oxygen levels need to be at least 10% for humans to not suffer severe medical issues, and carbon dioxide levels need to be below 5%. These conditions weren't necessarily satisfied at the beginning of the Great Oxygenation Event. However, they changed over time. Over the course of Earth's history, there were periods where these conditions became more (or less) conducive to human life. I'm reluctant to pick an exact date, so I'll instead post these graphs (taken from here, and found individually here and here). I'm not positive of their accuracy, but they'll do. I won't post them, though, because at least one is copyrighted, and so I can't reproduce it here.
Humans require an oxygen atmosphere to breathe, and require multicellular life to eat. They also require temperatures roughly similar to those found today.
It has been shown through geologic methods that the oxygenation of the atmosphere occurred during the Precambrian era, reaching levels possibly high enough to support human life around 1.9 billion years ago, though due to the low levels of oxygen, it would be like living at very high altitudes all the time. However, there is no evidence of large multicellular life, so humans deposited in this era would be reduced to filtering the sea and scraping stromatolites to obtain anything even remotely nourishing. Survival would be possible, but existence would be pretty boring and the potential for societal growth would be highly limited due to the lack of combustibles and the lethargy that the low oxygen levels would impose.
A more likely period for humans to be able to thrive in would be the Ediacaran era, 635-542 million years ago, when oxygen levels were closer to our own and there was an abundance of simple multicellular life. Earlier than this (The Cryogenian era, 850-635 million years ago), the earth was an iceball, with ice-caps reaching to or near to the equator, and the temperatures would likely have proved inhospitable.
Life for humans during the Ediacaran era would likely still be quite boring, as the life forms that existed then - other than potentially being pathogenic - were so simple that they would be easy prey for humans. Obtaining food would be simple, though possibly quite tedious. Since most life is thought to have existed in the sea, humans would be forced to live on coastlines. This era was also quite cold as earth was still warming after the Cryogenian era.
The earliest potentially interesting period would be the following era, the Cambrian, from 541 to 485 million years ago, during which all the modern phyla of life originated. However, most life was still in the seas during this time, and humans would likely still be forced to be a coastal species with a fairly boring existence.
The earliest period in which humans could live as a land-based rather than a coastal species would be the Devonian (419-358 MYA) or the Carboniferous (358-298 MYA) eras, during which land-based life spread out and became established. The appearance of large plants means that this is the earliest period in which humans could potentially establish proper civilisations.
I'd like to know more about your phrase "be dropped".
Does that mean with a group with technology (seeds & animals, oxygen concentrators, tent/dome cities, etc)? Or, just a group of people who were walking the street? Or, living in a prison (ie: Assiti Shard series: Time Spike)?
I've got issues with Time Spike; apple trees need frost to fruit, which probably won't occur in the Cenozoic era.
And without fruiting plants, where're you going to get edible plant food from? Ferns make spores, not seeds that we can eat (like grains; rice, wheat or corn), and tubers weren't mentioned at all.
An all-meat diet is going to be tough, without vitamins and roughage.
If you can take seeds with you, and have time enough to wait for stable levels of crops to grow, then you just need to be in the correct atmosphere. Temperatures are something you can deal with, if you've got shelter/technology to handle it (got a nuclear powerplant and plenty of fuel? No problem - greenhouse it all the way. Heck make grow lights and go totally underground). 'Handling it' means getting through the first couple of months/years while you set up your living situation; you'll need to have food sources to feed people for all of that time, until you can get crops in.
If you've got enough technology, you can go anywhen that's not molten (and the Hadean wasn't all molten), and generate your own oxygen; develop your own soil, grow your own plants and animals.
You're probably going to be fine with diseases. Most diseases require hosts in order to become really effective. The biggest issue is that our diseases are a lot more virulent than populations of primates (cough humans cough) in the past could have handled. We'd most likely wipe out them.
Water isn't going to be a big issue. Minus the fact that you need to treat it, some water sources now are naturally arsenic and will poison people. So water has, and currently can, be deadly. But if you do the right filtering, or distillation - not a problem. Drinking random water is ill-advised.
Hmm, my comment got deleted?
Inuit and Sami are the only ones that even come close, but only 6-9 months of the year, not year-around.
Yeah, you need to eat all of the animal, including boiling up the hooves.
Uhh, merge what? I don't have two accounts.
