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Related to How far in the past could unprepared humans survive?, but assumes a single survivalist, purpose-trained for his mission. He is to time travel as far back as he can and attempt to survive for an entire year. He is to be presented with extensive information about his chosen time period and has as much training as he'd like, although he's the first time traveler, so the training is limited by modern paleontology.

To travel time, he must provide the machine with geographical coordinates as well as a period of time in Mya (million years ago). The machine will place him at ground level at the nearest land to his specified coordinates, so he won't have to worry that he'll be placed in the middle of the ocean if our estimates of ancient geography are off. Unfortunately, he cannot bring anything with him.

How far back could he go? If he goes far enough back, he won't have to worry about predators, but if he goes too far back, everything will be too deep underwater, or there may not be enough oxygen. His immune system will be more than capable of adapting to ancient microbes that are hundreds of millions of years behind in the pathological arms race, so it's likely that he would not even need to purify water if he's as early as the Paleozoic.

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    $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – L.Dutch
    Nov 29, 2022 at 3:09

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My vote: 145 million years ago. Because fruit.

There are a lot of nutrients that we can't do without. The most prominent example is vitamin C, which is hard to find without fruit. Fruit didn't really exist until the Cretaceous Period. Prior to that, animals needed digestive systems that could wrench those nutrients out of less compact sources.

This is a less hard-core limit than oxygen, and could be hand-waved away for most audiences, but it provides another rational point of "earliest" that could be used.

Update: If you presume that all animals are basically deer with a slightly different body shape, you could find pituitary glands as early as 400m BCE to get Vitamin C from. That's a bit of a stretch, but again, no fossil evidence of soft tissues, so check the warranty on your artistic license.

Prior to that, land creatures were basically mites, centipedes, and spiders. You didn't even get the monster eight-foot-long N-ipedes until around 360m BCE. They might have been person-sized, but their entire gland structure is different.

Further update: When the OP said "highly trained survivalist," I'm presuming that whoever sent the survivalist back in time had the opportunity to sample the prehistoric food and figure out which plants and animals had nutritional value. From there, I'm taking a SWAG at whether the nutrients could be extracted from the food in adequate quantities by picking and choosing what they eat.

I feel comfortable saying that sending a person back even 50 million years with no knowledge of which foods are edible would guarantee death. That might make a good, if less focused, follow-up question.

It's not that the nutrition isn't available, it's that it would be hard to recognize. I wouldn't think that the equivalent of a pituitary gland would be identifiable without a lot of specimens and lab time. Remember that this is a survivalist, not a paleontologist.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – L.Dutch
    Dec 2, 2022 at 4:18
  • $\begingroup$ The survivalist may be highly trained, but remember that he is the first time traveler. No one has been able to go back to sample food. $\endgroup$
    – forest
    Dec 10, 2022 at 22:59
  • $\begingroup$ @Forest, He's the first person to go back. This doesn't mean that they haven't had the opportunity to pull things forward, or look around. $\endgroup$ Dec 11, 2022 at 17:11
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As far back as the oxygen content allows

The limit seems to be 500 Mya.

O2/time

(Image source: geologic history of oxygen)

500 Mya, an event known as SPICE caused a dramatic shift in atmospheric O2. Oxygen content in the oceans dropped and content in the air rose, probably due to a mass die-off of plankton where other photosynthetic organisms stepped in to fill the niche, pulling CO2 from the atmosphere and pumping out O2. During this 4-8 million year period, atmospheric O2 rose to 10-28% oxygen by volume, compared to the present 21%, and compared to the levels at what is likely the highest-elevation settlement possible for high-altitude acclimatized humans, La Rinconada, Peru, at ~10% oxygen by volume. (At less than this, hypoxia and cognitive failure sets in.)

500 Mya was during the Cambrian explosion when most animal phyla emerged. So, no dinosaurs to contend with. Your traveler will eat, drink, and thrive on plants and pond scum for a year, with an occasional arthropod snack (all assuming no toxic qualities).

If the necessary proteins or amino acids aren't present in Cambrian-era life (if inedible or without nutrients), then the traveler could possibly train for and endure a year-long fast, staking out in a cave somewhere wondering to himself why he signed up for this bullsh--
(In reality, this likely wouldn't work. The human body does not store all the nutrients that it needs.)


Other eras of high-oxygen content:

There is evidence that oxygen content 1,400 Mya was sufficient enough for animal respiration, at ~4% present-day levels, though not necessarily large animal respiration (such as us humans). It should be noted that those levels were appropriate for the animals of the time.

The oxygen levels required for early animal respiration were lower than those needed to sustain large motile animals and were probably ≤1% PAL (present atmospheric levels)

So, unless your traveler is an immotile bacterium, this likely isn't relevant.

Despite its name, the Great Oxygenation Event saw oxygen levels at 0.001% of their present-day levels (while air density was less than half what it is today). What makes the event noteable is that quantities of oxygen produced as a photosynthetic by-product of cyanobacteria began to exceed the quantities of chemically reducing materials, and not any particular overabundance of oxygen.

Finding information about the evolution of atmospheric density is hard (this isn't my subject), but the general trend seems to be that Earth's early atmospheres were less massive than the present-day. At least half to nearly a quarter as massive. Less oxygen mass at less oxygen concentrations per volume means less habitability for larger animals.

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    $\begingroup$ Note that the specialized training could include high-altitude survivalism which might make it more practical to push a little further back. $\endgroup$
    – forest
    Nov 28, 2022 at 0:35
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    $\begingroup$ @forest: 16% oxygen at 1 atmosphere pressure is not even very high altitude -- it is only about 2000 meters (6000 feet) above sea level. (The people in Aspen, Colorado feel fine with an oxygen partial pressure which would correspond to about 15% at sea level. People go there for fun.) $\endgroup$
    – AlexP
    Nov 28, 2022 at 0:41
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    $\begingroup$ @BMF: Your numbers are profoundly wrong. Don't believe what you read on random sites on the internet, especially when they make no sense whatsoever. What counts is the partial pressure of oxygen. For example, deep divers working at a depth of 90 meters usually breathe a mixture of 98% helium and 2% oxygen. (Which is fine, because 2% of oxygen at 10 atmospheres is the same partial pressure as 20% oxygen at sea level.) $\endgroup$
    – AlexP
    Nov 28, 2022 at 0:49
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    $\begingroup$ I have an intuition that taking the O2 partial pressure in isolation is insufficient, and you'd need to account for CO2 partial pressure as well to allow proper blood gas exchange in the lungs $\endgroup$
    – g s
    Nov 28, 2022 at 1:26
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    $\begingroup$ Sometimes we're too literal and too much physics lite. This is a great practical answer. Basically it says, "skills don't matter, that's up to the author. What matters are conditions the survivalist can't control with skills provided by the author, here's one such example." Bravo! This is good worldbuilding. $\endgroup$
    – JBH
    Nov 28, 2022 at 1:50
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How far in the past could a highly-trained survivalist live?

I can't come up with any sensible number. Consider this more as a reframing suggestion, looking at two aspects in particular.

His immune system will be more than capable of adapting to ancient microbes that are hundreds of millions of years behind in the pathological arms race, so it's likely that he would not even need to purify water.

That is an incredible assumption. Even just in modern life, visiting a different country often means catching a cold, or a parasite, or a life-threatening illness. Heck, just riding the bus opens you up to all kinds of infectious "fun". And about purifying water, consider the advice given to gringos traveling to various areas in Mexico.

Think too about historical parallels -- when Europeans were farting around in the 1400s, everyone there got smallpox as kids and just dealt with it. But when they showed up in the Americas – where no one had any prior exposure or immunity — the death toll was in the millions, all in one horrible mess.

Now you're talking about taking a person from today and sending them back to who-knows-when. If you go far enough back, the microbes prevalent in the environment will be markedly different from what this person is used to. And "far enough back" might only be a few decades, if we're talking about things like smallpox or polio. Consider also other "fun" stuff further back, like the Black Death, or the 1911 Spanish Flu, or all the god-only-knows plagues that were rampantly commonplace in the Middle Ages. Disease-induced quarantines and city shutdowns were so common in Middle Ages Italy that people built special windows so they could sell food and wine while maintaining social distancing. Given how bacteria change over time, there's no guarantee that our time-traveler's modern-day immunizations would be effective, leaving them susceptible to tetanus, mumps, measles, whooping cough, scarlet fever...

And that's just thinking about infectious disease.

But what about food?

As soon as you start talking about sending someone back millions of years, you have to also worry about how this person is going to eat. What plants and animals and mushrooms are safe to eat? How do you prepare them to ensure food safety? Fugu can be quite delicious, but if you don't cut out certain organs just the right way, it'll kill you. Cassava requires special processing to avoid cyanide poisoning. And how long do you have to cook them? Some toxins only break down over time, so just reaching a certain temperature isn't enough. Botulism is a thing. And some toxins don't break down no matter how long you cook them. Any such survivalist knowledge from the here-and-now becomes less and less reliable the farther back-and-away you go.

→ The "modern paleontology" mentioned in the OP isn't going to address either the immunity or food challenges in sufficient detail. Staying alive for any extended period becomes iffier the further back you go. And depending on what you are exposed to (say, a variety of smallpox that doesn't respond to modern medicines), you might not be welcome back to the present!

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    $\begingroup$ @Mark, that applies to the here-and-now. Like I said above, the further you get from here-and-now, the less that knowledge applies. Much like in John O's answer about hunting proficiency—the further you get from known environments, the more vulnerable you become. Someone who knows the Smokey Mountains like the back of their hand is going to struggle in Amazonia or Alaska. Even more so when heading back in time by millions of years—go back far enough, and that's essentially visiting another planet. Past a certain point, survivalist knowledge is context dependent. Change the context, and ... $\endgroup$ Nov 29, 2022 at 5:47
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    $\begingroup$ It sounds like the idea with the microbes was that, as they'd been evolving for less time, they'd be weaker/less virulent microbes that had never come up against the more advanced features that modern animals' immune systems have. Whereas the microbes that gringos catch in Mexico or the microbes that the Native Americans caught from the Europeans were "modern" relative to them. However, that's theoretical (since we have no time-travelling Jurassic bacteria to examine!), and having less immunity to unfamiliar germs, on the other hand, seems to be a known fact. $\endgroup$
    – A. B.
    Nov 29, 2022 at 6:56
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    $\begingroup$ @Mark not without lab equipment. Sure, he may know how to identify edible plants and mushrooms going back a few hundred years but more than that and much of that knowledge is lost, if it ever was written down in the first place. And there are likely many more toxic and poisonous things around back in prehistory that we just don't know about at all because those tissues didn't fossilise, and even if they did we don't have an accurate image of what they plants and animals looked like except vague indications of shape and size (and even today there are many similar looking things out there). $\endgroup$
    – jwenting
    Nov 29, 2022 at 7:29
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    $\begingroup$ With infectious disease in mind it may be that there isn't a linear survivability (i.e. the further back in time the less likely they are to survive) but that instead we get peaks an troughs and other factors combine. If we go far enough back there will be a point where viruses hadn't yet adapted to infect humans. Things like smallpox, measles and even the common cold hadn't evolved yet. Go back 200 years ago and you're more likely to have a virus hanging around ready made to target humans than you are if you went back 200,000 years ago. $\endgroup$ Nov 29, 2022 at 9:41
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    $\begingroup$ @A.B. Your idea would be correct for microbes that evolved in humans. However, most of our recent diseases transferred from other animals where they had been evolving for ages. What happens is that they are most deadly when first transferred. Then we get some immunity to them, and the microbes get less virulent. Syphilis killed quickly when first introduced to Europeans. The death rate for Europeans in the southern colonies was staggering at first due to exposure to new diseases. $\endgroup$
    – David R
    Nov 29, 2022 at 16:47
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Several thousand years

You stated that "the time machine works only on living human flesh". Assumed blood is also transported, otherwise such a time traveller would plain outright die, there will be an issue that would immediately cause troubles to him. And this is regardless of anything else!

Dysbacteriosis

Symbiotic bacteria living in your intestines that help us digest food would not be transported, as they are distinctly different from the host organism. Therefore the inside of that poor bloke would end up sterile. More, there would be no semi-digested food inside, causing immediate sense of hunger. Then he would have to quickly find something to eat, and reinforce his set of symbiotes, the best source for them being milk, as it would contain compatible bacteria. If he would fail to amend his microflora, he would die of inability to digest food.

Can't go further? Most likely

Without bacteria in one's intestines, whatever local microbiome specimen that are able to live in the intestines of some other animal (mammal) would take place inside that human, leading to unpredictable conditions, from partial compatibility to parasiting, the latter leading to death due to no immunity to ancient intruders and overall weakness of the host due to indigestion. Therefore this time traveller is most likely limited to time ranges close enough to modern times, just because of microflora evolution.

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    $\begingroup$ How several is several? Homo-Sapiens started to show up >200,000 ya. "Modern" man >70,000 ya, and languages started showing up ~50,000 ya. $\endgroup$
    – CGCampbell
    Nov 28, 2022 at 10:59
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    $\begingroup$ An excellent point, but the presumption of a few thousand seems wildly out of whack. Anatomically modern humans alone would take you back over a hundred thousand years, and there isn't really any evidence that we are so closely died to our microflora that we wouldn't be able to go much further back than that: I'd posit at least tens of millions (i.e. the age of Mammals). $\endgroup$ Nov 28, 2022 at 10:59
  • $\begingroup$ Humans get their gut bacteria nuked by antibiotics on a regular basis and don't die from it. Gut bacteria certainly provide useful functions but are not actually essential to life. $\endgroup$ Nov 28, 2022 at 20:07
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    $\begingroup$ @LawnmowerMan, 1) Antibiotics don't sterilize our guts, whereas any proposed "only human tissue" transporter would. 2) We rely on microbes for much more than digestion: consider the microbiome specific to the skin, and how that helps prevent infection. $\endgroup$ Nov 28, 2022 at 21:11
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    $\begingroup$ Ah, the logical problems with teleportation. What is a human? Hair? Dead skin layers (you'd be pretty sore without them)? Saliva? (OK, saliva stays -- in the mouth. Now I'm drooling. Where does my mouth end?) Lymph? What about the air in my lungs? If that doesn't go with me, I die from implosion. Contents of my bladder and intestine. Fluids lining those mucosal tissues? If that doesn't come, I probably die in a strange fashion. OK, everything inside a close body envelope goes, mine or not. No clothes, shoes? Who defines that line? $\endgroup$ Nov 29, 2022 at 18:03
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If the traveler was prepped before the journey with a wide range of good gut biota he could possibly live further back in prehistory than only a few thousand years. Otzi the Iceman lived about 5,000 years ago and had a Helicobacter pylori bacteria in his gut still found in people today. https://en.wikipedia.org/wiki/%C3%96tzi#H._pylori_analysis ... A good thing might be that the time traveler's cultural learnings to do with first aid and cleanliness would presumably be going with him.

If the traveler also spends a bit of time studying the edibility of plants and fungi and small animals and what they looked like in their undomesticated and non hybridized state before he goes, food won't be much of a problem. None of these have changed much in the last, say 100 000 years, and humans have grown up with them.

In a sole survival situation, he'll be wise not to hunt big game. If he did succeed in bringing down a large animal, he'd be attracting multiple large predators and scavengers always ready for a free feed. That is not to say that he himself can't scavenge when it's safe. Bones and sinews make good tools.

80% of his sustenance will probably be plant and fungi based, the rest small game and eggs. His cultural learnings from present day Indigenous cultures would include how to make and set snares (First Americans), catch small lizards and fishing with scoop nets (Australian Aborigines) and thorough knowledge of edible fungi. If he ensures that his journey sets him down near a water source, he'll have shell fish laid on. [Early human migrations are said to have followed coastlines where possible.1

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    $\begingroup$ Earth's first "civilization" was somewhere around 7000 ya, wasn't it? $\endgroup$
    – CGCampbell
    Nov 28, 2022 at 11:04
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    $\begingroup$ I think any number depends heavily on what you consider "civilization". $\endgroup$
    – chepner
    Nov 28, 2022 at 14:47
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    $\begingroup$ If I'm not totally off the hook here - the first agricultural societies formed in mesopotamia (ancient iraq to syria) about 12000 to 14000 years ago and in the indus valley (pakistan / northwest india) - they planted older versions of grain species that are still used today - though heavily modified by breeding over the centuries $\endgroup$
    – eagle275
    Nov 28, 2022 at 15:14
  • $\begingroup$ A "civilization" is a culture that uses writing. The first know writing systems are about 5400 ya in Sumer. $\endgroup$ Nov 29, 2022 at 15:24
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    $\begingroup$ @MartinBonnersupportsMonica, depends on how we define "civilization". 😄 While writing is a common benchmark, not all definitions of "civilization" include literacy as a criterion. $\endgroup$ Nov 30, 2022 at 1:09
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The two greatest challenges for this man would be as follows:

  1. Avoiding predators (and dangerous herbivores).
  2. Hunting for meat.

It is possible to train for the first issue effectively. Much is known about (in no particular order): lions, tigers, bears, orcas, crocodiles/alligators, hippos, etc. However, if he time travels to a period with radically different predators (or large herbivores), these lessons might be useless. They're highly dependent on animal behavior, which is simply too variable between species to be reliable. Can anyone really tell us how a smilodon differed from a tiger? Probably not alot, but if there are differences... our survivalist will probably discover those for himself in the worst way possible.

The second issue isn't entirely the same as the first. While he will need to deal with differing animal behaviors if he wants to hunt a mastodon or whatever, hunting successfully relies on a sort of (for lack of a better term) institutional knowledge of local conditions. A great hunter in the Alaskan wilderness might be no better than a rookie hunter if dropped off in Africa or the jungles of southeast Asia.

There are a host of other issues as well, but these are minor in comparison. Dealing with novel (natural) poisons, or strange geology (who knew that earthquakes could be a problem here?!?!) is something that luck alone might successfully navigate over a period of 12 months.

So, considering this, the safest maximum is probably on the order of 1000-3000 years. Possibly up to 8000 (there will be some new megafauna if he goes that far back). Surviving any further back will be pure luck.

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