Realistically, if you were the last person on Earth how long could you survive?

Provided you knew what you were doing, indefinitely. Or until you died of old age or managed to contract a deadly disease/cancer/manage to injure yourself in a life threatening way that you can't treat yourself.

You might not be happy about it. But growing basic crops, storing away dried grains for the winter, hunting and dressing animals, making a fire to cook them, etc. should be well within the reach of anyone capable of reading this. Neanderthals could do it, so can you.

If you didn't know what you were doing, well, until you ran out of easily scavengable canned goods and/or died of botulism from eating one of them that wasn't as canned as you thought.

Yeah my plan (dream) has always been like this:
Use the internet while I have it (assuming people just all disappear suddenly) to download survival guides, solar panel repair/installation PDFs, maps, etc. Anything I can think of, I'll download
Gas only lasts so long. I can use chemicals that extend it, but it's definitely limited. I'd start with a gas powered truck and eventually move into electric vehicles. Batteries aren't forever either... But I'd try.
I'd move to a warm, temperate climate
I'd find a building that claims it is powered by solar panels most of the year. I'd use that as my home
I'd immediately begin trying to farm. I have a black thumb so this would take me some years to get done correctly. But I'd hopefully have some potatoes and grain growing by the end of a year
In the meantime, I might find things to occupy my time such as: finding videogames to play, raising chickens, fishing, collecting guns/ammo, collecting books to preserve, storing solar panels, backup equipment, etc.
My end goal would be to survive as long as I'm happy. I'm pretty introverted so that would last a while. I'd use animals to keep me company. I believe nature would take us over pretty quickly. It would be hard to maintain the house, solar, etc. forever. But if I could, I would.
My wife and I already do a lot of foraging in our area and we have several guides for edible food. We also do some canning and prepping for disasters.
I don't think a disaster would be a picnic. People are the problem. But if they disappeared suddenly, I think it would be pretty livable.
- Use the internet while I have it (assuming people just all disappear suddenly) to download survival guides, solar panel repair/installation PDFs, maps, etc. Anything I can think of, I'll download
  AI would be really useful in this scenario. Have a whole internet's worth of information at your disposal with a fraction of the storage costs.
  Probably still want the actual manuals and guides for the important stuff tho...
Neanderthals (and humans for that matter) did indeed but they did it in groups, usually. Being alone really stacks the odds against you I think.
- Well, you're going to have a much harder time taking down wooly mammoths, that's for sure.
etc. should be well within the reach of anyone capable of reading this. Neanderthals could do it, so can you.
Oh you greatly overrate us internet people... 🧌

Honestly, the biggest issue would be mental, trying to figure out if there was any reason to continue.

Luckily I have a lot of experience with that debate.
- So the last survivor will be chronically depressed?
I've always wanted a zombie movie that focused on the absurd boredom and how you might combat it. Like setting up Rube Goldberg killing machines
- I'm pretty sure I've read some zombie stories that brought up that very idea, killing the zombies in fun ways just because they were bored. One I can think of would be Newsflesh. Not about boredom per se, but humanity has learnt to more-or-less coexist with the Z virus. Post-post-apocalyptic setting. Killing zombies in fun and novel ways isn't the theme, but comes up now and then. Really interesting novel.
- They kinda did that in I Am Legend

19 years, 5 months, 21 days, 16 hours, and 23 minutes.

If the material world survived, living in a large city, I could probably scavenge for non-perishable foods for quite some time.

If it's a nuclear wasteland and I'm fending off mutants, I guess it depends on how many caps and nuka-cola I can find.

19 years, 5 months, 21 days, 16 hours, and 23 minutes
someone did the math

Indefinitely. Just punch a tree; punch a sheep; punch a rock. Y’all so lazy these days.

In complete isolation I think a lot of people (including me) would probably kill ourselves just after few months. We all need at least some social interaction to stay with healthy mind.

Aside from that, what the hell is even the point of living if you are the only person left?

Aside from that, what the hell is even the point of living if you are the only person left?
Visiting museums naked? Cmon dog
- Just when you think the "last person on earth" fantasy has been fully explored!
Plenty of time to read.
- Until you tread on your eyeglasses.
I'm assuming my dog would still be alive. So I'd just go on adventures with him.
OMG, so many things would improve!
OK, healthcare is an issue. But if only humans disappeared? There's so much canned and freeze dried food; I could probably live off a single REI for a couple of years. Pharmacies to be raided, although I'd have to find an old-school reference for drugs, uses, and dosages. Hell, I'd spend a couple of years just outfitting a survivalist cabin near Yosemite or Frank Church Wilderness, and then a couple more backpacking out of the cabin, returning only for provisions.
I've lived without prior before; it's not that hard. I know there would be psychological challenges, but the physical ones really do compensate quite a bit.

I suppose it really depends on why I'm the last person on earth. If it was nuclear fallout, I'd probably be dead pretty quick.

If we're under the assumption that everybody just poofed away and I'm the last person, absolutely a few months minimum. I'm within easy walking distance of some stores that have plethora of canned and preserved foods and liquids. I also know some places that already have full solar infrastructure already integrated.

All that being said, I'd probably off myself. I'm fairly non-social to begin with but I'd still be unhappy never being able to communicate with anyone ever again on top of major resource struggles in the future. Never find a partner, probably not even a domesticated pet as they'd all likely die within a week or so if I don't prioritize that. Sounds incredibly sad and lonely

Probably depends on how everyone else disappeared

Round about a day or two. I'd likely make an oopsie while letting the zoo animals out and get myself slaughtered.

On the pro side, I live in southern California where I'm not going to have problems surviving the weather, even without electricity. I don't get bored easily, I like to read, and I'd probably entertain myself exploring. As others mentioned, food probably is not an issue, though I'd have to make sure I got the right combination of proteins and amino acids once meat wasn't available (or bring myself to hunt).

On the con side, I'm in my early 60s and, though I'm reasonably healthy, it's only going to be so long before I have some health condition that, if nothing else, makes it hard for me to go out and about for food.

The only issue I'd have is mental health. Pets can only ease the need for social needs to a certain extent.
- I would miss my wife and kids horribly, but I'm one of those folks who always feels kind of alone, even when around other people, so I think I'd be okay.

Depends on what happened to the world, what resources I have, time of year, etc.

Assuming decent conditions and luck (nothing saving you from a severe infection gotten because wrong place wrong time), likely a fair few decades. I have a lot of survival knowledge, both theory and practical - and I refresh myself on a regular basis

Probably until my first paper cut and infection after all the antibiotic starts to expire, it’s likely game over some time soon after.

You are way overestimating how often antibiotics are needed. I've used them once in my life and it was only a precaution, probably not needed. People who get antibiotics for every little thing are largely responsible for their loss in effectiveness.
- I don’t disagree with you. Antibiotic overuse is a real problem. I’ve had surgeries without needing antibiotics afterwards. I try very hard not to rely on antibiotics but have allergies and shitty sinuses. I had an on and off sinus infection I couldn’t clear for something like 9 months. Used every trick in the book and I’d think I’d finally cleared it all, but no.
  Happy that you seem more healthy overall than me. Good fortune for you.
How often do you take antibiotics now?
- As infrequently as possible. Maybe a year ago last time for a sinus infection I couldn’t clear on my own.

About a week until I couldn't even stand up anymore due to Long COVID.

Probably not for long. I'm on immune suppression medication, if it's a post apocalypse then I'll die from infection and if I somehow survive that then my ulcerative colitis will do it for me when it flares up.

Similar boat, I would last about 4-8 days without medical intervention (dialysis and other regular treatments I need)

Most of my problems are caused by other people being evil. I'd take being the last person on earth over that any day. I wouldn't live a super long time just because of lack of access to healthcare but I can't afford that today anyway.

After getting over the urge to kill my self…

Longer then one would expect

Most of us don’t have or are only kinda aware of real survival skills. - farming, hunting, fishing — okay there’s your food

Next thing is shelter and clothing- are the people just gone? Or is shit really messed up?

you can get good clothes and if housing is still standing you’ll be fine for a long time.

Holding out hope that there are any other survivors would help you go on, I believe as you’d want to find them.

So yes there are no numbers, but if you’re in a good place mentally. You can survive a long time

It's not even necessarily hunting/farming, it's being able to safely preserve it and survive a winter without starving or botulism.
- If you can boil water, you can can. Jars are reusable. Lids will be your most scarce commodity after a while (you shouldn't reuse the lids after they have been under pressure). Build a little smokehouse and you can preserve your meat.
  I don't even worry about downloading how to's. There are libraries and book stores that have this information that isn't dependent on any kind of electrical device to access it. These sites probably won't be high priority scavenge sites to the average survivor either.
  Winter meals will be mostly canned veggies, bread, and beans for protein. Salt or smoke cured meats infrequently. You'll learn to trap small game or hunt deer. Surviving will be a lot of work but it's totally within reach of anyone who can camp or is crafty/handy.

Probably until I get sick or die of old age.

I'll probably do things like ransack my local library for book like the SAS handbook, plant identification, etc to bring with me, hijack a truck that's already ready to go, load all useful stuff including a small backup car/motorbike and head for the sub-tropics somewhere to a forest next to a city.

Just make sure you do it fast and remember that, contrary to what walking dead would have you believe, gasoline goes bad.
- Solar chargers are great, but I couldn't imagine a portable one being any good for use with an EV. Gas would probably be the best bet for long-distance or hauling.

I could survive the rest of my life.

Probably a long time. I can learn how to run essential systems and live on non-perishables. Or hunt and farm.

I'm more likely to die of sheer fucking boredom, because there won't be anyone to make new games or movies or books or music and no reason to post comments on Lemmy so I'd just want to off myself.

I actually think that boredom wouldn't be that much of an issue though, most of your days will be filled with scavenging, foraging and in a later stage farming.
- All work and no play...

If I were the last person on earth, I'd probably kill myself pretty quickly

About as long as content servers stayed up before I went crazy from boredom.

After the internet fails, you'd quickly learn how valuable the public library is. They have lots of disc based games for Xbox and Playstation meaning no internet required. Lots of movies on DVD and Bluray, and still pretty good collections of music on CDs.
The smallest public library also probably has more books than you could read in your entire lifetime.
- "Time Enough at Last"
- For the majority of disc-based media, any large store would do the job too if we're talking post apocalypse situations but a lot of games these days are absolute crapfests without the plethora of updates. As for servers, I did specifically mean places to download from, not stream. I'd already be downloading every film and TV show I could think of I might like or have already seen and think I'd like to rewatch someday.
  That said, the internet is also my means of satisfying my social needs since all of my friends live in different countries. Without that and them, the dire loneliness sets in real quick like.
  And libraries have a lot of books, I agree and I already enjoy reading but I have relatively niche tastes that would likely only be catered to by large libraries so the vast majority of a small library's collection would be worthless to me. I know the end of the world is more a "any port in a storm" kind of situation but if I'm just running down the clock before the last human alive becomes the last human to have lived, I'm not really looking to pad that time out by reading How to Win Friends and Influence People or 15 different DIY books.

By playing beatbox music and making everyone stay away from me while I grew corn and ate it slowly in front of them while they watched me cautiously from a distance.

I misread the question.
- I'm intrigued. What question did you answer?
By everyone, I mean nonhumans (nonhuman animals).

Something completely unforeseeable would off me within the first six months, is my guess. That tiny hangnail that breaks some skin which leads to an infection would be my demise. Assuming I didn’t have access to antiseptic, antibiotics, and the like.

Well, I'd guess no one would keep you from going "shopping" at the nearest pharmacy.
- True! But with my luck this would all happen whilst camping in the wilds.

I think I would try everything to launch myself into space. I know it's most likely impossible, but fuck it imma die anyway

Until I finished eating the second to last person.

At least three days, I reckon. No water infrastructure workers here means no water around these parts - period.

Id probably get a fast car or bike and speed and end up crashing and dying or something

Why crash a car when you can crash a plane? I always considered this to be my final quest in this kind of scenario. Learn to fly a plane as big as possible. Then probably get lost and crash while looking for an airport.

A month tops

Long enough to teach some monkeys how to make fire.

Or just set them on fire....

Probably about 5 years. I'd have to teach myself how to live in the rough by then and I don't think I could learn everything.

I don't think I'd even make it through a full day

It really depends on a handful of things. For context, I’m in northern Ontario.

I’m an introvert, and I spend most of my time alone. I thrive by myself, so being alone (for a while) won’t be an issue. However, I would bet over time I might lose my mind with zero presence of people.

Farming is possible, but the area isn’t great. Most of my food would be from hunting and fishing, which I have experience with already.

I’m also surrounded by lots of fresh water, so that is not a concern. My house is heated by a mix of propane and wood, so heating in the winter is not an issue.

Winter's will be tough, because roads won't be plowed. Eventually fuel will go bad and I won't have any running vehicle, including snowmobiles. So I would likely be walking with snow shoes, or staying inside. This would then require me to stock up all my food, which is its own issue.

Electrical grid would likely go down. If I want to preserve my food using freezers before the winter, I will need electricity, which would require me.installing some sort of solar system. I'm not knowledgeable on the subject, which would require me to go physically searching for books or written documentation.

Electricity is paramount for any future long distance travel, as combustion engines would not be operable, and electric vehicles would replace them.

Moving south to a warmer climate where hunting/fishing is still possible, and surrounded by fresh water would probably be best.

Long story short, loneliness would likely kill me before anything else does.

Assuming the food and water situation isn't too bad because of non-perishables in the many stores around me, it would be however long it would take a lack of immunosuppression drugs to allow my body to reject my heart.

People forget that even though it may not expire, a lot of food will get ruined by rodents within a few weeks of no one is around for pest control.

I could last a long time until all the nuclear plants melted down and turned the earth into a radioactive death ball.

It would automatically shutdown long before it got close to melting down

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Considering the number of nuclear power plants that would explode without someone to take care of them. Not long after the one about 45 minutes from my house goes up. It would be a painful death. Even if I somehow survived that, there would be LOT of radiation that would be put into the environment. Yeah I'd die pretty quick from radiation poisoning.

I don't think they work like that...
- They need heavy water. After a while the heavy water would boil away / evaporate.
  That would leave the rods exposed. Of everything I've been told there is a very loud bang at that point.
  What would prevent a catastrophe from happening?