Do you ever get the feeling that human beings were built for a different universe? Sometimes this world plays out exactly like a video game: you're dropped into a place you don't know and everything is inexplicably trying to kill you. If this is a video game, we're all glass cannons at best. We like to celebrate the toughness of the human spirit or our adaptability to hostile environments, but individually we are really easy to kill.
Get hit on the head just a little too hard? You're dead. Fall off a ledge higher than like five feet? You could die even from that if you land funny. Run into a wild animal? Good luck - most of them can easily kill a human and we don't have any natural defenses. Caught outside on a bad day? Turns out the weather on our planet can get extreme. If it's too cold, you die. If it's too hot, you die.
Your body can only heal small wounds, and only on certain places. Did something puncture your neck? Say goodbye. Did you lose a limb? Yes, survivable if you're treated immediately - otherwise you die of blood loss. Did you break a bone? Well, that can heal if properly set, unless it's your back, in which case you're paralyzed and will die unless you receive immediate attention forever. Maybe you got a minor cut. Even then you better get that washed out right away or some kind of dirt or bacteria will get in and kill you even after the wound heals. If your wound/broken bone can heal at all, sometimes your body will never make it back to 100%. A series of small wounds will eventually end your life.
Our bodies also require a lot of maintenance. We need to eat food every single day, and a lot of it, in order to keep us going. If you don't eat, you die. You need water, too. If you don't drink, you die faster than if you don't eat. Oh, and while you're not eating or drinking, you suffer extreme agony the whole time. You also need air to breathe and if you're not in open, clean air, you will die quicker than the time it takes to boil an egg. If the food, drink, or air you consume is not clean, you die. You need vitamins and minerals, most of which are found in food and drink, but one of them requires you to get out in the sunlight. By the way, sunlight causes cancer. If you get cancer, you eventually die. Every day you need sleep. Sometimes you can go a day or two without it, but all your functions are significantly reduced. And if you don't sleep at all, eventually you die. You also need shelter, though it's possible in some climates to live out in the open. In extreme heat, however, you need some kind of protection from the sun or you will burn or suffer heat stroke. In extreme cold you need four walls and a roof to keep the temperature up. If the weather is at all stormy, you die if you have no shelter.
Protect your eyes - they are the only way to see the world. You get two of them and you are severely disadvantaged if you lose even one. They do not repair themselves beyond superficial injuries and there is nothing stopping them from being punctured aside from a thin membrane. Unless of course you are born with imperfect eyes, in which case you're screwed from the get-go. If you're lucky, glasses or contacts can augment your sight so that you can function in society. Protect your ears too - they are the only way to hear the world. You get two of them and you are severely disadvantaged if you lose even one. The eardrum can repair itself from minor injuries, but your hearing can be permanently damaged if you hear too many things too frequently! Also, even if you protect yourself perfectly, your eyes and ears will begin to lose function as you age.
Perhaps the only body part that grows back when lost is your teeth. But that only happens once and it's a part of growing up. If you're an adult and you lose a tooth, it's gone forever. Teeth are necessary to eat most food, and if you remember from a couple paragraphs above, you need food to stay alive. If you do not regularly clean your teeth, they will eventually break or rot. This happens very quickly if you eat sugary foods. Sugary foods are those that we are all conditioned to seek out for our survival. While your teeth decay and break they will cause you great pain. There is no way to heal them. You will be in pain until they fall out. If the tooth decay is bad enough you may have a disease that will ultimately kill the whole you.
Your fingers do not grow back once severed. Hopefully you start with ten, five on each hand. While it is possible to live with none, the less fingers you have, the more you are dependent on people with all ten in order to survive. Lose your thumbs, for instance, and you can no longer carry things. Like your fingers, your toes similarly never regrow. Toes are necessary for you to balance yourself while standing. Standing is something you have to do in order to commence walking, which is how you move from one place to another.
You need your blood. It is a liquid substance inside your body. If you ever see your blood outside your body something is wrong, unless it is coming from your genitals and you are female, in which case something only might be wrong (the body is weird). In order to keep your blood inside your body, you are protected by a layer of skin. This skin is easily punctured by many, many things, including but not limited to your own fingernails, which are necessary to protect your fingers. If your skin is broken blood will start to flow. If the cut is deep enough your body will not have time to repair the skin before you die because too much blood is on your outside instead of your inside. And, as mentioned above, if something aside from blood gets into the cut, you can die.
In order to get all your blood to flow through your body, you have a heart. The heart pumps the blood at a consistent rate without you having to do anything. However, that rate must be maintained within a small window. If your heart pumps too slowly, you will die. If your heart pumps too quickly, you will die. Sometimes you can control your heartrate by eating the right foods and exercising, but you could just be born with a heart condition and die for no reason. If the heart is damaged by trauma in any way, you die.
Your brain is the organ that controls pretty much everything in your body. If this organ is slightly damaged, you might become a person whose title is considered an insult. If this organ is majorly damaged, you die. This organ is very easy to damage. Fortunately it is protected by a very thick casing of bone - your skull. But if the skull cracks from trauma, you have no protection anymore. Also, your brain can injure itself against your own skull if you move too quickly or come to a stop too quickly. These are both things that happen when you are hit in the head by, well, anything.
This is all not to mention the various diseases in the world. Without modern medicine, most diseases kill. With modern medicine, some of them merely make you wish you were dead. Diseases can be bacterial or viral, and they can spread by multiple means. Some are transferred via the air, meaning you will catch them if you are breathing in a space where a diseased person has also breathed. By the way, if you stop breathing you'll die, so that's not an option. There are other ways to transmit disease, but none is so powerful as the airborne virus.
If that's not enough, even a perfect body that contracts no disease and suffers no trauma will die of aging. Aging happens as your body replaces old and dead cells. When it does so, the replacement cells aren't quite 100% what the old ones were. Your functions will get weaker over time. It will hardly be noticeable until you are around 40 years old. Each decade the rate of decay will drastically increase. This rate of decay also increases with each trauma you induce, as you will never heal to quite the level you were at before.
Oh, and you'll essentially "age faster" - or decay quicker - if you live certain lifestyles. If you don't get good enough sleep, you will die sooner. Bad sleep can happen during stressful situations, which are impossible to avoid. Or you could have a sleep disorder from birth. If you don't eat the right foods, you will eventually not have enough vitamins and you'll contract a disease. If you eat too much food, you will gain a lot of weight and your body's ability to conduct business will be shot, speeding up the aging process. Gain enough weight and you die immediately. If you consume alcohol or drugs or smoke, you might die sooner than if you don't. But if you want to avoid hard drugs, remember that all the things drugs do to your body could also happen by random chance, so good luck out there!
We're also emotional beings, and our emotional states affect our health just like our physical states. Under extreme stress, the body can develop ulcers, you can lose sleep, or you could just have a stroke and die in your living room. Stress can be caused in emotionally charged circumstances such as someone else dying. In other words, when you die, you will take other people out with you. Stress can also be caused simply by being concerned about the survival or well-being of someone you care about. Oh, and when you don't get enough food, water, sleep, shelter, and so on, you undergo increasing stress until the need is acquired - yet the effects of the stress linger.
In addition, we're social beings. Without good social bonds during early development, we suffer terrible health problems or social disorders that cannot be cured. If your mother and/or father abused you, you will carry bad memories forever that will cause - let's all say it together - stress, as well as severely diminish the ability of you to begin your own healthy relationships. This means that those relationships you choose to start will be tainted, causing further stress upon you and your partner. Terminating the relationship will cause even more stress. If you stick together though, eventually one of you will die. The closer the bond, the more likely your death will stress your partner to death.
Even if you have a good upbringing, you can suffer if you don't find love as an adult. Usually this means a sexual partner though that's merely the most common way to manufacture a strong social bond. Finding a sexual partner the healthy way is a matter of being born with genes that make you sexually attractive. If you are severely deformed or are an unpleasant person to be around you will not find a partner, which will induce - you guessed it - stress. Single people actually live shorter lives than married ones, believe it or not. Of course you could always pursue a sexual relationship the wrong way. You could pay someone for sex, but in practice this is a good way to catch a disease, which, if you've been paying attention, you want to avoid. Or you could just forcibly take sexual activity from someone. This is called rape, and if you can do it without causing yourself stress due to the fact you are a psycho piece of human garbage, you'll severely traumatize the other person, and if they and their social contacts find out, it could cause you severe bodily harm when they catch you.
As social animals, we tend to form large social groups. These groups need a leader or leaders, and whoever leads the social group will likely be as physically, mentally, and emotionally screwed up as you. They will either deliberately or accidentally implement bad ideas and introduce problems into your society. If the leaders are powerful enough and/or rule enough people, they can directly or indirectly cause the death of thousands or millions from a number of the following methods: lack of adequate and clean food or drink, lack of medical care to keep all the blood on the inside instead of the outside, lack of protection from murderers who would like to see all your blood on the outside, reduction of necessities so as to induce massive stress, and - when all else fails - directly murdering people. Societies that do not do any of these things are lucky, but eventually leaders die because the position of leader is stressful. The new leaders are not likely to be as wonderful, so all societies eventually fall into disrepair.
All that to say the earth is not our home. We might live here for now, but there's no way we were meant to. Human survival is the story of extreme stubbornness, of a people that moved into a neighborhood that didn't want them and did its best to kick them out.