Why Aliens Might Exist
And Why We Might Be Them
Nobody who knows the subject well is really saying aliens do not exist. What we are actually saying is that so far, we have not found any solid evidence of aliens. That being said, however, most scientists believe simple life is probably common in the universe, while complex life is likely rare.
Why do we think that? Because that is what happened here on Earth. And since Earth is the only place where we know life exists, we have to start with what we know. If other planets follow the same pattern, we would expect plenty of simple life, but not much complex life. So yes, aliens may exist, but they might mostly be single-celled organisms.
Life appeared on Earth not long after the planet cooled, but it stayed simple for billions of years. The leap to complex, multicellular life took a very long time. That suggests the jump from simple to complex life is much harder than just forming life in the first place.
One reason for that is something called endosymbiosis. Roughly 2 billion years ago, one single-celled organism swallowed another one. But instead of digesting it, the two cells formed a partnership. That is how the first eukaryotic cells were born, which later led to all complex life, including plants, animals, and humans. This partnership created mitochondria inside cells, and later chloroplasts in plants. But this major event seems to have happened only once in Earth's history. All complex life descends from that one fluke.
By the time endosymbiosis happened, Earth was already around 2 billion years old. For almost half its existence, life on Earth was nothing but single-celled organisms without a nucleus. In about another billion years, Earth's surface will likely become too hot for life as the oceans evaporate. After that, the planet will drift through space for several billion more years, once again lifeless.
Since Earth is our only example, most scientists think life elsewhere is probably common. But that life is likely to be simple, like microbes, rather than advanced creatures like animals or people.
Could we be aliens? In a way, yes, we are all aliens. We are made from atoms and dust created in ancient stars that exploded long before Earth existed. That material drifted through space and eventually came together to form our solar system, our planet, and us. So even though we were born on Earth, our raw ingredients were forged in the hearts of stars. In that sense, yes, we are all aliens.