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Originally Posted by blankall
If life were to happen it probably wouldn't survive very long, as the existing life would take it out. For example, you wouldn't have a large stew of amino acids and carbohydrates without existing life consuming it.
In order for life to involve a second time, it would have to happen in a sterile environment that is also conducive to life evolving.
There also is some ongoing research to show that life may have evolved more than once on Earth. Most life is microbial, and we have a very poor knowledge of that life. We haven't sampled and then molecularly examined every microbe from every nook and cranny. In fact, we've likely only examined a very small fraction and largely from the most common of places. Secondly, the Earth's environment of lots of nitrogen, water, and carbon may lead to a co-evolution scenario, where life evolves to becomes relatively similar, consisting of DNA, proteins, carbohydrates, etc...
There's also some suggestion that viruses may have evolved independently of the main branch of life.
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It's only theoretically possible, but not scientifically proven. We only have evidence that it happened once. Scientists have never been able to recreate it or completely understand the circumstances that cause abiogenesis. At the very least, we know that the process is very rare and not something that happens easily.
The conditions that allowed it to happen on Earth also aren't the same conditions that exist today. 3.5-4 billion years ago when it is believed to have happened, the Earth had a dense methane, water vapour, and ammonia atmosphere. The planet was getting bombarded and the system had a lot more energy at the surface. There were polycyclic aromatic hydrocarbons in vast amounts. It could very well be these things that kicked off the process, and not just the existence of liquid water.
Having said that, maybe Mars had a similar eon in it's early days.