Life has found a home on Earth for around 4 billion years. That's a significant fraction of the universe's 13.77 billion-year history. Presumably, if life arose here, it could have appeared anywhere. And for sufficiently broad definitions of life, it might even be possible for life to have appeared mere seconds after the Big Bang.
To explore the origins of life, first we have to define it. There are over 200 published definitions of the term, which shows just how difficult this concept is to grapple with. For example, are viruses alive? They replicate but need a host to do so. What about prions, the pathogenic protein structures? Debates continue to swirl over the line between life and nonlife. But for our purposes, we can use an extremely broad, but very useful definition: Life is everything that's subject to Darwinian evolution.
