I'm familiar with two arguments for the nihilistic, natural creation of life.
1. Chaos theory/Simple rules lead to incredible complexity given enough time.
Take Conway's Game of Life. Citing wikipedia, there's only four simple rules.
>Any live cell with fewer than two live neighbours dies, as if by underpopulation.
>Any live cell with two or three live neighbours lives on to the next generation.
>Any live cell with more than three live neighbours dies, as if by overpopulation.
>Any dead cell with exactly three live neighbours becomes a live cell, as if by reproduction.
With this, people have been able to make literal computers in the game. People have simulated The Game of Life within The Game of Life. However, this supposes the intervention of some kind of intelligence. If The Game of Life had some sort of mechanism to randomly generate positions, and over a large enough grid and with sufficient time, it wouldn't be unreasonable to assume that it could generate some incredibly complex mechanisms.
That being said, reality has many orders of magnitude more complexity than the Game of Life, and each level of complexity adds more ability for things to grow, reproduce and change. By that logic, it's only a matter of time that life spontaneously generates given enough time and with enough random interactions occurring between atoms. Ergo, life spontaneously generated.
2. Reality is biased towards entropy
This one I'm much less familiar with. Essentially, if the universe somehow WANTS homogeneity and the highest level of entropy possible, sentient beings are pretty much the best way to increase entropy as rapidly as possible. If no life ever existed, then Earth would've slowly withered away all the while the Sun burns it away into ash. With humans though, we getting that process started, and doing so at an exponential rate. By that logic, the universe is BOUND to generate life as it's the fastest way to go to the highest level of entropy achievable.
So it goes.
