[oskarkk]

To go through all 10^84 possible combinations in a billion years, around 10^67 combinations would have to be "tried" per second. So yeah, it doesn't seem feasible to have one and only correct combination of 140 nucleotides spontaneously appear.

But if the "solution" could be composed of a couple of separate smaller parts, that would be stable and linger for a long time, it would be much easier. 40 nucleotides have 10^24 combinations, so only 10^7 tries per second would be needed... over a billion years. And all of the necessary parts would need to be created and then meet in the same place and somehow combine. So, still not easy, but this case doesn't sound so outrageously improbable.

In the end, maybe it is extremely improbable for life to happen, and only one in 10^n suitable planets develops life, and Earth was just very lucky to experience this peculiar phenomenon.