On timescales of billions of years, we don't know if any of those steps are "hard," "easy," or "inevitable." As the first commenter to that article notes at the link, we have only a single datapoint.<p>It's certainly unclear that photosynthesis is an ironclad requirement for life, and, in any case, it evolved multiple times in parallel on Earth. (Anoxygenic photosynthesis in fact evolved, <i>de novo</i>, more than once.)<p>I'd add that it's also by no means a given that life, broadly defined, can only evolve around a sunlike star.<p>Speculation around "hard steps" is pseud naval gazing until we gather more datapoints. It's not even valuable as speculation -- except perhaps as an exercise in countering sloppy thinking.
- <i>"The fact that it did happen here tells us nothing more than that, and until we dig out evidence of a ‘second genesis,’ perhaps here in our own Solar System inside an icy moon, or on Mars, we can form no firm conclusions."</i><p>I'm convinced the absence of evidence is, itself, valuable evidence. The (apparent) single origin of Earth life is a remarkable statistical observation hiding in plain sight.
The steps don't have to be that hard for us to be unique.<p>There are 10^22 to 10^24 stars in the Universe (a big range of uncertainty there but it doesn't matter for this point). That sounds like a lot but anyone with knowledge of combinatronics would immediately say holy shit that's a small number. Cryptography buffs are probably looking at that going "that can't be right, it's a tiny number", you divide it by 10 repeatedly, 24 times in fact and you're down to nothing. 24 not-at-all 'hard' steps and we're probably alone.<p>You've probably heard "there's soooo many stars in the universe there has to be other life out there". There really isn't that many in terms of probability.
This topic is near and dear to me. It is so important that I'm working full-time on a game about how a first contact with an extraterrestrial could look like. If you are interested, I just released a free demo with ~2 hours of content: <a href="https://store.steampowered.com/app/3040110/Outsider/" rel="nofollow">https://store.steampowered.com/app/3040110/Outsider/</a>
"Carter was motivated by the timing of our emergence, which we can round off at 4.6 billion years after the formation of our planet. He reasoned that the upper limit for habitability at Earth’s surface is on the order of 5.6 billion years after Earth’s formation, a suspicious fact – why would human origins require a time that approximates the extinction of the biosphere that supports us?"<p>My absolute favorite Fermi's Paradox theory is that life on Earth was evolving too slowly to survive, so aliens sped up the process while hiding themselves, but that came with a variety of problems that humanity bumps into, and the aliens are trying to clean up the mess without destroying humanity's potential for a unique identity.<p>The sci-fi novel practically writes itself.
The planetside wet carbon-based chauvinism is outstanding.<p>We don't even know what goes on inside our own Sun (a place with immense energy flux ripe for sustaining lifeforms, whatever they might be) and these people are looking for a second Earth with little green men. I don't think it's as much scientific curiosity as some deep-seated psychological issue.<p>Life is sustained fire, I don't think it much matters what is burning, just that there is infinite fuel available and a remotest possibility of a spark.
Here's a parameter I don't think the Fermi Equation takes into account:<p>I call it "Fragile Universe", based off of the notion of the "Fragile" or "Vulnerable World Hypothesis" [1].<p>f_c is the parameter for the fraction of "civilizations that reach the technological level whereby detectable signals may be dispatched", so that handles the "Fragile World Hypothesis". Alien civilizations can wipe themselves out, exhaust resources, etc. before we get to see their detectable signals [2].<p>What the equation doesn't take into consideration is the possibility that an advanced species can trigger the destruction or total reset of the entire universe, eg. by nucleating the vacuum collapse. The first advanced species to reach that point could kill every species in the universe and start the whole thing over from scratch.<p>This is an extreme version of the anthropic principle [3]. We exist because the universe hasn't been reset yet. By us or otherwise. Presumably we might be the first to get there.<p>Universe fragility could be the reason we don't see aliens. We could be the very first, and we could wind up hitting the reset button.<p>[1] <a href="https://nickbostrom.com/papers/vulnerable.pdf" rel="nofollow">https://nickbostrom.com/papers/vulnerable.pdf</a> ; tldr: advanced species can trivially wipe themselves out and will tend to do so. Not just nukes, but in the extreme, your average citizen can create grey goo at home with the press of a button that will turn the entire planet into paper clips.<p>[2] It could be that the time frames in which advanced civilizations emit detectable signals are so geologically small as they shift into non-detectable modes (eg. dark forest, disinterest in expansion, shift into a higher plane of existence, etc.), but that's orthogonal to the discussion.<p>[3] <a href="https://en.wikipedia.org/wiki/Anthropic_principle" rel="nofollow">https://en.wikipedia.org/wiki/Anthropic_principle</a>