I think this is why those of us interested in this are very much hanging out for results from our explorations of Mars. It's pretty much our only opportunity to see if life emerged twice in the same planetary system under similar conditions.

What are the chances?

On the positive side Mars had pretty much all the ingredients for life (chemically) and had a long enough period for at least something intersting to develop. On the negative side, the environment was still different to ours (gravity, climate, whether, etc).

We still aren't clear on what conditions need to met for like to get going. This is why both positive or negative results on Mars are exciting.

My favourite theory is that life actually takes ages to get started so much so that it started on Mars and finished on Earth (from Mars being impacted and debris making it to Earth). Kind of like a passing of the torch.

Mars: "Crap. I'm dying. Hey Earth, can you take this thing I've been working on?"

Earth: "Yep. No worries. I got this. Will try to send some back in a few billions years. Take care."

Well there's also Titan, Enceladus, and Europa right? Titan is by far the most exotic but it could serve as an example of what happens when life evolves under a radically different environment. It has some complex organic molecules there and a very interesting/varied geology and topology. Enceladus and Europa could exhibit very similar conditions to those of the deep sea on Earth as well, which could very well be where life originated. Also at one point even Earth could have became a "snowball" planet and still able to sustain life: https://en.wikipedia.org/wiki/Snowball_Earth

You mean carbon based life. Methane based life may be a possibility, and you wouldn't have found it on Mars. You might find it on Titan.

Titan would have a low-temperature carbon-based chemistry, but it would remain carbon-based if focused on methane.

Alternative hypothetical biochemistries usually focus on silicon.

Mars alone does not provide the answer about whether or not life evolved independently in our solar system.

I just can't understand what the chemical difference is. Are these differences because of oxygen? I.e. life forms of C+H vs lifeforms of C+H+O?

What temperatures are we considering?

It is not my suggestion. It's suggested by scientists that Titan may host methane based life.

Life adapts.

Similarly, Earth wasn't particularly earthlike, in contemporary terms, for much of its history -- sometimes far hotter, frequently far colder.

No, it just means that only one lineage did well enough in competition that we can find examples to study its biomarkers.

Or that there are factors we don't yet understand that favor similar biomarkers in independent lineages.

Or...luck.

> if biogenesis happened only once (for all of our bio examples), then it is either a rare event that just happened to occur very shortly after the planet cooled, or it happened sometime before Earth was hospitable for life and elsewhere in the universe.

Or biogenesis itself changed the environment in a way which made it no longer conducive to biogenesis.

But I would be cautious calling a billion years or so "shortly" even in the context of a four-something billion year old planet. A billion is a very big number, especially when you multiply it by how many trillions of little potential multi-dimensional petri dish situations were around for each second of each year of those billion years.

And it could have happened only once because once established, it tends, on a timescale orders of magnitude shorter than a billion years, to squelch the appearance of other forms, at a minimum by guiding them along with its template, or alternatively by smothering them.

But this all says nothing about whether it happened on Earth or elsewhere. Either one could still be true.

I think we can wrangle a few more datapoints if we are creative with perspective, for example, we have extremely good evidence to suggest that the planet Mars is currently inhabited by alien robots.

Consider for example that abiogenesis is probably impossible in an atmosphere containing oxygen, because everything gets oxidized too fast, although free oxygen in the air happened quite a long time after life arose.

To abiogenicly form long chain carbons you need a free oxygenless reducing environment, oxygen from the new microbes could have easily poisoned all the viable sites for abiogenesis.

Or life can only appear on young planets (maybe while it's hot), or some astronomical event triggered it (Moon?), or there are enough hard steps on the path to intelligence that it couldn't appear if life took too long to form. It's a good question anyway.

> So that means that biogenesis happened only once.

Life has repeatedly shown a tendency of consuming all available raw matter and populating every niche. Biogenesis happening only once is completely non-surprising.