There are only four ways the answer could be yes:

1) Life on Mars originated from Earth (e.g., via contamination from one if our probes).

2) Life on Earth originated from a common source with life on Mars, or from Mars itself (panspermia).

3) Life is common in the Universe, so it was able to arise independently on both Earth and Mars.

4) Life is uncommon, but through an unlikely coincidence, it arose on both Earth and Mars.

4) is inherently very unlikely, and 3) seems extremely unlikely both on a priori and empirical grounds: Life emerging requires a lot of unlikely things to happen, regardless of what kind of life or how it first comes to be, and I would intuitively expect that the chances of life ever appearing at all, let alone surviving, on a given planet are extremely low. Furthermore, the Fermi paradox counts as evidence against life being common, since, if life was common enough for the existence of life on Mars to be anything other than a super unlikely coincidence, I would expect intelligent life to be common enough that we would have found it by now.

I also find 2) to be extremely unlikely, since it requires life capable of surviving on both the primordial Earth and Mars, two completely different environments, to have evolved in yet another different environment, made its way to both planets, and survived the journey. On top of that, Occam's razor tells us we should disfavor panspermia and favor the hypothesis that abiogenesis happened on Earth, and there is a lot of evidence that early Earth had the right conditions for abiogenesis to occur, which again increases the probability that this is where it happened.

Finally, 2), 3), and 4) are all made less likely since we haven't any life on Mars yet, and all three of these option imply that life has probably been on Mars for a long time and should therefore have spread pretty far across the planet and possibly evolved into macroscopic life.

So, in my estimation, the only possibility with a non-negligible chance of being true is 1), and the only plausible way I could see of Earth life getting to Mars is on something we sent there. But even if that has happened, it is hard to imagine that the life we inadvertently sent there will have replicated and spread far from its source, and that we will have detected it doing so beyond serious doubt, in such a short time span.

Technically, there is a fifth possibility, which is that life is uncommon in the Universe, but there is some common factor between Earth and Mars that made it likely to arise on both. This would still fall under the "unlikely coincidence" possibility (no. 4), unless there was some causal relationship between the factor occurring on Earth and on Mars, since otherwise we would just be moving the unlikely coincidence of life arising back one step, to an unlikely coincidence of a factor that makes life possible arising. So the common factor would have to be something that has to do with the two planets being in the same solar system, or forming from the same protoplanetary disc, etc. I find it extremely unlikely that any such factor could affect the probability of life arising by much, though, since planets in the same solar system aren't really any more similar to each other than planets in different solar systems.

@JosephNoonan I had some chat with Bing AI, and it's unclear to me how strong the evidence is for life being uncommon. Seems like our ability to detect planets at all is very low, let alone to find life on them.

• There are only 5000 planets that are confirmed to be planets at all How many exoplanets are there? – Exoplanet Exploration: Planets Beyond our Solar System (nasa.gov)

• The furthest potentially habitable planet confirmed is only 2,870 light-years distant List of exoplanet extremes - Wikipedia

Does “currently” refer to the time of market creation or resolution?

@JimHays the word 'currently' refers to the time of resolution, as opposed to consensus that there was once life on Mars, millions of years ago. I would not count a human Mars base, for instance.

But if the next Mars rover accidently seeds a bunch of earth bacteria that reproduces and spreads on Mars, that would resolve to 'yes'. Likewise, if one of the Mars landers in the '70s contaminated the planet with earth bacteria, and that bacteria has multiplied and spread beyond the lander to a large region of the planet, that would resolve to 'yes'.

Does that help?

If someone contaminates Mars with Earth bacteria, as you've phrased it, this would still resolve "yes". Is this intentional?

@EvanTh I would resolve that to yes if it survives and spreads across the planet, beyond whatever spacecraft it came on. The question I care about is whether there is life on Mars that we didn't put there on purpose, that will survive even if Earth were wiped out or something. I don't think it is a very interesting question if there could be life on an earth thing on Mars.

I didn't try to make this a trick question, I swear! Does this clarify things in a manner that feels true to how the question was originally stated?

This study seemed to indicate that there were moving white spots on Mars that could be fungi. “Although similarities in morphology are not proof of life, growth, movement, and changes in shape and location constitute behavior and support the hypothesis there is life on Mars.” https://www.researchgate.net/publication/351252619_Fungi_on_Mars_Evidence_of_Growth_and_Behavior_From_Sequential_Images

I fixed the spelling error in the title