It's pretty unlikely that life emerged independently on two planets in adjoining orbits.

The much more reasonable explanation is that life emerged on one planet and transferred over. Earth and Mars aren't particularly close, but they're close enough for material to transfer between them, particularly early in the solar system when there were far more asteroid impacts kicking rocks and dust out into space.

Earth is strange - a billion years of water and chemistry on every grain of sand on every beach. Every combination of temperature, sunlight, wetting and drying as tides slosh things around in every phase.

It's like a bioreactor for creating life. One could argue, it's highly unlikely given all those experiments and all that time, that life would avoid being created.

And Mars? It has a moon too, two of them. Tides as well, lunar and solar. Pretty much the same deal.

Why would you say it's unlikely? For all we know simple life could emerge very easily if conditions are right and it's the step to complex life that's the hard one.

The transfer of life from one planet to another("panspermia") seems plausible in principle. However, the immense energy released during asteroid impacts makes survival unlikely in many cases, as the very organisms hitching a ride may be destroyed in the process. That said, evidence suggests that microbes shielded deep within rocks could potentially endure the violent stages of ejection, space travel, and planetary re-entry.

Beyond panspermia, I lean toward another perspective:

Life exists because it serves as a natural means of accelerating "entropy" production. In an otherwise relatively stable system, life provides a shortcut, catalyzing processes that dissipate energy gradients far more efficiently than non-living chemistry alone.

At the microbial level, metabolism drives the dissipation of redox gradients, pushing chemical systems toward lower free-energy states. While cells locally maintain order and complexity, their activity increases the overall entropy of their environment. In this sense, life is not a violation of thermodynamic laws but a direct expression of them.

If this is true, life may be extremely common in the universe, arising naturally wherever the right conditions exist to favor energy dissipation.

What may be rare, however, is complex life. Complexity requires not only a lucky balance of stability and change, but also the ability to endure, or even be forced forward, by catastrophic events such as mass extinctions, without wiping out all of it. Earth's active techntonic plates provided yet another means to enable evolution.

My hunch was completely the opposite

Two planets in the "habitable zone" both receiving "ingredients" from the same stream of comets/meteors/cosmic dust, both with what we believe to be environments conducive to life (liquid water, surface temperatures/atmospheric pressures that aren't like Venus..:)

In fact, I'm more surprised that life didn't start independently on both planets at roughly the same time (it could turn out that life did indeed start at the same time on both planets too, just it didn't "take hold" or "last as long" on Mars)