You would have to ask those aquinted with the evolution theory at which point did the chickens become true chickens.
Everything is constantly evolving, it is just a very slow process, which we humans given our short lifespans doesn't really like, for us 20 years is a long time, for evolution it is barely a blink of an eye.
Think about this, homosapiens are estimated to be around 200-300 thousand years old, which is quite some time. Now to put that into perspective, lets look at the dinosaurs:
Dinosaurs went extinct about 65 million years ago (at the end of the Cretaceous Period), after living on Earth for about 165 million years.
Think about that for a second and let the number sink in.. 165 million years, it's almost impossible to even get your head around such number. Even though it is not evolution, just try to compare it to what humans have accomplished in just 2000 years and how much we have changed the Earth, societies, technologies.
Now imagine going 200 years into the future and humans living on Mars, those people born there, most likely won't be able to return to Earth due the difference in gravity, because their bones, muscles might not be strong enough including all other kinds of health issues. So it probably wouldn't take that long before humans on Mars might change into something else, that might at some point not be able to procreate with humans on Earth, if they were left isolated.
So they would still be part of the homosapiens, but might be called homomartians or something and would be a new branch of the great apes
. And given enough time they might change enough so they hardly look humans as we know them now, depending on how the environment etc. would evolve them on Mars. But again, we are not talking 20 years or something. The scale we are talking here are thousands and thousands of years.
Even looking at humans living on Earth today, you can clearly see how they look differently, africans, asians, europeans etc. Now obviously since we travel all over the world with ease now, there is probably a good chance that we won't see a new branch coming off from homosapiens like in the past and it will just be a continuous development for us here on Earth, but colonizing space and given enough time, that could probably change.
So in regards to your true chicken question, at which point wouldn't the homomartians be homosapiens anymore? It's a long evolving process.