Mars gets somewhat nice
In about 1.6 billion years, the sun's habitable zone will have moved outward, increasing the carbon dioxide in Mars's atmosphere. According to Wikipedia, its surface temperature will rise "to levels akin to Earth during the ice age".
Combining a thick carbon dioxide layer to trap the heat (and the atmosphere), and the future extent of the habitable zone of the sun which will continue to warm the planet, it's reasonable to think that the Martian ice caps will melt - filling the northern, low-lying plains, and creating a primitive ocean. Assume, for this question, that that happens, 1.8 billion years from now, and that the atmosphere won't leave for billions more years.
BOOM! Panspermia occurs.
The Earth, now desolate and devoid of humans, is hit by something large (it'll happen eventually), throwing material into orbit, along with some hitchhikers. You are free to choose what exactly is on this material, but it should be able to survive a trip through space to the Martian ocean. Consider an extremophile organism, or a tardigrade, or a newly evolved something, which thrives the hot-Earth present.
That material lands in the sea, and life starts to evolve based on whatever just came from Earth;
Which organisms will survive, and what traits will start to develop among the population(s)?
Can a complex ecosystem eventually be built upon this?