Plausible evolutionary pressure for an organism to evolve space flight?

Question

A recent question asked for a natural mechanism for a creature to escape one planet and go to another. The answers prompted me to this question.

Given any answer to this, is there any plausible case for evolution to enable a creature to leave a planets biosphere and go to another, not by begin blasted from the surface by a meteorite or titanic volcanic explosion, or genetic engineering. I suppose in other words; is naturally evolved panspermia. possible?

My guess is no, but cannot help but think there may be perhaps some cyclic devastating conditions that eventually leads to this possibility. "Life finds a way."

I would prefer the creature be multicellular. But would accept answers for spores or seeds type transmission.

EDIT: To make things even easier, we can assume a minimal gravity to sustain a life bearing atmosphere.

Answer 1

Yes, but not on a planet

First off, it is impossible to me that life evolves naturally to panspermia on a planet. The energy requirements for launching yourself into space are astronomical, the further travel from the edge of space into the rest of the star system even more so. There is simply too much energy that needs to be gathered and stored safely for a long time. Even if a creature can do this, how will it safely ignite the fuel and fly up into space?

Most plausible would be a microorganism that blasts it's offsping in excessively protected cocoons. Some bushes hold their seeds until a forest fire. These eggs could hatch after exposure to extreme heat and cold. The creatures would blow up a large portion of their offspring, but in so many cases there's the 'get many offsping and see what survives' strategy. It still requires too much energy, an implausible explosion or ignition and a protective shell. It seems much more plausible such huge stores of energy would be used for movement, protection or much, much more offspring.

That being said, I can see two playsible ways for panspermia to evolve. One is still being blasted into space at a regular basis that space flight is a great way to survive. The other is to not start on a planet to begin with. If you start at asteroids there are plausible mechanisms for many sizes of creatues to get off them. Then they can fly towards other asteroids, moons or other suitable celestial bodies. It isn't hospitable, but the same can be said of hot deserts or the icecaps in the Arctic circle. You just need to be adapted well enough.

Answer 2

Assuming a minimal gravity to retain a life-sustaining atmosphere does not make things easier!

Retaining an atmosphere requires a certain minimum escape velocity, and that's exactly what you want to minimize. Life, fortunately, does not require an atmosphere. Life requires liquid, but deep ocean life would get along just fine if there were no atmosphere above it, and plenty of organisms (famously including tardigrades) can survive in stasis in vacuum. Humans can even operate for up to 90 seconds (in the ideal case) in vacuum without permanent injury, so it's not that hard to imagine an organism that has developed in an ocean (or at least, say, a sub-surface lake) and then evolved full resistance to vacuum, maintaining its own internal liquid environment. Really, it's not that much different from Earthling creatures moving from the ocean onto land, and developing impermeable skin to take our watery environment with us.

Liquid environments can exist under icy or rocky shells, with no need for an atmosphere, on bodies with much lower escape velocities. The ideal environment for evolving an organism that naturally propels into space is, therefore, probably something like a small ice moon like Enceladus--which does, in fact, have geysers that expel material above escape velocity--or a large cometary / Kuiper-belt object like Pluto or Arrokoth--large enough to have significant internal heat, and retain liquid underneath an ice shell, but small enough to easily escape from.

Robert Forward describes this kind of organism in Camelot 30K. (Spoilers for a 29-year-old SF novel incoming.) In that novel, intelligent life is discovered on a Kuiper-belt object at 30K. The animal-analogs are endotherms, which actively maintain their own pressurized internal liquid environment against the low-temperature, hard vacuum environment of the world's surface. The ecosystem survives on the energy of reactive chemical radicals left in the ice by the passage of cosmic rays, and the heat released by bioaccumulating radionuclides.

The power input from cosmic rays is not huge, and radionuclides are rare, so they are using up the energetic resources of their tiny world far more quickly than they will be replenished--which means that they cannot survive indefinitely on just one world. They must travel between low-gravity icy bodies. And in fact, it turns out that every organism in the alien "ecosystem" is actually genetically identical--the plants, animals, people, everything, are actually just clones of a single organism, exhibiting extreme phenotypic plasticity (like different casts of ants) in order to instinctively build a structure that will launch their colony into space--which kills most of the individuals, but allows fertile spores to drift to other bodies where they can germinate and start the cycle over again.