Panspermia is pretty much irrelevant to the actual question though; even assuming life got to Earth the hitchhiker way, it would have to have developed on another planet, and we’re back to square one.
Panspermia is kind of weird to think about IMHO. Because, it likely took a long time to develop and a long time to travel here. So it must have started a long time ago. Before the Solar system was created. But Sol is pretty old. How early could life have started really?
That gets into astrobiology. Theoretically life could have started as soon as its basic ingredients were ready. So not until things had cooled down a bit after the Bang, when there were heavier elements than hydrogen and helium; and some kind of land and water.
> In the redshift range 100 . (1 + z) . 137, the cosmic microwave background (CMB) had a temperature of 273–373 K (0-100◦C), allowing early rocky planets (if any existed) to have liquid water chemistry on their surface and be habitable, irrespective of their distance from a star.
> In the standard ΛCDM cosmology, the first star-forming
halos within our Hubble volume started collapsing at these redshifts,allowing the chemistry of life to possibly begin when the Universe was merely 10–17 million years old.
In comparison, our beloved sun is estimated to have been born 9.2 billion years after the Big Bang, a third of the way into the universe's history so far.
> The Sun is approximately 4.6 billion years old, while the age of the universe, based on current estimates, is about 13.8 billion years.
So our solar system is not that old, relatively speaking. We're among elders, some stars are three times older than the sun.
> In the future, however, life might continue to emerge on planets orbiting dwarf stars, like our nearest neighbor, Proxima Centauri, which will endure hundreds of times longer than the sun’s.
> Ultimately, it would be desirable for humanity to relocate to a habitable planet around a dwarf star like Proxima Centauri b, where it could keep itself warm near a natural nuclear furnace for up to 10 trillion years into the future.
The earliest galaxies formed when the universe was just a few hundred million years old, which means there may have been planets that are 3x times older than Earth.
Several caveats apply, chiefly that heavy elements weren't produced for a while in significant quantity, but were produced fairly early on due to large stars exploding relatively quickly, when they were merely tens of millions of years old, if that.
