Probability of Pre-Intelligent Life Seeding Between Solar Systems

That probability is increasing. Scholz's Star passed well within the outer edge of the Oort Cloud a mere 70,000 years ago. It's important to keep in mind that one theory for why we should expect to see life on Europa is that impacts on Earth must have thrown biological material into space, which must eventually impact on Europa. We shouldn't hold our breath for a whole giraffe to make it to Europa just yet, although it's worth recalling that the C. elegans worms (not giraffes, but at least metazoans!) on the Columbia survived re-entry and were found alive 3 weeks after impact (link here). While many outlets are covering the "Neanderthals must have seen this!" angle of the Scholz's Star near-miss, a more important take home is that if biological material can plausibly mix between bodies in the same solar system, it is not much less likely to mix between solar systems.

Scholz's Star is not unique. If this just happened 70,000 years ago, we can reasonably infer that this has happened frequently. 70,000 years is not a long time in astronomical terms. A number of known stars have come or will come within Oort-mixing distance in this 100,000 year period. In point of fact, the Stardust mission - which returned physical material from Wild-2 - showed based on isotope ratios that Wild-2 must have originated in a different solar system besides our own. We have classically thought about life moving between solar systems in terms of intelligent aliens building ships, but it may be more plausible to expect that something at the level of unicellular organisms or even simpler than that is what usually moves back and forth. The ideas is not new (probably Fred Hoyle articulated it first mid-20th century) but we now have more data to support the ideas as plausible.