I think Panspermia makes the most sense as an explanation for the emergence of life on Earth, here's why.<br>Here's the timeline (approximately):<br>[0 years] Big Bang ... [10 billion years] Creation of Earth ... [10.5 billion years] First Single Celled Organism ... [13.5 billion years] First Multi Celled Organism<br>It is commonly accepted that the increasing complexity of life from 10.5 billion years to present is due to selection/evolution, and I accept that. The problem I have is the complexity it takes to go from basic molecules to single celled organisms seems to be at least as big a leap as going from single celled to multi celled organisms.<br>Also, the most beneficial trait for an organism to adopt is the ability to evolve faster. Evolution has been speeding up over time, and modern biology provides many examples of evolutionary enhancements to evolution itself.<br>Given these two observations, it makes MUCH more sense to assume that the leap from basic chemistry to single celled life took longer than the leap from single celled to multi celled organisms.<br>So, I think the most likely explanation is that life took the leap from basic chemistry to either single celled organisms, or at least complex self reproducing molecules, over that first 10 billion year interval. This highly suggests that there should have been an RNA world stage of evolution that took place before Earth even existed.<br>Another strength of the RNA world hypothesis is that it provides an interesting solution to the Fermi Paradox. Life (and intelligent life) just takes way longer to emerge than originally thought, and it requires at least two planetary lifetimes to get from basic molecules to single celled life.<br>It's not that it's impossible for it to happen another way, but propositions for alternative explanations inevitably rely on non evolutionary mechanisms.<br>It's hard to explain why, if it's easy for life to develop, the universe isn't teaming with intelligent life already. You'd think an intelligent civilization would have colonized the galaxy by now if single celled life had emerged within, say, 1 billion years of the Big Bang. That's the complexity of human civilization plus 9 billion years of additional evolution. That's a lot of evolutionary complexity!<br>An even bigger problem is that it's hard to explain, if there was pre cellular life on Earth, where that pre cellular life is hiding now. If Earth had an RNA world stage, then you'd think it would have survived in some evolutionary niche that cellular life was unable to fill, or metabolically inefficient in filling. For example, fitting in tiny crevices in rocks. If there was pre cellular life on Earth, then where's the evidence for it, and in absence of evidence, where's the evidence for (what seems unlikely to me) the total extermination of pre cellular life? If Earth had an RNA world stage, it seems much more likely that our ecosystem would include pre cellular life.<br>I see single celled life as a molecular ecosystem captured in a membrane. When considering pre cellular life, I think it makes more sense to think in terms of ecosystems, rather than simply thinking of self reproducing molecules. It's not enough to have one self reproducing molecule to produce single celled life, instead I expect a large, diverse, mature ecosystem of self reproducing molecules in order to produce single celled life.<br>One more problem is that if Earth did have an RNA world stage, and single celled life emerged from this ecosystem, then why did this happen exactly once? It seems much more likely that we would have multiple lineages from pre cellular life to cellular life if that leap happened on Earth.<br>The Panspermia from RNA world hypothesis is an outgrowth of a confidence in the evolutionary explanation, along with a recognition that the universe is substantially older than early biologists realized. Also, there are far more Earth like exoplanets than early biologists realized. This hypothesis couldn't have emerged before the age of the universe was established, and the existence of exoplanets was confirmed, and that only happened relatively recently.