...certainly less speculative than believing extra-terrestrial influence in the process...

I'm just curious, how would you justify it is less speculative in any sort of manner that isn't a logical fallacy?

At the heart of that argument, should it be boiled down far enough, you find yourself addressing "First cause".

The primary argument in favor of science can represented as "there must be a physical cause and effect in all systems". Macroevolution is the effect of mutations (the cause), which in turn have inner cause/effects, which have inner cause/effects. Every observable, natural behavior is deterministic to some degree or another and science is the art of figuring out what the factors are. Religion states that things are supernatural, that things merely are. If you follow either path, you're left with a paradox. If everything is a cause/effect, then its inherently paradoxical leading towards the infinite. However if there is a "first cause", what came before that, who decided that is the nature of things?

I'd argue that saying either is more speculative than the other is pretty hard to determine.