A theoretical physicist at Dartmouth, Marcelo Gleiser, observes in an NPR commentary just how unlikely it is, even under Darwinian assumptions, that complex life exists other than on Earth:
A very clear distinction must be made between simple, unicellular life and more complex life forms. It’s hard not to doubt that Earth is the only planet where life took hold. After all, we have seen how resilient it is here, with extremophiles defying our previously held assumptions of where life can thrive. However, there is a huge difference between simple life and complex life. Contrary to what many believe, evolution doesn’t lead to complex life forms: evolution leads to well-adapted life forms.
We had unicellular algae here for about 2.5 billion years and nothing more. The jumps from simple to complex life are many and still poorly understood. For example, on Earth simple prokaryotic cells had to incorporate outside structures to become eukaryotic cells — with nuclei protected by bags; life then had to go from single-celled to multicellular creatures; it had to somehow develop differentiated organs that were nevertheless integrated by functionality; it had to adapt to water, air and land, and to multiple environmental cataclysms; finally, it evolved creatures capable of higher brain functions.
This interestingly turns Steve Meyer’s argument in Signature in the Cell on its head. Let’s assume we get the first, simple life as a free gift. Gleiser says it’s everything that happens after that that is the real enigma.