[JacobAldridge]

Nick mentions Environment Disaster as a possible existential risk, but perhaps the environment is a more mundane and yet still 'Great Filter' that befalls expansionist civilisations.

What if the resources required to leave the home planet (including here the resources required for generations of technological steps to reach that point) are more than an Earth-like planet can sustain? In other words, of necessity or as close to it as probability allows, intelligent life must destroy the sustainability of its home planet before it achieves the technology capable of leaving it.

Of course, by leaving it I'm talking on the scale required to explore and interact with neighbouring solar systems, not just sending Voyager 1 out there with a picture on its side.