One of the interesting ones I've heard talk about lately is the "interstellar travel" step - Earth is on the lower end of the size range for rocky planets that can support an atmosphere and magnetic field to shield surface life from radiation. And because of the tyranny of the rocket equation, life developing on larger rocky planets may have a much harder time reaching orbit.

A good demo video: https://www.youtube.com/watch?v=amjuJJwI3iM

On a planet with twice the Earth's mass, it would take a larger-than-Saturn-sized rocket to put a Mercury-sized payload into orbit.

EDIT: And said Mercury-sized payload would have to spend more of its mass on heat shielding than the human Mercury capsule if it's intended to come back down.

Just to clarify for anyone unfamiliar with the US space program in the 50s-70s, or those just briefly confused by the discussion about planet sizes using planet names for things that aren't planets...

Mercury was a small space capsule containing 1 person designed to last a few hours in space. The Saturn V rocket was until recently the most powerful rocket ever launched, and took 3 crew, a crew module, a lander module, and payloads, to the moon.

This wouldn't stop von neumann probes https://en.wikipedia.org/wiki/Self-replicating_spacecraft

The upmass requirement for bootstrapping that kind of system would be quite large.

Yes but the rocket equation wouldn't stop intelligent species from going "loud" with radio signals.

The rocket equation itself, no, but the planet mass would also have drastic effects on the evolution of species. On our planet, high density fibrous plants like trees have an advantage when it comes to access to the sun but what if on a bigger planet, that advantage goes to plants than can emit a spidersilk-like material to float on the wind in colonies? There goes a practically infinitely renewable building material that can be used for almost anything until the industrial age. How can a civilization get to the electronic age when they spend all their time between extinction events hauling rocks and mining ores to build 2 story dwellings?

There's just so many unknowns.

Not only dwellings but fuel as well. I mean, if you don't have trees how are you going to invent fire...

Some people consider cooking a critical step in our evolution (cooking enables harvesting more calories from food).

With no good fuel available cooking could also be ... off the menu.

Dry grass catches fire quite easily and high gravity shouldn't hinder the growth of low grass.

If not grass, dried escrements are also good fuel. People use them in areas of this world with little or no trees.

Someone evolved a multicellular yeast with a similar approach back in 2011 or so. They used centrifuging to select for larger 'clumps' of yeast, rather than a predator to eat the smaller ones, but still pretty similar.

https://www.newscientist.com/article/mg21028184-300-lab-yeas...

Exciting, but also slightly terrifying. I know this isn’t universal, but I, for one, very much hope that the filter is behind us and not in front of us.

That being said I’m not too shocked. The transition from unicellular to multicellular has always seemed like less of a hurdle to me compared to the start of unicellular life itself.

Not to be too pessimistic, but there's no reason that there should only be one "Great Filter". There could easily be multiple astronomically improbable events barring interstellar civilizations from bearing fruition. It might be just as improbable for unicellular life to form as it is for unicellular life to beget multicellular life, so on and so forth.

Or even reason to assume that exobiotic life is cellular, and needed to jump from singular to multi-cellular structures like ours did on this planet.

I mean, in principle, but that's probably like assuming that exobiology is carbon based. Carbon and cells are both really super useful.

The great filter exists at all times as cited by the fact that the longevity of a species does not guarantee their future survival. All life may die off at any time.

The jump to the start of unicellular life seems like the big stumbling block. It might be so incredibly rare that it's lucky for it to even happen once in the universe.

Evolution is a process rooted in randomness. The more faces a dice has, the longer you need to throw it to get the highest number. Or, reversely: the longer you have to throw your dice to get the highest number, the more faces it generally has.

It took "only" < 400 million years for life to develop, meaning unicellular life, but it took 3 billion years for multicellular life to develop. If we apply this basic rule, multicellular life seems the hard thing. Of course though, statistics wise this is pretty crappy ground to stand on because we only have one sample :).

The hard part is (probably) the transition from Prokaryotes to Eukaryotes. The transition from unicelular Eukaryotes to multicelular Eukaryotes is (probably) easy [1] .

Edit: [1] From https://en.wikipedia.org/wiki/Multicellular_organism#Occurre...

> Multicellularity has evolved independently at least 46 times in eukaryotes, and also in some prokaryotes, [...] . However, complex multicellular organisms evolved only in six eukaryotic groups [...]

One sample is hard to read because even low odds can happen. Picture the following statement: "The chance of life forming anywhere in the universe over a 100 billion year period is equal to 1 in Graham's number".

If this statement is true then it is incredibly unlikely life forms in our universe at our current stage exist. Low odds can occur and the only reason anyone is here to observe them is because they did occur. Since we only know of life forming once we cannot make assumptions about the odds of it happening.

"It took "only" < 400 million years for life to develop"

Yes, that's what people assume, but the universe existed for billions of years before that, so what if life might have used all that time to evolve before it landed on Earth deep inside some rocks?

Unicellular in the form of bacterium may in fact not be that much of a jump. It may even have happened twice in the form of archaea and bacterium.

The jump from prokaryotic to eukaryotic is far more intimidating.

That's actually happened a bunch of times, too. I'm just going to link to a comment I made a while back: https://news.ycombinator.com/item?id=18456049

"Diatom-derived plastids" blew my mind.

You need heredity, differential reproduction and selection. We really just need to start with a single case of a self-replicating system. Hard yes, but with countless trials running in parallel for millions of years it seems intuitively possible

If the universe exists for eternity, and in approximately the same form, then even very small odds may approach near certainty.

Is it really that much of a paradox?

- "Metallicity": We probably need a recent / "Population I" star, i.e. it' s probably unlikely that there can be civilizations much older than us (in astronomical dimensions): https://en.wikipedia.org/wiki/Stellar_population#Population_...

- In 1.1 billion years from now, the Sun will be 10% brighter than it is today, and this increase in luminosity will also mean an increase in heat energy, which Earth’s atmosphere will absorb. This will trigger a moist greenhouse effect here on Earth that is similar to the runaway warming that turned Venus into the hellish environment we see there today. So we have appeared only after 80% of the "usable" life span of our star has already been over -- a window that might be easy to miss (an larger stars burn even faster)

You don't need an astronomically old civilization for it to have galaxy-wide impact. Already a "few" million years should be enough to colonize a dead-except-for-you galaxy.

Yes, although keep in mind we already knew of three independent evolutions of multicellularity — animals, plants, and fungi — so it arguably couldn’t be an improbable enough transition to qualify for great filter status.

Don't forget the humble Volvox!

https://en.m.wikipedia.org/wiki/Volvox