In Part I I introduced the Hard Steps model, and last time we looked at why its central pillar is dubious.

Then we asked: With no puzzle about timescales to solve, why believe in Carter’s solution to that puzzle?  The easy answer is that we do see evolutionary singularities, which suggests that the path Earth took to generating humans is very unlikely and contingent.  Even if Carter’s particular motivation for the model is wrong, surely evolutionary biology supports the existence of “hard steps” independently of that?

So next, Dan looked at the candidate hard steps.  He did a lot of work to compile various proposals for the hard steps, and found they vary a lot.  People seem to agree on abiogenesis, eukaryogenesis, and something special about humans or primates as hard steps, but other guesses are all over the map.

One error he found was people mistaking “Major Transitions in Evolution” for candidate hard steps.  This concept has a well defined meaning in biology, and eukaryogenesis is one of the major transitions, but he points out it’s a category error to conflate the two: despite the name, these major transitions don’t need to necessarily be hard or singular. This is why it’s good to collaborate with people in the fields you’re trying to publish in!

Anyway, Dan looks at what might constitute a hard step and one key is that the event be singular: unlike the evolution of the eye, which is convergent and has happened many times, a super unlikely evolutionary innovation would only be expected to happen once, perhaps because it is so complex.

But Dan points out that even though unlikely innovations will be singular, there are other explanations for singular innovations.

My favorite analogy is abiogenesis: we don’t actually know that life only started on Earth once!  Imagine that some lifeless organic matter on Earth got a fragile start on a brand new form of life going in some corner of a cave pond somewhere. What would happen? It would immediately be lunch! Life on Earth is really good at spreading everywhere and eating everything.

Life-as-we-know-it has fully occupied every available niche on Earth, so a second (or hundredth) abiogenesis has no hope of catching on and surviving.  This is sometimes called an incumbancy effect or pulling up the ladder.  In other words, just because we see only one tree of life on Earth today, all sharing a common ancestor, that does not at all mean it only evolved once.  Being singular does not necessarily imply being “hard”.

Another example of how this could happen is if a species, like cyanobacteria, so fundamentally changes the environment that it prevents an existing innovation from evolving again because the conditions that would induce it are no longer present (like oxygenating the atmosphere).

So many of the apparently singular events in the record might actually not be unique, it could just be that the first time it happened it somehow prevented it from ever happening again.

It’s also possible that many singular events are actually only apparently singular because the other instances have died out.  Dan points out an example of the evolution of plastids, organelles used in cells for photosynthesis.  This was widely regarded as a singular evolutionary event, but recently analogous and unrelated organelles called chromatophores in a particular species, which evolved only a few Myr ago.  He points out that if this rare lineage had died out, we would never know that the event wasn’t really singular and asks: how many other singularities only look that way because we haven’t found (or cannot find) any of the other lineages?

Indeed, we can’t really test the lineage of things in the fossil record: perhaps things that look like eukaryotes are actually from totally independent examples of the form that simply haven’t survived to present day!

The bottom line is that it’s not clear that there are actually any singular events in the evolutionary history of humans—which means there might not be strong evidence for “hard steps” even independent of Carter’s original argument for them.

But surely, you say, humans and our technosphere is singular?  There have never been cities and rockets on Earth before!  But Adam Frank and Gavin Schmidt are way ahead of you on that and have shown that we actually don’t have evidence that’s true!  An ancient technosphere would not be obviously technological in the geologic record.

And we’re not the only technological life on Earth! If we disappeared tomorrow, who knows what the octopi or beavers would get around to doing in a few million years…

Anyway, there’s a lot more in the paper, which I encourage you to read!  You can find it here:

http://arxiv.org/abs/2408.10293

The bottom line is that the hard steps model is dubiously justified on anthropic grounds or evolutionary grounds.  It might be right—in which case we might not expect to find other human-like species int he Galaxy—or it might not. Dan points out what work we can do to try to get to the bottom of this, but in the meantime: don’t worry about the Great Filter! There’s little to no reason to think it exists.

Enjoy!