In their 2016 journal article “The Cosmic Zoo: The (Near) Inevitability of the Evolution of Complex, Macroscopic Life“, Dirk Schulze-Makuch and William Bains write:

An important question is … whether there exists what Robin Hanson calls “The Great Filter” somewhere between the formation of planets and the rise of technological civilizations. …

Our argument … is that the evolution of complex life [from simple life] is likely … [because] functions found in complex organisms have evolved multiple times, an argument we will elaborate in the bulk of this paper … [and] life started as a simple organism, close to [a] “wall” of minimum complexity … With time, the most complex life is therefore likely to become more complex. … If the Great Filter is at the origin of life, we live in a relatively empty universe, but if the origin of life is common, we live in a Cosmic Zoo where such complex life is abundant.

Here they seem to say that the great filter must lie at the origin of life, and seem unclear on if it could also lie in our future.

In the introduction to in their longer 2017 book, The Cosmic Zoo: Complex Life on Many Worlds, Schulze-Makuch and Bains write:

We see no examples of intelligent, radio-transmitting, spaceship-making life in the sky. So there must be what Robin Hanson calls ‘The Great Filter’ between the existence of planets and the occurrence of a technological civilisation. That filter could, in principle, be any of the many steps that have led to modern humanity over roughly the last 4 billion years. So which of those major steps or transitions are highly likely and which are unlikely? …

if the origin of life is common and habitable rocky planets are abundant then life is common, and we live in a Cosmic Zoo. … Our hypothesis is that all major transitions or key innovations of life toward higher complexity will be achieved by a sufficient large biosphere in a semi-stable habitat given enough time. There are only two transitions of which we have little insight and much speculation—the origin of life itself, and the origin (or survival) of technological intelligence. Either one of these could explain the Fermi Paradox – why we have not discovered (yet) any sign of technologically advanced life in the Universe.

So now they add that (part of) the filter could lie at the origin of human-level language & tech. In the conclusion of their book they say:

There is strong evidence that most of the key innovations that we discussed in… this book follow the Many Paths model. … There are, however, two prominent exceptions to our assessment. The first exception is the origin of life itself. … The second exception … is the rise of technologically advanced life itself. …The third and least attractive option is that the Great Filter still lies ahead of us. Maybe technological advanced species arise often, but are then almost immediately snuffed out.

So now they make clear that (part of) the filter could also lie in humanity’s future. (Though they don’t make it clear to me if they accept that we know the great filter is huge and must lie somewhere; the only question is where it lies.)

In the conclusion of their paper, Schulze-Makuch and Bains say:

We find that, with the exception of the origin of life and the origin of technological intelligence, we can favour the Critical Path [= fixed time delay] model or the Many Paths [= independent origins] model in most cases. The origin of oxygenesis, may be a Many Paths process, and we favour that interpretation, but may also be Random Walk [= long expected time] events.

So now they seem to also add the ability to use oxygen as a candidate filter step. And earlier in the paper they also say:

We postulate that the evolution of a genome in which the default expression status was “off” was the key, and unique, transition that allowed eukaryotes to evolve the complex systems that they show today, not the evolution of any of those control systems per se. Whether the evolution of a “default off” logic was a uniquely unlikely, Random Walk event or a probable, Many Paths, event is unclear at this point.

(They also discuss this in their book.) Which adds one more candidate: the origin of the eukaryote “default off” gene logic.

In their detailed analyses, Schulze-Makuch and Bains look at two key indicators: whether a step was plausibly essential for the eventual rise of advanced tech, and whether we can find multiple independent origins of that step in Earth’s fossil record. These seem to me to both be excellent criteria, and Schulze-Makuch and Bains seem to expertly apply them in their detailed discussion. They are a great read and I recommend them.

My complaint is with Schulze-Makuch and Bains’ titles, abstracts, and other summaries, which seem to arbitrarily drop many viable options. By their analysis criteria, Schulze-Makuch and Bains find five plausible candidates for great filter steps along our timeline: (1) life origin ~3.7Gya, (2) oxygen processing ~3.1Gya (3) Eukaryote default-off genetic control ~1.8Gya, (4) human-level language/tech ~0.01Gya, and (5) future obstacles to our becoming grabby. With five plausible hard steps, it seems unreasonable to claim that “if the origin of life is common, we live in a Cosmic Zoo where such complex life is abundant”.

Schulze-Makuch and Bains seem to justify dropping some of these options because they don’t “favour” them. But I can find no explicit arguments or analysis in their article or book for why these are less viable candidates. Yes, a step being essential and only having been seen once in our history only suggests, but hardly assures, that this is a hard step. Maybe other independent origins happened, but have not yet been seen in our fossil record. Or maybe this did only happen once, but that was just random luck and they could easily have happened a bit later. But these caveats are just as true of all of Schulze-Makuch and Bains’ candidate steps.

I thus conclude that we know of four plausible and concrete candidates for great filter steps before our current state. Now I’m not entirely comfortable with postulating a step very recently, given the consistent trend in increasing brain sizes over the last half billion years. But Schulze-Makuch and Bains do offer plausible arguments for why this might in fact have been an unlikely step. So I accept that they have found four plausible hard great filter steps in our past.

The total number of hard steps in the great filter sets the power in our power law model for the origin of grabby aliens. This number includes not only the hard filter steps that we’ve found in the fossil record of Earth until now, but also any future steps that we may yet encounter, any steps on Earth that we haven’t yet noticed in our fossil record, and any steps that may have occurred on a prior “Eden” which seeded Earth via panspermia. Six steps isn’t a crazy middle estimate, given all these considerations.