An exchange between Astrophysicist Charles Lineweaver and myself:

In their 2019 paper “The Timing of Evolutionary Transitions Suggests Intelligent Life is Rare”, Snyder-Beattie, Sandberg, Drexler, and Bonsall argue that the expected time for “intelligent life” to appear on Earth “likely exceed the lifetime of Earth, perhaps by many orders of magnitude” which “corroborate[s] the original argument suggested by Brandon Carter that intelligent life in the Universe is exceptionally rare.”

In a Feb. 2022 comment in Inference, “A Lonely Universe”, Charles Lineweaver disagreed:

The Snyder-Beattie et al. result depends on the assumption that … the major transitions that characterize our evolution happen elsewhere. There is little evidence in the history of life on earth to support this assumption. … transition to human-like intelligence or technological intelligence occurred only about 100,000 years ago and is species-specific. The latter trait is strong evidence we should not expect to find it elsewhere.

It [is not] reasonable to argue that … the features of life on earth … most likely to appear in life elsewhere are those that have evolved independently many times, such as complex multicellularity, eyes, wings, and canines. … [because] these … have only occurred within a unique [never-repeated] eukaryotic branch that represents a tiny fraction of the diversity of life on earth. …

Attempting to compute the probability of human-like intelligence elsewhere based on our lineage is akin to analyzing the evolution of the English language on earth and trying to use the timing of the Great Vowel Shift to estimate its timing on other planets

My July 2022 reply, also in Inference, says:

Lineweaver suggests that without good reasons to think “the major transitions that characterize our evolution happen elsewhere,” estimates regarding Earth do not allow us to make estimates regarding other planets.

On the contrary, I see two ways to compare planets so that Earth estimates become relevant for other planets, allowing us to infer a low overall rate at which advanced life appears elsewhere. First, if Earth is a random sample from planets that succeed in making life at our level, the success rate on Earth cannot be too different from the typical success rate on other such planets. Second, if there is a substantial chance that our descendants will soon become very visible in the universe, the fact that no other star in our galaxy has yet done so can set a low upper bound on the fraction of such stars that can have reached our level by now. …

Let R be the chance of life at our current level—i.e., controlling nuclear power and practicing spaceflight—appearing on a particular planet within some fixed planet habitability duration. … chance Q that, within the following ten million years, a planet at our level would give rise to a civilization that becomes permanently visible across its entire galaxy. [I elaborated with math examples for both these approaches.]

In that same place, Lineweaver then responded:

I don’t believe in the general group that he and many others call “advanced life.” … No other life-forms in the universe will be genetically or phenotypically more similar to us than chimps, bonobos, gorillas, naked mole rats, or frogs. Since Hanson and many others exclude our closest relatives from “advanced life,” they are—by their definition—not talking about a generic group with other members. …

On Earth, humans are the only ones who have become humans at our level of technology. To then conclude that among all species, our species had an average chance of becoming humans at our level is meaningless. …

Morris … argues that strong selection pressure leads to convergent evolution which then produces human-like intelligence. Hanson and most physicists subscribe to this view, but most biologists and I don’t. … Hanson refers to … life at our level … I … ask: If we exclude our species from consideration, does this talk of levels make any sense when applied to the rest of life? Are dogs or red oak trees at a higher level?

Reading Lineweaver’s response, I see my reply was off target; his issue is with the very idea of “life at our level”. So let me try again.

A key datapoint is this: we do not now see any big visible civilizations (BVC) in the sky who have greatly changed the natural universe into something more to their liking. In order to explain this fact, we must postulate a “great filter”, i.e., a process whereby simple dead matter might give rise first to simple life, and then to a BVC, or various filter obstacles might end this progress, so that it never produces a BVC. We must conclude that so far, averaging across the universe, this filter process has a very low total pass-through rate to a BVC. After all, no dead matter in the entire universe has yet given rise to a BVC we can see. That is, this great filter is on average very large.

In contrast, Earth today seems to plausibly have a much higher rate for creating BVC. I’d say we have at least a one in a million chance of doing so within the next ten million years. (This isn’t value judgement, just an estimate.) As Earth is now thus much closer to this BVC endpoint than it was originally, there is a sense in which Earth has now passed through part of the great filter, so that a substantially smaller filter lies before us than once lied before a simple dead Earth.

To talk about how much of the great filter we have so far passed, we’d like a way to talk about where we “are now” in this filter process. And this is where we can want to talk about our current “level” along some linear path from dead matter to BVC. But, as Lineweaver points out, evolution is in many ways a tree, instead of a line, and we cannot construct such a level concept merely by creating a conjunct of various random specific features of our species and planet.

Even so, I do think there are useful ways to define “our level” (OL) within the great filter. What we want is an equivalence class OL of alien civilization-moments such that (a) Earth today is in OL, (b) almost all BVC were once in OL at some prior point in their history, and (c) OL covers only a short “time slice” during which few civilizations go extinct. If we have more choices, we’d further like to pick OL so that (d) it minimizes the variance in the (coarse-grained) chance that each civilizations in OL later gives rise to a BVC. The lower this variance, the more it makes sense to talk in terms of the average chance within OL of giving rise later to a BVC.

One option would be to just define OL as the class that meets criteria (a,b,c) and actually minimizes (d). But while this might be well defined, it seems unwieldy. Which is why I tried above to define OL above in terms of a civilization having just mastered the basics of both nuclear power and spaceflight. It might be reasonable to add a few other techs to this list, such as computers.
Sure, we’d define somewhat different OL sets if we added or cut techs from this list. But the key point is that any civilization that had mastered all of them would be well on its way to being able to start a BVC soon. And most likely the chance of extinction is low between the point of having mastered half of these techs and mastering all of them. Thus the exact list of techs in our OL definition probably doesn’t make that much difference.

Yes, this way to define OL can let humans pass through OL, while chimps never do. But I just don’t see why that’s a problem. There is in fact a big important difference between what humans and chimps have accomplished, and I’m fine with our OL definition reflecting that.