Last time I introduced the “hard steps” model of Carter and the “Great Filter” model of Hanson.  Now on to the new stuff.

Dan Mills is an affiliate member of the PSETI Center, and a geomicrobiologist. Discussing the hard steps model with him, he became interested in diving in and seeing how it holds up to the scrutiny of someone with deep expertise in these matters, which hitherto had been the province of physicists, economists, and others without any formal biology training.

[I’ve written before about a specific affliction common but not unique to a certain kind of physicist or engineer that because what we do is hard, we can just waltz into other fields and contribute.  As xkcd put it:

And my favorite takedown of the type is from SMBC (go read it!)]

What Dan Mills found was that while there has been a lot of things right in the development of these ideas, the Hard Steps model is actually largely unjustified.  It could be right—but there’s not much reason to think so.

One big flaw in the argument is that there actually is a good reason that evolution on Earth might operate on a timescale coincident with Solar evolution.  One that had been in the literature is that the sun changes luminosity on that timescale by a bit, so that could couple the biosphere to its evolution, but the bigger one is the (somewhat recent) understanding of how tightly coupled geological processes are to the biosphere.

The biggest and most famous is probably the oxygenation of the Earth’s atmosphere.  For most of Earth’s history, the atmosphere had virtually no oxygen, but eventually oxygenic photosynthesis evolved, and began “polluting” the atmosphere with oxygen molecules.  Eventually the oxygen levels got so high that life began evolving to adapt to the new environment.  This is called the Great Oxidation Event.

And animal life on Earth requires oxygen! There’s a whole literature on this, but essentially oxygen provides a way for large animals like us to metabolize our food and get the energy we need to operate, one that’s hard to replicate any other way, biochemically speaking.

Dan charts major milestones in evolution with time against oxygen abundance on Earth here:

In this figure, the past and future of life on Earth is charted.  Oxygen makes a big jump after cyanobacteria show up, allowing eukaryotes to evolve, and then as oxygen goes up even more animals (metazoans) show up.  We are thus today in a window of habitability in which humans could have evolved.  The “death of the biosphere” in 0.5–1 Gyr is from solar evolution which will eventually boil off the oceans.

Dan’s point is that animals show up right when they’re “supposed to” (or, perhaps, “allowed to”), and humans show up shortly thereafter.  This isn’t “late” or weirdly coincidental at all!  The coincidence that Carter based the hard steps model on then isn’t between evolutionary timescales and solar nuclear timescales, but between the latter and the time it takes life to change Earth’s atmosphere, which is set by geophysical factors, which can be billions of years. That’s an interesting coincidence, but that doesn’t seem to be much importance behind it, and, more importantly, it doesn’t implicate human evolution at all!

So the fundamental underlying puzzle that the Hard Steps model was designed to answer might not exist!

With no puzzle to solve, why believe in the solution of the hard steps model?  Well, primarily because we do see evolutionary singularities, which suggest 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 it independently of that?

Next time: challenging other aspects of the model