When did recursion evolve? (part 2)

Chomsky and Behavioral Modernity

This is part of a series on the evolution of recursion. The first post explained why so many psychologists, philosophers, and linguists believe recursive thinking is the ability that makes us human. The second presented early proposed dates for when this evolved. The main takeaway is that if recursion was fully present more than 200,000 years ago, it is not central to what makes us human. It can’t explain why we outcompeted the Neanderthals, for example. The following dates allow recursion to take center stage, the first is from the man who introduced recursion to linguistics.

100,000-50,000 years ago (Chomsky)

Chomsky makes two reasonable assumptions: that recursion must have evolved before humans left Africa, and that it will be accompanied by large cultural changes. From there he argues that recursion was the result of a single gene that mutated 50-100kya. Or in graph form:

Importantly, he really does commit to a single, species-defining change right before we left Africa, and then nothing important since¹. I have two problems with that.

It’s not how evolution works

Every animal with a spine evolved from the same common ancestor. The code to create a spine is complex, involving many genes. These instructions do not pop up willy-nilly. A giraffe has the same number of neck vertebrae as you or I. Even when there is strong selection for a longer neck, it tends to be on continuous variables—the length of each vertebra—rather than on additional vertebrae. You see the same thing in the preservation of the number of finger bones in whales even though they have approximately 0 fingers.

Now, recursion may be simpler to code than our skeletal system. But it boggles the mind that it can be defined and integrated into the many cognitive traits it affects by a single gene. Recursive functions are liable to be unstable, it would be a great surprise if that was worked out in one fell swoop. What’s more, we have now sequenced the genes of millions of people, including hundreds of prehistoric humans. In the words of population geneticist and Nobel Laureate Robert Reich, if there was a “single critical genetic change” it is “running out of places to hide.”

Where’s the recursion?

To my knowledge Chomsky does not specifically argue that the complexity before OoA requires recursion other than by reference to the “Cultural Revolution” described by archeologists. But even Corballis notes “The African record prior to the exodus certainly suggests the beginnings of modernity, although the development of technology and cultural complexity seems relatively meager compared with what was to come in the Upper Paleolithic”. It would instill more confidence in the date if there were claims about specific technologies that 1) emerge in this time period 2) require recursion to produce 3) and are not used by Neanderthals.

Other models

That model seems unlikely, but this time period is where many researchers believe recursion emerged. We start to see the first glimmers of art, technological complexity steadily increases, and there is a massive migration out of Africa (requiring many behavioral changes wherever humans went). Things seem different, even if the genetic changes must have been more gradual. To get a more even-handed picture of the time period, I recommend archeologist Stefan Milo’s YouTube overview of life 100,000 years ago. His video on evolution 1,000,000-30,000 years ago is also great. Both cover art and burial practices which start in this time period, and were shared with Neanderthals. (Though the art is, generously, “abstract”; maybe at about a toddler level.).

40,000-12,000 years ago

The Upper Paleolithic is the first time that most everyone can agree we see the full range of the human spirit. Herzog explains why he chose to film Cave of Forgotten Dreams, a documentary about the art found in Chauvet Cave:

The quality of the art, which is from a time so far back in history [20 kya], is stunning. It's not that we have what people might call the primitive beginnings of painting and art. It is right there as if it had burst on the scene fully accomplished. That is the astonishing thing, to understand that the modern human soul somehow awakened. It is not a long slumber and a slow, slow, slow awakening. I think it was a fairly sudden awakening. But when I say "sudden" it may have gone over 20,000 years or so.

Archeologists, too, have interpreted this period as the first evidence of the modern mind. (See: The Mind in the Cave: Consciousness and the Origins of Art.) The idea that our minds took their now-current form 40-20 kya is termed the “classic Upper Paleolithic model” by the critical paper Behavioral Modernity in Retrospect. It summarizes the situation:

In the 1990s, many archaeologists shared the view that there was a significant change in our species' behavior following its incursion into Europe. (For example, see Mellars and Stringer 1989; Mellars 2005; C. Renfrew 2009; Henshilwood and d'Errico 2011a; Cook 2013, and for commentary Conard 2010; Nowell 2010.) This lightbulb moment was thought to be so marked that some postulated that it was the result of a mutation that changed the organization of the brain (see Klein 2002; Curtis 2007). Multiple suggestions have been made about this: that the mirror-neuron system matured in immediately prior millennia (Ramachandran 2003), or that working memory was enhanced (Wynn and Coolidge 2007), or that domain general intelligence improved (Mithen 1996), or that parallel processing came into play (Solso 2003).

Crucially, much of this work was done before recursion was considered fundamental. Chomsky’s The evolution of the language faculty was only published in 2005. None of these researchers argued the new ability was recursion, but it is relevant to the general discussion of when we became fully human and whether that was genetic. 

Even Corballis, who argues that we were fully modern 170,000 years ago, says: “The Upper Paleolithic marked nearly 30,000 years of almost constant change, culminating in a level of modernity equivalent to that of many present-day indigenous peoples.” Note how strong the claim is: it is only by the end of this period, 12 kya, that we see societies of the complexity of current indigenous people. Put another way, the most remote tribes in the world today (or in 1800 when much of the contact was happening) are living a life more complex than perhaps anyone was 20,000 years ago. Go to the mountains of Papua New Guinea, or the outback of Australia, and you will find people the same as you and I, telling the same types of stories, engaged in the same sort of rituals². But if you had a time machine and did the same 20,000 years ago, all bets are off. In the words of Jacques Coulardeau:

“If we want to understand the phylogeny of language or any human production, we have to keep in mind the following timeline. What is most important is that an essential divide occurred around 15,000 BCE, but it took several thousand years before being effective, and in many areas in the world the transformation may have started later and may have taken longer to become effective.” 

And it is not just our stories and technology that have changed recently, the shape of our skulls have as well³. All modern skulls are “globular”—rounded on top—as shown below.

We do not see modern levels of globularity until 35,000 years ago, and the evolution in that direction was not complete even 10,000 years ago. The authors hypothesize that the skull may have changed shape in order to accommodate a growing precuneus, “a central node of the default-mode network and an important hub of brain organization.” Many studies connect this region to consciousness.

Drawing on this evidence of cognitive adaptation two linguists propose a four-stage model for the evolution of language, with recursion only present from 10kya onward. This model’s relationship to skull changes is made more explicit in their paper The Shape of the Language Ready Brain.

I want to reiterate that, whatever else recursion does, we are most certain that it allows self-awareness and it seems that is unique to humans. Despite this, the authors push back on whether their recent date implies a fundamental psychological change. Of course, it could have been phenomenologically incremental, but then when was “I” discovered? Gotta put a date somewhere. And the worldwide similarity of the first person singular suggests it could have been invented around this time. 

In conclusion, many people look at our past and only see full human behavior in the last 40,000 years. The counter-position is that everything was there earlier, but either the evidence was destroyed, or the abilities were lying fallow. But the shape of our skull, too? A latent trait waiting for the moment to go full globular? Seems like some brain reorganization was underfoot. 

The most difficult question is how that would have spread. Was there some gene (or many) that made it around the world? Which ones? These questions are enough to temper the theory that we became “us” in a genetic sense in the last 40,000 years.

Summary

Recursion is understood to be the superpower underlying language, self-awareness, and perhaps even subjectivity. But when dating its evolution researchers often look for things that are not recursive (eg. masturbation) or cannot be measured (eg. grammatical language 2 million years ago). Generally speaking, there is a bias to err on the side of more distant evolution, as that is seen as politically safer. This creates paradoxes of its own.

• 200,000,000+ years is a common estimate for when recursive subjectivity emerges. This requires chickens—but not octopi—to “Carry their sense of their own identity forward”. It implies an impassable evolutionary chasm separates the self-making recursion present in animals, and the type of recursion described by Descartes or Chomsky.

• 2,000,000 marks the beginning of the genus Homo. Some have argued that full language was present then. Others see recursion in the stone tools. This downplays the importance of explaining how Homo Sapiens outcompeted Denisovans, Neanderthals, Homo Erectus, Homo Florensis, Homo Longi, and Homo Luzonensis all at about the same time. If we all had the special sauce, why is there only one lineage standing? 

• 200,000 is officially about the time our species emerged. Corballis makes the case that by this point recursion was fully in place. As a psychologist and linguist, he also takes a strong stance on everything recursion allows: language, mental time travel, self-awareness, counting, and storytelling. The evidence is rather scant: a stone tool complex mostly associated with the Neanderthals, and mostly static for a hundred millennia. It’s the Sapient Paradox on steroids. If humans were fully formed, then where are all the things that make us human?

• 100,000-50,000 is how Chomsky threads the evolutionary needle. It satisfies taboos about evolution and the human brain by making the moment an almost hand-of-god mutation with no follow-ups. It satisfies the genetic data by placing the event in Africa. And it’s mostly in line with the archeological record, though it fails to explain why the most direct evidence of recursion is 10,000 years later on a different continent.

• 40,000-12,000 we see the first art you can make a compelling documentary about, more complex societies, and changes in skull shape. Many have argued there was some genetic change, but generally point to an incremental improvement in working memory or the like. Some have argued for recursion emerging in this period, but downplay the phenomenological nature of that change. It’s strange how recent these changes are, while also being global.

1. Truthfully, I’m not sure if he thinks there was much evolution before the fateful recursion mutation; maybe the level should be higher before 50kya. But I’m trying to be generous by making it symmetric before and after, and he says there was nothing important after. The plot would be even more implausible if evolution slows down after humans 1) enter a new cognitive niche 2) pick up a bunch of genes from Neanderthals and Denisovans 3) explode in population size 4) enter a bunch of new environmental niches

2. One possible exception is the Piraha tribe in the Amazon studied by linguist Dan Everette. He claims they have no creation myth and make no art. Amazingly, he also claims that their language is not recursive, they can't fathom numbers, and their pronoun system was borrowed from a neighboring language (though Piraha is otherwise an isolate).

He accounts for all of this with the Immediacy of Experience Principle: they have strong cultural values to not discuss or think about anything that is not in immediate experience. Recursive sentences allow that type of thinking, so recursion violates the IEP. As counting requires recursion, it is also off-limits. Despite this, they attended his nightly "learn to count" classes for 8 months. Nobody could learn! They sure do a good job of abstracting and enforcing that one rule about recursion, eh?

He makes very clear that all of this can be explained by cultural forces, and as such we should discard recursion from our definition of what it means to be human.

3. It will always be funny how much this debate hinges on skull shape. Lots of high-sounding rhetoric about what it means to be human, and then invariably researchers whip out the calipers. The people who prefer 200kya are no exception, their date is based in part on the emergence of a "gracile" skeleton. Humanness is stored in our thin torso, I guess.