The Molecular Genetics of Cold Winters Theory

When did Man eat from the the Tree of Knowledge?

Here feel we but the penalty of Adam,

The seasons’ difference, as the icy fang

And churlish chiding of the winter’s wind,

Which when it bites and blows upon my body

Even till I shrink with cold, I smile and say

“This is no flattery. These are counselors

That feelingly persuade me what I am.”

Sweet are the uses of adversity,

Which, like the toad, ugly and venomous,

Wears yet a precious jewel in his head.

- William Shakespeare

In Genesis, Satan and Eve seduced Adam into eating from the tree of knowledge. Then God punishes Adam by expelling him from the Garden of Eden, forcing man to farm the land according to the seasons. 

Frankly, I’ve always found the story very frustrating. As many have noted, it’s bizarre that the desire for knowledge is presented as evil whilst God is on the side of ignorance. But it may be that the story gets another issue back-to-front, the origin of intelligence. Rather than the harshness of nature being a punishment for intelligence, intelligence was a response to the harshness of nature. 

Whilst there are many explanations for group differences in IQ, the only attempt at a general theory is Cold Winters Theory (CWT). It posits that intelligence is selected for in seasonal ecologies with cold winters. Leaving a luscious Garden of Eden will select for brains. The hypothesis in its modern form came from Richard Lynn who noticed a striking pattern in his analysis of global differences in intelligence - smarter people tended to be further from the equator. 

There are broadly four lines of argument for the theory. 

1. Intelligence is higher among people who evolved further from the equator. In fact, the strongest correlate for national IQ is skin reflectiveness (r > 0.9). National IQ correlates very highly (r = -0.7) with winter temperatures and only weakly with summer temperatures. 

2. Archaeological evidence shows hunter-gatherers further from the equator developed more tools.

3. Humans further from the equator have larger brains. A trait which is known to be *causally* responsible for higher intelligence.

4. Within species, animals further from the equator have larger brains

Emil Kirkegaard summarises the first four lines of argument here and a history of the theory here. There are perhaps some other smaller lines of argument, such as the relationship between IQ and “delay discounting”/patience which helps explain why IQ would be useful in preparing for the seasons. There are, of course, some counter-arguments. I don’t find them convincing and Emil gives them a fair fight in his summary.

I wonder however if there might be a crucial fifth line of argument? Evolutionary theories are often considered ‘just so’ stories that are difficult to test, so Emil suggests we run an experiment, exposing animals to a controlled seasonal environment to see if there is any selection pressure. Alas, such experiments may be extremely impractical, expensive and of course too controversial. But it may also be unnecessary - nature has already run the experiment for us. 

Climate change and migrations are akin to the randomized intervention of an experiment. With molecular genetic techniques, we can simply test whether genes for intelligence increase in frequency the more humans were exposed to cold winters. Let’s first look at the “interventions”. 

In terms of climate change, assuming lower temperatures lead to more Cold Winter exposure, we should expect changes in intelligence surrounding the Last interglacial period 140-110,000 years ago and the Holocene interglacial period where temperatures shot up 30-20,000 years ago.

And of course, we have the very famous out-of-Africa migrations, where humans left a lush Garden of Eden for the seasons and cold winters. Although hominids had made the journey before, our ancestors are thought to have moved around 60,000 years ago. 

To summarise CWT’s predictions, assuming the earlier out-of-africa migrations did not have a genetic effect, IQ should be rising 130-110,000 years ago and after 60,000 years ago. It should be falling or stagnating 140-130,000 years ago and 30-20,000 years ago.

Now, let’s look at the data. 

Our first study “Genetic timeline of human brain and cognitive traits” was published as a pre-print last month. The paper estimates when common genetic variants (SNPs), which cause ‘brain and cognitive traits’, appear in the human genome. They estimate this temporal emergence by combining estimates from a ‘recombination clock’ and a ‘mutation clock’. In the paper itself, the scientists study genes that are euphemistically related to intelligence such as genes causal for the neocortex, the cerebellum or the general “human-phenotypic SNPs”. Instead, the scientists present the real juice, the genes for intelligence in supplementary materials. They know what they are up to… 

The red line shows the density of genes for intelligence appearing over time. Showing that the genes for intelligence started showing up around 200,000 years ago, before reaching a maximum rate of selection less than 100,000 years ago. Note that it’s the genes for intelligence that appear at the fastest rate around that time, not the other traits like sociability. But maybe this is confounded with homo sapiens simply going through a lot of genetic changes at that time? 

In the below graph, the authors compare the frequency of brain-related genes appearing to that of randomly selected genes. Thus we have a case and a control. Now we see that the relative selection for intelligence was peaking in the last 50,000 years, matching the Out-of-Africa migration. There’s no observable effect of the changes in climate, however.

Our next study, “Evolutionary Trajectories of Complex Traits in European Populations of Modern Humans”, was published last year. It estimates polygenic scores (ie. genetic predictors of traits) for ancient genomes. Then it looks at whether the polygenic scores were increasing or decreasing. Nice and simple.

But there’s a few problems. Firstly, the authors only look at the European genomes. Thanks political correctness. Secondly, the sample size really isn’t good enough yet. The older the DNA the harder it is to sequence, so we only have good sample sizes since 10,000 years ago. We can at least say intelligence has been increasing since then. Hopefully, if and when the sampling problem changes we will be able to test Cold Winters Theory with the polygenic scores.

So far the origins of human intelligence seem consistent with Cold Winters Theory.

Human intelligence did seem to increase after we left Africa. There are no traces of the effect of climate change, but the data is not refined enough to detect it. There are a few ways we can improve our analysis of this question. We can try to attain a more granular measure of Cold Winter exposure than temperature or the Out-of-Africa migration. Ideally, we would know which biomes individuals were living in at different times. It’s possible that climate change did not do much to change the ecologies in which humans were living since we were so close to the equator before leaving Africa. With more ancient genomes, better dating of pre-history and improvements to the genetic clock techniques we should attain more accurate estimates of the exposure and response of cold winters. 

Ideally, we would probably need improvements in the methods for testing selection. For example, it is known that a larger population means more mutations. I imagine that biases the mutation clock. It’s also known that applying polygenic scores trained in one race to more distant races, such as our ancient ancestors, results in a bias of some sort.

Although there are some ways we could improve a molecular genetic test of CWT, perhaps the biggest problem is how well migrations or climate constitute natural experiments. Climate change will result in selection, but it will also cause people to migrate, attenuating its effect. For migration, it is difficult to distinguish cause from effect. We might expect smarter people to be more likely to perform migrations. This is a particular problem given there’s some uncertainty about when the genes for intelligence arose and when the Out-of-Africa migration occurred.

Who knows, maybe the Book of Genesis is right after all?