For most of he last century, the received wisdom in the social sciences has been that human evolution stopped a long time ago–in the most up-to-date version, before modern humans expanded out of Africa some 50,000 years ago. This implies that human minds must be the same everywhere–the “psychic unity of mankind.” It would certainly make life simpler if it were true.
Thus Cochran and Harpending fire the opening salvo of The 10,000 Year Explosion: how Civilization Accelerated Human Evolution. (If you haven’t finished the book yet, don’t worry–we’ll discuss one chapter a week, so you have plenty of time.)
The book’s main thesis–as you can guess by reading the title–is that human evolution did not halt back in the stone age, but has accelerated since then.
I’ve been reading Greg and Henry’s blog for years (now Greg’s blog, since Henry sadly passed away.) If you’re a fan of the blog, you’ll like the book, but if you follow all of the latest human genetics religiously, you might find the book a bit redundant. Still, it is nice to have many threads tied together in one place–and in Greg & Henry’s entertaining style. (I am about halfway through the book as of this post, and so far, it has held up extremely well over the years since it was published.)
Chapter One: Conventional Wisdom explains some of the background science and history necessary to understand the book. Don’t worry, it’s not complicated (though it probably helps if you’ve seen this before.)
A lot of of our work could be called “genetic history.” … This means that when a state hires foreign mercenaries, we are interested in their numbers, their geographic origin, and the extent to which they settled down and mixed with the local population. We don’t much care whether they won their battles, as long as they survived and bred. …
For an anthropologist it might be important to look at how farmers in a certain region and time period lived; for us, as genetic historians, the interesting thing is how natural selection allowed agriculture to come about to begin with, and how the pressures of an agricultural lifestyle allowed changes in the population’s genetic makeup to take root and spread.
One of the things I find fascinating about humans is that the agricultural revolution happened more or less independently in 11 different places, all around 10,000 years ago. There’s a little variation due to local conditions and we can’t be positive that the Indus Valley didn’t have some influence on Mesopotamia and vice versa, but this is a remarkable convergence. Homo sapiens are estimated to have been around for about 200-300,000 years, (and we were predated by a couple million years of other human ancestor-species like Homo erectus) but for the first 280,000 years or so of our existence no one bothered to invent agriculture. Then in the span of a few thousand years, suddenly it popped up all over the darn place, even in peoples like the Native Americans who were completely isolated from developments over in Asia and Africa.
This suggests to me that some process was going on simultaneously in all of these human populations–a process that probably began back when these groups were united and then progressed at about the same speed, culminating in the adoption of agriculture.
One possibility is simply that humans were hunting the local large game, and about 10,000 years ago, they started running out. An unfortunate climactic event could have pushed people over the edge, reducing them from eating large, meaty animals to scrounging for grass and tubers.
Another possibility is that human migrations–particularly the Out of Africa Event, but even internal African migrations could be sufficient–caused people to become smarter as they encountered new environments, which allowed them to make the cognitive leap from merely gathering food to tending food.
A third possibility, which we will discuss in depth next week, is that interbreeding with Neanderthals and other archaic species introduced new cognitive features to humanity.
And a fourth, related possibility is that humans, for some reason, suddenly developed language and thus the ability to form larger, more complex societies with a division of labor, trade, communication, and eventually agriculture and civilization.
We don’t really know when language evolved, since the process left behind neither bones nor artifacts, but if it happened suddenly (rather than gradually) and within the past 300,000 years or so, I would mark this as the moment Homo sapiens evolved.
While many animals can understand a fair amount of language (dogs, for instance) and some can even speak (parrots,) the full linguistic range of even the most intelligent apes and parrots is still only comparable to a human toddler. The difference between human language abilities and all other animals is stark.
There is great physical variation in modern humans, from Pygmies to Danes, yet we can all talk–even deaf people who have never been taught sign language seek to communicate and invent their own sign language more complex and extensive than that of the most highly trained chimps. Yet if I encountered a group of “humans” that looked just like some of us but fundamentally could not talk, could not communicate or understand language any more than Kanzi the Bonobo, I could not count them members of my species. Language is fundamental.
But just because we can all speak, that does not mean we are all identical in other mental ways–as you well know if you have ever encountered someone who is inexplicably wrong about EVERYTHING here on the internet.
But back to the book:
We intend to make the case that human evolution has accelerated int he past 10,000 years, rather than slowing or stopping, and is now happening about 100 times faster than its long term average over the 6 million years of our existence.
A tall order!
To summarize Cochran and Harpending’s argument: Evolution is static when a species has already achieved a locally-optimal fit with its environment, and the environment is fairly static.
Human environments, however, have not been static for the past 70,000 years or so–they have changed radically. Humans moved from the equator to the polar circle, scattered across deserts and Polynesian islands, adapting to changes in light, temperature, disease, and food along the way.
The authors make a fascinating observation about hunting strategies and body types:
…when humans hunted big game 100,000 years ago, they relied on close-in attacks with thrusting spears. Such attacks were highly dangerous and physically taxing, so in those days, hunters had to be heavily muscled and have thick bones. That kind of body had its disadvantages–if nothing else, it required more food–but on the whole, it was the best solution in that situation. … but new weapons like the atlatl (a spearthrower) and the bow effectively stored muscle-generated energy, which meant that hunters could kill big game without big biceps and robust skeletons. Once that happened, lightly built people, who were better runners and did not need as much food, became competitively superior. The Bushmen of southern Africa…are a small, tough, lean people, less than five feet tall. It seems likely that the tools made the man–the bow begat the Bushmen.
Cro-magnons (now called “European Early Modern Humans” by people who can’t stand a good name,) were of course quite robust, much more so than the gracile Bushmen (Aka San.) Cro-magnons were not unique in their robustness–in fact all of our early human ancestors seem to have been fairly robust, including the species we descended from, such as Homo heidelbergensis and Homo ergaster. (The debate surrounding where the exact lines between human species should be drawn is long and there are no definite answers because we don’t have enough bones.)
We moderns–all of us, not just the Bushmen–significantly less robust than our ancestors. Quoting from a review of Manthropology: The Science of the Inadequate Modern Male:
Twenty thousand years ago six male Australian Aborigines chasing prey left footprints in a muddy lake shore that became fossilized. Analysis of the footprints shows one of them was running at 37 kph (23 mph), only 5 kph slower than Usain Bolt was traveling at when he ran the 100 meters in world record time of 9.69 seconds in Beijing last year. But Bolt had been the recipient of modern training, and had the benefits of spiked running shoes and a rubberized track, whereas the Aboriginal man was running barefoot in soft mud. …
McAllister also presents as evidence of his thesis photographs taken by a German anthropologist early in the twentieth century. The photographs showed Tutsi initiation ceremonies in which young men had to jump their own height in order to be accepted as men. Some of them jumped as high as 2.52 meters, which is higher than the current world record of 2.45 meters. …
Other examples in the book are rowers of the massive trireme warships in ancient Athens who far exceeded the capabilities of modern rowers, Roman soldiers who completed the equivalent of one and a half marathons a day, carrying equipment weighing half their body weight …
McAllister attributes the decline to the more sedentary lifestyle humans have lived since the industrial revolution, which has made modern people less robust than before since machines do so much of the work. …
According to McAllister humans have lost 40 percent of the shafts of the long bones because they are no longer subjected to the kind of muscular loads that were normal before the industrial revolution. Even our elite athletes are not exposed to anywhere near the challenges and loads that were part of everyday life for pre-industrial people.
Long story short: humans are still evolving. We are not static; our bodies do not look like they did 100,000 years ago, 50,000 years ago, nor even 1,000 years ago. The idea that humans could not have undergone significant evolution in 50–100,000 years is simply wrong–dogs evolved from wolves in a shorter time.
Dogs are an interesting case, for despite their wide variety of physical forms, from Chihuahuas to Great Danes, from pugs to huskies, we class them all as dogs because they all behave as dogs. Dogs can interbreed with with wolves and coyotes (and wolves and coyotes with each other,) and huskies look much more like wolves than like beagles, but they still behave like dogs.
The typical border collie can learn a new command after 5 repetitions and responds correctly 95% of the time, whereas a basset hound takes 80-100 repetitions to achieve a 25 percent accuracy rate.
I understand why border collies are smart, but why are bassets so stupid?
Henry and Greg’s main argument depends on two basic facts: First, the speed of evolution–does evolution work fast enough to have caused any significant changes in human populations since we left Africa?
How fast evolution works depends on the pressure, of course. If everyone over 5 feet tall died tomorrow, the next generation of humans would be much shorter than the current one–and so would their children.
The end of the Ice Age also brought about a global rise in sea level. … As the waters rose, some mountains became islands.. These islands were too small to sustain populations of large predators, and in their absence the payoff for being huge disappeared. … Over a mere 5,000 years, elephants shrank dramatically, from an original height of 12 feet to as little as 3 feet. It is worth noting that elephant generations are roughly twenty years long, similar to those of humans.
We have, in fact, many cases of evolution happening over a relatively short period, from dogs to corn to human skin tone.
No one is arguing about the evolution of something major, like a new limb or an extra spleen–just the sorts of small changes to the genome that can have big effects, like the minor genetic differences that spell the difference between a wolf and a poodle.
Second, human populations need to be sufficiently distinct–that is, isolated–for traits to be meaningfully different in different places. Of course, we can see that people look different in different places. This alone is enough to prove the point–people in Japan have been sufficiently isolated from people in Iceland that genetic changes affecting appearance haven’t spread from one population to the other.
What about the claim that “There’s more variation within races than between them”?
This is an interesting, non-intuitive claim. It is true–but it is also true for humans and chimps, dogs and wolves. That is, there is more variation within humans than between humans and chimps–a clue that this factoid may not be very meaningful.
Let’s let the authors explain:
Approximately 85 percent of human genetic variation is within-group rather than between groups, while 15 percent is between groups. … genetic variation is distributed in a similar way in dogs: 70 percent of genetic variation is within-breed, while 30 percent is between-breed. …
Information about the distribution of genetic variation tells you essentially nothing about the size or significance of trait differences. The actual differences we observe in height, weight, strength, speed, skin color, and so on are real: it is not possible to argue them away. …
It turns out that the correlations between these genetic differences matter. … consider malaria resistance in northern Europeans and central Africans. Someone from Nigeria may ave the sickle-cell mutation (a known defense against falciparum malaria,) while hardly anyone from northern Europe does, but even the majority of Nigerians who don’t carry the sickle cell are far more resistant to malaria than any Swede. They have malaria-defense versions of many genes. That is the typical pattern you get from natural selection–correlated changes in a population, change in the same general direction, all a response to the same selection pressure.
In other words: suppose a population splits and goes in two different directions. Population A encounters no malaria, and so develops no malria-resistant genes. Population B encounters malaria and quickly develops a hundred different mutations that all resist malaria. If some members of Population B have the at least some of the null variations found in Population A, then there’s very little variation between Pop A and B–all of Pop A’s variants are in fact found in Pop B. Meanwhile, there’s a great deal of variation within Pop B, which has developed 100 different ways to resist malaria. Yet the genetic differences between those populations is very important, especially if you’re in an area with malaria.
What if the differences between groups is just genetic drift?
Most or all of the alleles that are responsible for obvious differences in appearance between populations–such as the gene variants causing light skin color or blue eyes–have undergone strong selection. In these cases, a “big effect” on fitness means anything from a 2 or 3 percent increase on up. Judging from the rate at which new alleles have increased in frequency, this must be the case for genes that determine skin color (SLC24A5), eye color (HERC2), lactose tolerance (LCT), and dry earwax (ABCC11), of all things.
In fact, modern phenotypes are surprisingly young–blond hair, white skin, and blue eyes all evolved around a mere 10,000 years ago–maybe less. For these traits to have spread as far as they have, so quickly, they either confer some important evolutionary benefit or happen to occur in people who have some other evolutionarily useful trait, like lactose tolerance:
Lactose-tolerant Europeans carry a particular mutation that is only a few thousand years old, and so those Europeans also carry much of the original haplotype. In fact, the shared haplotype around that mutation is over 1 million bases long.
Recent studies have found hundreds of cases of long haplotypeles indicating recent selection: some have reached 100 percent frequency, more have intermediate frequencies, and most are regional. Many are very recent: The rate of origination peaks at around 5,500 years ago in the European and Chinese samples, and at about 8,500 years ago in the African sample.
(Note that the map of blue eyes and the map of lactose tolerance do not exactly correlate–the Baltic is a blue eyes hotspot, but not particularly a lactose hotspot–perhaps because hunter-gatherers hung on longer here by exploiting rich fishing spots.)
Could these explosions at a particular date be the genetic signatures of large conquering events? 5,5000 years ago is about right for the Indo-European expansion (perhaps some similar expansion happened in the East at the same time.) 8,000 years ago seems too early to have contributed to the Bantu Expansion–did someone else conquer west Africa around 8,500 years ago?
Let’s finish up:
Since we have sequenced the chimpanzee genome, we know the size of the genetic difference between chimps and humans. Since we also have decent estimates of the length of time since the two species split, we know the long-term rate of genetic change. The rate of change over the past few thousand years is far greater than this long-term rate over the past few million years, on the order of 100 times greater. …
The ultimate cause of this accelerated evolution was the set of genetic changes that led to an increased ability to innovate. …
Every major innovation led to new selective pressures, which led to more evolutionary change, and the most spectacular of those innovations was the development of agriculture.
Innovation itself has increased dramatically. The Stone Age lasted roughly 3.4 million years (you’ll probably note that this is longer than Homo sapiens has been around.) The most primitive stone tradition, the Oldowan, lasted for nearly 3 million of those 3.4; the next period, the Acheulean, lasted for about 1.5 million years. (There is some overlap in tool traditions.) By contrast, the age of metals–bronze, copper, iron, etc–has been going on for a measly 5,500 years, modern industrial society is only a couple of centuries old–at most.
What triggered this shift from 3 million years of shitty stone tools with nary an innovation in sight to a society that split the atom and put a man on the moon? And once culture was in place, what traits did it select–and what traits are we selecting for right now?
Is the singularity yet to come, or did we hit it 10,000 years ago–or before?
By the way, if you haven’t started the book yet, I encourage you to go ahead–you’ve plenty of time before next week to catch up.