Monthly Archives: July 2019

African Exodoi

123-117 thousand years ago

Our genus, Homo, left Africa by 1.8 million years ago. On the latest genetic evidence, there was another Out Of Africa migration about 700,000 years ago, which led to the ancestors of Neanderthals and Denisovans splitting off from the ancestors of Homo sapiens (who stayed in Africa this time), and replacing Homo erectus in Eurasia. And there seems to have been another Out Of Africa migration around 200,000 years ago that introduced early Homo sapiens mitochondrial DNA into the Neanderthal gene pool. This early migration didn’t lead to major evolutionary change, however. Other Neanderthal genes (i.e. nuclear rather than mitochondrial DNA) show no evidence of it.

More consequential was another Out Of Africa migration by Homo sapiens around 120,000 years ago. This skull dates back to then.
skhul5
Skhul 5 was found back in 1935, in Israel, on the slopes of Mount Carmel, buried together with a boar mandible. It looked for a long time like the skull came late in the day, and might represent a transition from Neanderthals to Homo sapiens. But now that date has been moved back, thanks to the development of new dating techniques (thermoluminescence, electron spin resonance) that finally broke the 40,000 year limit for Carbon 14. Skhul 5 now looks like a representative of an early movement of Homo sapiens out of Africa. But the skull also has some Neandethal-like features (check out the “Neanderthal bun” at the back of the skull) and could have hybrid ancestry.

Until recently it looked like this and other very early Homo sapiens outside Africa were a side branch that left no descendants – think Leif Erickson, not Columbus – with the real move coming later. But recently there has been a recalibration of DNA mutation rates that suggests that the split between African and non-African branches of H. sapiens happened closer to 100,000 years ago than 50,000 years ago. And there have been discoveries of stone tools with African affinities in the Arabian Peninsula, in the United Arab Emirates (Jebel Faya, 125 kya) and Oman (106 kya). It may be that when Homo sapiens left Africa 125,000 years ago (perhaps across the Red Sea to Arabia, rather than across the Sinai), they spent a long time isolated in a corner of Southwest Asia before much later expanding more widely.

It’s even possible that more dramatic revisions of the Out Of Africa story will be called for in the future. It could be that the finishing touches in the evolution of modern Homo sapiens happened in North Africa or Southwest Asia, with the genes involved spreading to Sub-Saharan Africa by back migration.

Putting’ on the Ritz

173-164 thousand years ago.

It’s fancy dress day on Logarithmic History. Between 170 and 80 thousand years ago, people started wearing clothes. We know this from recent genetic studies showing that sometime during that period, probably closer to 170 kya, pubic lice and body (=clothing) lice diverged into two separate species. This is before the major exodus of modern humans from Africa, so it may mean clothing was not just about protection from high latitude winters. Or else Neanderthals or Denisovans were wearing clothes, and we got body lice from them. (There’s an even earlier split, three million years ago, between head lice and pubic lice, that probably means human ancestors had lost their fur. And there’s another story about the ancestors of East Asians picking up a different strain of head lice from non-sapiens humans that we may cover later.)

Lice are not just disgusting, but dangerous. On later dates, we’ll have occasion to see how louse-borne diseases like typhus have affected the course of history. But for now let’s forget about lice, and celebrate clothing, with Irving Berlin’s song about fancy duds, “Puttin’ On the Ritz,” covered below by a Moscow flashmob.

Also relevant to the video is Merlin Donald’s book Origins of the Modern Mind: Three Stages in the Evolution of Culture and Cognition arguing that dance is part of a mimetic mode of culture that came before language. And Barbara Ehrenreich’s book on dancing in history and prehistory Dancing in the Streets: A History of Collective Joy. is worth a look.

The Goodness Paradox

194 – 184 thousand years ago.

We’re now taking (pre)history ten thousand years at a time.

Earth Abides is an early (1949) entry in the genre of post-apocalyptic science fiction, with some haunting reflections on what it takes to keep a civilization going, or just a human community. In this case the apocalypse takes the form of a lethal infectious disease that wipes out well over 99% of Earth’s human population, leaving scattered survivors to try and put things back together.

The drama is low-key. If you want to read about the remnants of civilized humanity defending themselves against zombies, or venomous man-eating walking plants, or a horde of cannibal anti-nuke zealots, you’ll have to look elsewhere. The threat to civilization in Earth Abides is more subtle. The generation born after the die-off has no understanding of what they have lost, of what the collapsing factories and powerlines and machines around them really were, or how they worked, or how to get them back up and running. The older generation, without the institutions of a complex society backing them up, can’t supply enough discipline and punishment to pass on the arts of civilization. The young will grow up as illiterate scavenger-foragers, skilled with bow and arrow, well-adapted in their own way to a rewilding Earth. Ish, the protagonist, will end his days as the Last American (a fictional counterpoint to the real-life Ishi, the last Yahi Indian).

The community is nonetheless capable of reacting decisively when their survival is threatened.

One day a newcomer enters the scene. Charlie is talkative, forceful, charismatic. The kids adore him. It looks like he might even take over as leader of the little group. But it becomes clear that there is something off about him, even sociopathic. He can turn on the charm, but when thwarted he is menacing. He always carries a gun, but keeps it hidden. He is sexually abandoned. And, in a world without antibiotics, he is infected with a slew of venereal diseases.

Something has to be done about Charlie, and the elders of the group meet to decide what. Their options are limited. Keeping him locked up is not a practical possibility. They could banish him. But who is to say he won’t find a gun, and come back, looking for revenge? They could execute him. But what actual harm has he done, so far? They decide to settle the matter with a vote.

Em located four pencils. Ish tore a sheet of paper into four small ballots.

This we do, not hastily; this we do, not in passion; this we do, without hatred. …

This is the one who killed his fellow unprovoked; this is the one who stole the child away; this is the one who spat upon the image of our God; this is the one who leagued himself with the Devil to be a witch; this is the one who corrupted our youth; this is the one who told the enemy of our secret places.

We are afraid but we do not talk of fear. … We say, “Justice”; we say, “The Law”; we say “We, the people”; we say, “The State.”

Ish sat with his pencil poised … He could not be sure. Yet, at the same time, he knew that The Tribe faced something real and dangerous and even dreadful, in the long run threatening its very existence. … In that final realization, he knew that he could write only the one word there, out of love and responsibility for his children and grandchildren. …

“Give me your slips,” he said.

They passed them in, and he laid them face up before him on the desk. Four times he looked, and he read: “Death … death … death … death.”

Keith Otterbein is an anthropologist who did a study of capital punishment across cultures. He expected to find that capital punishment is limited to complex societies, used to enforce social hierarchy. Instead he found that capital punishment is a universal, present in societies from the simplest to the most complex. It is an option that even the smallest, most easy-going communities – like The Tribe of Earth Abides – may find themselves resorting to. We can infer that for tens or even hundreds of thousands of years, our ancestors have been carrying out group-sanctioned executions of individuals deemed anti-social and a threat to group harmony and survival. This is long enough to have had evolutionary consequences. Long before human beings domesticated the wolf, the wild sheep and goat, the aurochs, we may have been domesticating ourselves, weeding out the wildest and most dangerous from our midst, replacing the old tyranny of the alpha male with the tyranny of Custom and The Law.

The idea that human beings are in some ways like domesticated animals is an old one. It has recently returned to the spotlight. The extraordinary long-term experiment in artificial selection for tameness in foxes carried out by Nikolai Belyaev, Lyudmila Trut, and their coworkers in Russia has demonstrated that selection for tameness ends up selecting for a whole suite of anatomical characteristics as byproducts. Strikingly, many of the features that differentiate Homo sapiens hundreds of thousands of years ago from Homo sapiens today are also features that distinguish tame from wild foxes, and dogs from wolves.

Richard Wrangham’s recent book, The Goodness Paradox: The Strange Relationship Between Virtue and Violence in Human Evolution connects the dots, setting out one long argument that an evolutionary history of capital punishment has reduced our disposition for reactive aggression (the hot-blooded, spur-of-the-moment, volatile, antisocial kind), while leaving intact our capacity for calculated, cold-blooded, proactive killing (“not hastily, … not in passion, … without hatred”).

Perhaps this explains a very recent finding in human evolution. Apparently 200,000 years ago, we were about evenly matched with Neanderthals, sometimes replacing them, sometimes being replaced by them. By 40,000 years ago however, Neanderthals lose out decisively to modern humans. It may be that what changed in the interim to give us the edge is that we improved our ability to get along peaceably with insiders (including distant insiders we don’t know personally) without losing our ability to apply lethal aggression to outsiders.

Learn This One Weird Trick (Part Two)

… that humans use, and now you can too! (Continued from the previous post.)

recursion 3) Recursion. What if you have one mirror facing a second mirror, so the first mirror shows what’s in the second mirror, which shows what’s in the first mirror …? What if you take a chameleon, which tries to take on the color of its surroundings, and put it on a mirror? What if you point a video camera at the very screen that’s showing what the video camera is pointing at? What if (getting mathematical) you use a function in defining that same function? What if you use the cleaning attachment from your vacuum cleaner to suck dust off the vacuum cleaner itself? (Okay, the last one is a bit lame.) The basic idea in each of these cases is called recursion, which is a major concept in mathematics and computer science. Douglas Hofstadter’s Gödel, Escher, Bach is all about recursion. Some people think recursion – nesting ideas about ideas inside one another in a potentially infinite hierarchy, or (for syntax) phrases inside phrases — is central to human uniqueness. Noam Chomsky has lately been pushing a hard-core version of this argument. Here he is with Robert Berwick defending his view.

Related to the idea of recursion is the idea of “meta-representation”: not just having ideas about the world but having ideas about ideas, being able to put a box around a proposition, and then attaching a tag to it that says the equivalent of “This is true” or “This is false” or “This will be true later” or “Suppose this were true,” and then manipulating it accordingly. A nice little essay in “imagination,” elaborating this idea, is here from Simon Baron-Cohen, best known as an authority on autism.

4) Shared intentionality. Suppose you and I are friends with a couple, Fred and Wendy Smith. I tell you “I saw Wendy Smith kissing a man in the park yesterday.” Logically speaking, there’s nothing to say the man wasn’t Fred. But you’ll probably assume that I meant she was kissing someone other than Fred. Why? Well if the man had been Fred I could just as easily have said “I saw Wendy Smith kissing Fred in the park yesterday.” Since I didn’t say that, you assume I mean to convey the man wasn’t Fred. Note this only works if both of us try to pack as much relevant information into our sentences as possible and know the other person is doing the same. (If you think this sounds like recursion, you’re right.) Back in the 1950s, Paul Grice, a philosopher, worked out a lot of how we pack non-literal meanings into sentences. But the same principles are at work even when people are communicating non-linguistically. This leads to another theory of human uniqueness: human beings are uniquely good at developing shared intentions with one another: each party knows the other party is trying to communicate something, so they converge on the correct answer. People may have been doing this even before language evolved. Following up on this can quickly get you into game theory, where a central concept is “common knowledge”: not just “I know X” and “You know X,” but “I know X,” and “I know X is common knowledge to us,” and similarly for you. The cc option on your email generates common knowledge: if you see someone’s address there then they see yours, and you know they know you know, etc. Here’s a philosophical treatment.

scleraBut you can skip the philosophy if you want and move on to a telling little piece of anatomy that’s relevant here. In most mammals, including chimpanzees, the sclera (white of the eyes) is not visible. It’s hard to tell where a chimpanzee is looking, easy for a human. Human eyes make it easy to cooperate in sharing attention, a first step in developing shared intentions. If you know your card games, chimpanzees are playing poker, humans are playing bridge.

Our discussion of human uniqueness on Logarithmic History has been frustratingly short on specific dates. But human sclera are probably a fairly simple trait genetically, and we may soon enough discover the genes involved and even tell how long ago they mutated.

Learn This One Weird Trick … (Part One)

… that humans use, and now you can too!

There are people who think that human beings are nothing special. Sure (the argument goes) people have uniquely large brains. But all sorts of creatures have unique features. Elephants are the only animals with trunks. Tamarins and marmosets are the only primates that give birth to twins. Platypuses are the only venomous mammals. Spotted hyenas are the only mammals whose females sport pseudo-penises (through which they give birth!). And so on. If we could ask members of these species they’d claim that they’re the special ones.

But of course we can’t ask them, and in any case, this isn’t a very convincing argument. Human beings have an absolutely outsize impact on the Earth, and the advent of human beings looks like one of the major evolutionary transitions, comparable in importance to the origin of the eukaryotic cell or multicellular life. But even if we buy this, it still leaves open the question of whether there’s a key adaptation – a One Weird Trick – that accounts for the exceptional course of human evolution. Here are some candidates that being are being batted around these days:

1) The cognitive niche. The basic idea is at least as old as Aristotle, that human brings are defined by their capacity for Reason. A modern version of this is advocated by evolutionary psychologist John Tooby and cognitive scientist Steven Pinker. Pinker in particular has elaborated the argument that humans are uniquely adapted to acquire and share knowledge, by virtue of a suite of cognitive, social, and linguistic adaptations. We’ve already touched on several aspects of this: Human beings seem to have taken the capacity for thinking about physical space and retooled it for thinking about the abstract cognitive space of possession – a social relationship. (Other abstract cognitive spaces include kinship, time, and change-of-state.) And humans seem to harness the machinery for processing the sounds of interacting solid objects in creating major categories of phonemes. For a more complete exposition, here’s an academic article by Pinker, and a talk on youtube.

2) Culture. Rob Boyd and Pete Richerson, who’ve done a lot of mathematical modeling of cultural evolution, are skeptical about the “cognitive niche” argument. Too much culture, they argue, is things that have been learned by trial-and-error, and are passed on from one generation to the next without people understanding why they work. Boyd and Richerson appeal, as anthropologists have for generations, to the importance of culture. We mentioned earlier their argument that the frequency of climate change in the Ice Age was nicely calibrated to favor social learning rather than individual learning or instinct. Joseph Henrich provides a recent defense of the importance of culture. Contra Pinker, he thinks humans often don’t have a good cause-and-effect understanding of the things they do, but depend heavily on imitation and the accumulated wisdom of the elders. And see this post, for the importance of High Fidelity cultural transmission in the evolution of animal and human intelligence.

Coming up: Part Two. Recursion and Shared Intentionality

African geneses: Bushmen

271-257 thousand years ago

You’ve probably run into some version of the factoid that there is more genetic variation in Sub-Saharan Africa than in all the rest of the world. This assertion has to be handled with care. It doesn’t necessarily apply to genes that have been under strong diverging selection pressures on different continents. Consider skin pigmentation: there is not more variation inside Africa than outside it in skin color, or in genes for skin color. Obviously. Likewise for hair form. But it’s true for neutral genetic variation, which is most genetic variation.

The simplest way to account for the broad Africa/non-Africa distinction would be to assume a large homogenous founder population in Africa, with a smallish number of people leaving Africa and going through a genetic bottleneck, thereby reducing their genetic variation. But recently we’ve been learning that the African situation is more complicated. Specifically, there used to be a lot of genetic differentiation between different regions within Africa. Recent population movements have smoothed out some of that variation, but recent work on ancient DNA has been bringing this more variegated past to light.

A case in point: the latest data imply that the Bushmen of Southern Africa separated from other African populations (East African, West African) around 260,000 years ago (at least), long before the major Out Of Africa venture by modern humans. What’s more, the very latest data imply that the Bushmen have received outside genetic input pretty recently, in the last 1-2 thousand years. This admixture, 9-22% of the ancestry of modern Bushmen, is absent from a 2,000 year old skeleton from Ballito Bay, South Africa.

The intruding population were probably pastoralists whose livestock, and a fraction of their genes, ultimately derived from the Near East. Another fraction of their genes originated in the Sahel or East Africa. And they probably spoke a language in the Nilo-Saharan or Afro-Asiatic family. These language families pop up as a substrate in East Africa, although largely overlain by the later expansion of Bantu speakers.

One implication: Bushman groups like the !Kung have often been presented as models for our Pleistocene hunting and gathering ancestors. Yet the most recent findings imply that there has been substantial interaction, including gene flow, between Bushmen and non-hunter-gatherers for some time.

A relevant result from twentieth century anthropology: when Nancy Howell did her classic work on the demography of the Dobe !Kung Bushmen, she found that, when you look at female reproductive histories, the Dobe !Kung look like a growing population, but when you look at male reproductive histories, they look like a shrinking population. There’s no contradiction here: the women, but not the men, in the population were sometimes having children by outsiders, neighboring pastoralists. The pastoralists in question were Bantu, having arrived in the last few centuries, but the latest genetic data imply that something similar was going long before the Bantu showed up. Since it’s not clear what effect this subaltern sexual status might have had on Bushman social organization, the social life of historic Bushmen may not be a good model for hunter-gatherer life before agriculture.

Here’s the article on Ballito Bay Boy.

And a recent review article from Nature

African geneses

303 – 288 thousand years ago

Our picture of human evolution in Africa around 300 thousand years ago has changed dramatically in just the last few years.

brokenhillHere’s something we already knew. This skull was found at Broken Hill, Zambia, in 1921, He (yes, “he,” he’s probably male) is sometimes known as Rhodesian Man. He looks like he’s a step away from Homo erectus, but not quite Homo sapiens. He’s heavily built, with massive brow ridges. (He looks like he could pass the “pencil test” for erectus: you could rest a pencil on those ridges. Of course, seriously, this isn’t enough to define a species.)  But he’s got a flat face and relatively large brain. He could be significantly younger than 300,000 years ago.

jebel irhoudBut now Rhodesian Man is bracketed both geographically and evolutionarily by some new finds. From Jebel Irhoud, Morocco, around 315 thousand years ago, come these skulls, which are more unequivocally Homo sapiens. pushing the fossil record of our species back 100 thousand years. The skull is still archaic – elongated rather than globular like a modern human – but the face is now tucked under the skull, as it is with us. The brow ridges are not as pronounced as with Rhodesian man, although still heavy for modern Homo sapiens.

naledi.jpgAnd we now have dates for Homo naledi, from South Africa, of 335-236 thousand years ago. This recently discovered species had a tiny brain, and may have been adapted for climbing trees, but still makes it into genus Homo based on other features (teeth and jaws, lower skeleton). The initial guess from a lot of folks was that this was a very early member of our genus, somewhere around early Homo erectus or earlier. But instead, Homo naledi looks to have been around at the same time as early Homo sapiens.

In other words, Africa 300,000 years ago was home to an impressive variety of humans – archaic Homo sapiens in Morocco, near relations in Zambia, and barely-humans in South Africa.