When did human evolution stop?

kw: book reviews, nonfiction, paleoanthropology, archaeology, historical linguistics, primatology, social anthropology, evolutionary psychology, population genetics, human origins, natural selection

The simple answer to this post's title: It hasn't. Among those who accept evolution of the human animal, it is common to think that we've reached some kind of pinnacle, and no evolutionary change has taken place in the last ten or twenty thousand—or perhaps 100,000—years (For those who don't, of course no evolution has ever taken place). The book Before the Dawn: Recovering the Lost History of Our Ancestors by Nicholas Wade presents a synthesis of what is known and strongly inferred in the disciplines related to human origins. The picture is full of surprises.

Item: When did people begin to wear clothing? Fabrics are so rarely retained in "fossil" deposits, we can't begin to guess. Body lice provide the answer (yuck!). Head lice, equivalent to head-and-body lice in apes, live in the hair. Body lice live only in clothing, and move to the skin to feed, but lay eggs in clothing, because there is too little hair on most human bodies to reliably protect the nits. So when did body lice evolve? Genetic dating from modern head and body louse populations provides the surprising answer: about 70,000 years ago. Such dates currently suffer from large uncertainties, so the actual development of the body louse, shortly (a few thousand years) after clothing became common, occurred before 40,000 years ago, and most likely 80,000 or later.

Item: When did proto-humans lose their body hair? Both chimps and bonobos (once called "pygmy chimps") both have fair skin under that black hair. Only the face, hands and feet are dark, because of a permanent tan. Dark skin over the body probably arose as hair was lost, to protect the skin from the sun. "Black" Africans can be sunburned, but it takes a lot more sun exposure compared to lighter-skinned people. When did darker skin over the whole body evolve? Genetic dating again provides an answer, this time from the genes that produce skin pigments: about 1,200,000 years ago.

Strangely, I looked for information on the "first use of fire", but Wade doesn't mention it. A BBC News article of 22 March 2004 states that early African humans, possibly Homo erectus, seem to have had hearths as early as 1.5 million years ago. Another article, dated 29 April 2004, about H. erectus in Israel, has them using fire in sophisticated ways 790,000 years ago. It seems to me that nakedness arose primarily as a result of fire use. Other explanations, such as sexual selection, seem less compelling.

What is interesting, though, is the million or more years that passed between the loss of hair and the development of clothing. Production of textiles must require a change in thinking that didn't occur earlier. The Acheulian "toolkit" was used by at least three species of Homo from 1.7 million years ago until it was supplanted 250,000 years ago by Mousterian (H. Neanderthalensis in Europe) and Middle Paleolithic (H. Ergaster in Africa) technologies. These related technologies show two things: that there was contact between African and European species of Homo; and that one had attained sufficient brain power to greatly improve their use of stone tools, which the other quickly adopted (it takes less wit to copy than to invent). So, apparently, clothing other than cured skins was an invention later yet.

I've known the term "anatomically modern human" (AMH) for decades. I confess that I didn't think about it much. But later, the term "behaviorally modern human" (BMH) gave me pause. Is there a difference? Sure is, by tens of thousands of years. It seems AMH's, which arose in Africa 100,000 years ago, could pass for human today, as long as they didn't do anything. But they didn't live like BMH's, who lived like the hunter-gatherer groups of today, such as certain "primitive" tribes in South America or New Guinea. Evidence of BMH living appears just after 50,000 years ago in Africa, and about 45,000 years ago in Europe.

AMH living was by foraging and minor hunting. Family groups stayed together, in bands of related families that scattered and coalesced on a daily basis, numbering 50 or fewer. Few activities other than food gathering and preparation, procreation, and sleeping, occurred.

BMH living included much more hunting, of larger game, and specialized plant gathering. A great increase in the use of ornaments and artful trinkets arose. The stone toolkit, called Upper Paleolithic, was much more complex, with specific tools for specific uses, and complex tools made of bone, antler, and ivory. Musical instruments, such as bone flutes, are found for the first time in BMH leavings. The people ritualized burial. And they traded among groups, much more than AMH's, to obtain materials and products found far from their home ranges.

Add one item: About 50,000 years ago, BMH's from the "ancestral human population" (AHP) in Africa migrated beyond Africa, apparently for the first time since 1.8 million years ago, and established themselves eastward and northwestward to the limits of the Eurasian continent in a few thousand years. They seem to have supplanted and replaced Neandertals in Europe and "archaic Homo" people in Asia, in less than ten thousand years. During this time, some reached Australia, something the archaics didn't do.

When I saw all these together, I was convinced for the first time that the "multiregional" hypothesis, which requires persistent gene flow among the African, European, and Asian species of Homo, must be wrong. What was it that kept Africans from continuing to migrate to Eurasia, at least successfully, between 1.8 million and 50 thousand years ago? The most likely answer is that the land was occupied by Neandertals and archaics, who either drove out or destroyed invaders. In the same way, the AHP in Africa filled at least the Northeast part of that continent and kept the other species from invading.

Once the AHP developed BMH living, they could make better use of resources, had better weapons, and could stay together in larger groups, compared to the other species, and could successfully invade Eurasia. However, even with these advantages, it wasn't a simple walkover. The number of BMH's who became the ancestors of all non-African people was less than 500, and possibly as few as 150.

OK, so what happened since 50,000 years ago? Are we BMH's who have somehow adapted to settled life? Are today's remaining hunter-gatherer (H/G) groups BMH's? Not really. Although BMH's were anatomically very nearly the same as AMH's, there are differences between today's people, both in Africa and Eurasia, and the people of the African AHP. If you were to take a range of modern people, of all races, of various sizes and ages, and pair them up with AHP folks of similar size and age, you'd find the earlier people had thicker skulls and heavier bones at all ages. They were not as heavily built as Neandertals, but were definitely more like American Football players than like Soccer (European Football) players. The AHP's also stayed in smaller groups than members of modern H/G groups do.

The main reason is inter-group combat. There has been a strong tendency among anthropologists to understate the level of aggressiveness and prevalence of warfare among "innocent savages." An unbiased look at the record reveals that the greatest single danger to humans through all ages was and is other humans. Xenophobia is the natural state of the H/G mind, the BMH mind, and the AMH mind, as it was for all earlier species of Homo.

It is now known that groups of chimps engage in wars of extermination against neighboring groups, and that the leading cause of death among males is "chimpicide". Bonobos, with a female-dominated society, do not engage in such warfare...a good reason for turning over all politics to women!

The greatest single cause of both male and female death in most H/G groups is warfare and killing raids between groups. So, while trade arose at least 50,000 years ago, it probably occurred primarily among groups that couldn't exterminate one another, just as it does today among all the mutually warring tribes in New Guinea and South America, for example. "You aren't my brother or cousin, but unless I judge I can destroy you, I'll call you friend...for a while."

My father's cousin, while a missionary in New Guinea in the 1950s, saw frequent warfare and some cannibalism. Two groups of men might trade one day, and slaughter one another the next. A smaller village might have all its men killed, whereupon all their children would be killed, plus most of the women, and the rest taken as concubines.

Let's look at a few numbers. About a third of adult male chimps die in combat. The number is the same among the Yanomamo, a Brazilian tribe of H/G's. One study showed that a typical tribal society lost, on average, 0.5% of its population yearly to warfare (that's ~1% if the men and a smaller proportion of women; and among men, that's a 26% loss per thirty years). However, averages tend to hide incidents. Most inter-tribe warfare is carried out by small battles and raids a few times per year, or perhaps monthly. Pitched battles are much less frequent, but cause huge carnage: A 30% loss on both sides is typical. Modern societies prior to 1900 AD could produce similar death tolls: At the battle of Gettysburg, the Union side lost 21%, and the Confederates lost 30%.

However, the two World Wars of the 20th Century resulted in losses of less than 10% among the soldiery, and a percent or less of the engaged nations' total citizenry. Stalin's purges of the 1940s and '50s were actually worse than either World War in terms of pure carnage...they were wars of another sort. But in 2002, though a number of wars were in progress around the globe, including America's war in Afghanistan, 0.3% of deaths were due to warfare.

But these latter figures are comparing recent wars among settled societies, with wars among H/G societies, BMH societies...and apes! They are probably not entirely fair. Wade doesn't go into this, but by my own study, I find the following: History is full of war stories, from Troy to Sparta to Carthage and so on. But it seems the total toll of warfare among settled societies is typically about half that among H/G societies.

I wonder what is cause and what is effect? Did settlement and agriculture result in a preference for negotiation? Or did the development of more negotiation-prone people—perhaps first among those who profited most from trading—allow settlement to occur?

I'd say it is a coevolutionary change. Our skeletons reflect the fact that we have less reason to fear being bonked on the head some random night, compared to the AHP, and less need for face-to-face combat. Although today's H/G members live very similarly to the AHP, a Yanomamo or Dani person can be socialized to live in a city. I suspect a person brought in a time machine from 40,000 years ago would be unsocializable, or only partly so.

The brain has changed since that time. So far, two genetic changes that determine brain size, and probably size in specific areas, have been found and genetically dated. One occurred 37,000 years ago, the other 6,000 years ago. The people of 50,000 years ago were, from archaeological evidence, able to live in larger groups than earlier peoples. They were able to extend the idea of "kin" to a larger group than before. But each change to the brain seems to have increased the number of "others" a person could tolerate.

By 15,000 years ago, when settlement began, it seems people could tolerate even greater numbers, and additionally, accept direction from leaders. Both are required for a few hundred people to live in continuous close proximity. It seems strange that these first settlements were not primarily agricultural. They seem instead to have been "places to be" between hunting and collecting expeditions, and trade centers. Specialized skills proliferated; the best hunters got their spear and arrow points from the best stoneworkers, as the best clothing makers got needles and other tools from bone workers...and in turn traded clothing to those less skilled.

The later development of agriculture led to much larger settlements, about the time of the 6,000-year-ago bump in brain size. A larger brain adapts one to the need to remember a larger number of people. It was not much later than this that lactose tolerance evolved among those who kept herds of large mammals. To this day, lactose tolerance or intolerance indicates whether your ancestors were farmers or ranchers.

Now think, today an office worker can step out of an apartment and essentially vanish into a city of millions of strangers, and return safely hours later! Rather extraordinary, wouldn't you say?

What other genetic changes have arisen recently? Malaria resistance among those living in subtropical areas has evolved at least twice since 10,000 years ago (one such change causes sickle-cell disease). Our sense of smell is much reduced compared to just 15,000 years ago, because most of the olfactory gene set has become inactive. Whether this is because of the stench of early cities, of because of less need to smell out game, I forbear to speculate!

The changes to brain and body that occurred prior to 50,000 years ago are found in all humans. Those that arose later are found in larger or smaller numbers depending on where a mutation arose and how strong a benefit it ensured.

This leads us to the most visible differences among human populations, those that correlate best with the continent one's ancestors inhabited: we call them races. While skin color is often considered to be the main indicator of race, it is not really the best. The Caucasians in southern India are darker than some Africans; some Japanese (and I don't mean Ainu, which are very pale) are paler than most Caucasians; many Australians (aboriginals not Austro-Brits) are darker than most Africans or Indians. If you take almost anyone, and paint their skin white (or any solid color of your choice), most people can tell at a glance what continental type they typify, and often the subtype (pygmies and Inuit are quite distinctive, for example).

Regardless how extensive "racial" differences are, it seems they arose no more than 12,000 years ago. They reflect, as do many other indicators, that most people in history seldom moved far from the place they were born. The subtypes within continental groups emphasize this. So, rather than the "not politically correct" attitude toward any mention of racial genetics, it would be better to look upon the diversity of human types as a library of the history of humanity.

One thing is sure: We continue to evolve. We are more gracile than the AHP. Gracility may continue, but it has probably already reached it logical limit among East Asians. Even though Chinese fed a "Western" diet grow taller than their parents, they remain on average much slenderer than Europeans. The continually increasing complexity of modern life will, unless civilization collapses, lead to people with larger brains and more multitasking skills, first behaviorally, and over a few generations, genetically. We won't become the bare brains of science fiction, but we may become smaller, thinner, quicker, and less aggressive in the future. Should Homo sapiens survive another 50,000 years, no doubt our descendants will look back at us as quizzically as we look back at the people in Africa, 50,000 years ago, some of whom managed to escape that continent to colonize the rest of the planet.