by montrose on 6/1/18, 6:29 AM with 80 comments
by Mbioguy on 6/1/18, 6:54 PM
Here's the original article that caused such a stir in 2015. Figure 2 shows the sudden drop in the reproducing Y-population globally (meaning it cannot be explained by genes or migration).
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4381518/
The paper cited in the article alters the date of the event, but really there's a lot of uncertainty remaining.
https://www.nature.com/articles/s41467-018-04375-6
The best current hypothesis to explain this drop (and that no similar one occurred for the reproducing X-population) is the conflict between predominantly agricultural societies versus predominantly hunter-gatherer societies. Until sufficient evidence has been found to rule out this or alternatives, take any explanation with a grain of salt.
Look at Figure 2, and you'll notice the Y-axis are different. Between 50-10kya, the effective reproductive population was 3-4 times larger for women than men, globally. This fits with modern anthropological evidence of polygyny in early hunter-gatherer cultures (loose polygyny with on average 3-4 wives per successful male over a lifetime, but with limited ability to enforce fidelity). Y chromosome diversity tends to accumulate, albeit at a lower rate than the X.
An agricultural community is likely to be much more homogenous in terms of Y-chromosomes, than a hunter-gatherer one. Power is much more effectively concentrated in these communities, allowing leaders to amass more wives and enforce fidelity much more strictly than in hunter-gatherer societies. Stories of King Solomon's wives, or Sultan Moulay Ismail of Morocco (who reportedly sired hundreds of children) are an easy way to visualize this.
While man-for-man, a hunter-gatherer may be healthier and stronger, a hunter-gatherer society may find themselves vastly outnumbered by an agricultural community. Over time, hunter-gatherers would find themselves pushed off of prime land onto marginal land. The newer article mentions a founder effect. Where are these Neolithic pioneers coming from and where are they going to? From agricultural communities, expanding into territory previously held by hunter-gatherers. While certainly many deaths occurred due to combat, Y-chromosomal diversity loss also would have occurred to disease and famine. The agricultural population would continue to rise, while the hunter-gatherers would struggle to maintain on more marginal land. History is replete with stories of taking women, so if this scenario is the best explanation, it is unsurprising that there was not a corresponding drop in X-diversity.
This sort of scenario occurred globally. Agriculture independently arose in many places: the near-east, sub-saharan Africa, China, Mexico, the Andes, and possibly others. We've seen what happened to the Americas after Columbus. Similar mechanisms help explain the population-level Y-cide on smaller scales that probably occurred during each of the agricultural expansions above.
This hypothesis, while probably the most widely-accepted at present, is challenged by some of the evidence in the newer paper. It will be interesting to see how it falls out once the original authors have a chance to respond or additional voices join the conversation.
by nostrademons on 6/1/18, 7:30 PM
Assume that humans are living in patrilineal clans of roughly 20 males and an equivalent number of females. All males are genetic descendants of the clan patriarch and share the same Y-chromosome markers. All females are born outside the clan and marry into it.
Now assume that 95% of clans are wiped out through a couple millenia of warfare. That Y-chromosome is now extinct; all male-line descendants of the patriarch are dead. However, genetic markers carried by the female are not extinct, because the 20 daughters born into the clan have married into 20 different clans, and at least one of them has survived.
Note that the population doesn't actually have to drop in this scenario! 95% extinction of clans over 2000 years implies only 0.15% extinction annually, assuming an exponential decay. If warfare is continuous and resources go to the victor, then one clan is exterminated, but the victorious clan quickly doubles in size as it takes the dead clan's resources (and oftentimes, womenfolk). Total population remains roughly constant, but all living descendants come from a tiny percentage of male ancestors.
Other articles about this study have made this distinction explicitly (or at least hinted about it), but it's totally missing from the headline.
by jdfellow on 6/1/18, 5:51 PM
The neolithic period was one of agriculture and early civilization. Stonehenge was built by neolithic peoples, and Egypt, Mesopotamia and China were embryonic civilizations in the neolithic era as well. Cavemen, these were not.
by mabbo on 6/1/18, 5:59 PM
Maybe the reason civilization is even possible today is that the majority of our potential ancestors who couldn't deal with civilized life wiped each other out. Or alternatively, the males who survived were really good at warfare and surviving warfare.
by jbattle on 6/1/18, 7:54 PM
What kind of World War Zero could possibly explain this? I really can't picture how a state of sustained (for thousands of years?) high-intensity warfare over an area spanning three entire continents could have worked. How many historical instances are there of a population decreasing to 1/20th of previous levels? The 20th century had a couple of instances but that required totalitarianism and modern communication, logistics, and industrial capabilities.
The other historical instance I know of is the decimation of New World populations after contact with Europe (through disease). I don't know what other evidence they've assembled, but disease feels like a much better way to explain this population drop than warfare.
But what diseases only target males? Dunno. Maybe the early domestication of livestock introduced some kind of chickenpox or dogpox that killed (or sterilized!) males dramatically more frequently than females.
by varjag on 6/1/18, 5:14 PM
by dkoubsky on 6/1/18, 5:16 PM
by pvaldes on 6/2/18, 9:15 PM
by olivermarks on 6/1/18, 6:11 PM
Article 'A SUDDEN and dramatic drop in the number of human males living in Europe, Africa, and Asia 7,000 years ago is evidence of brutal warfare spanning multiple generations, a new study has suggested.'
Theory is fact in click bait tabloid headlines...
by galieos_ghost on 6/1/18, 5:37 PM
by z3t4 on 6/1/18, 9:13 PM
by kristaps on 6/1/18, 8:26 PM
by rjsw on 6/1/18, 8:29 PM