The piggish origins of civilization
September 10, 2010 Leave a comment
Follow the pig
For researchers interested in tracing the path of human civilization from its birthplace in the Fertile Crescent to the rest of the world, they need only follow the path of the pig.
Until this research was reported, humans agreed that pigs had fallen under our magical domestication powers only twice about 9,000 years ago, once in what is called the Near East (Turkey), and a second time in what is called the Far East (China). Morphological and some genetic evidence seemed to point to these two events only. That led human geographers to conclude that humans must have toted domesticated pigs around from the Far or Near East to other parts of the world like Europe or Africa, rather than domesticating the wild boars they encountered in every new locale.
Occam’s Razor violation
As it turns out, those ideas—which enjoyed the support even of Charles Darwin—were wrong. And they provide a nice example of a violation Occam’s Razor, the rule that scientists should select the explanation that requires the fewest assumptions. In the case of the pig, two domestication events definitely required fewer assumptions than the many that we now believe to have occurred.
Research published in the journal Science in 2005 has identified at least seven occurrences of the domestication of wild boars. Two events occurred in Turkey and China, as previously thought, but the other five events took place in Italy, Central Europe, India, southeast Asia, and on islands off of southeast Asia, like Indonesia. Apparently, people arrived in these areas, corralled some wild boars, and ultimately domesticated them, establishing genetic lines that we have now traced to today.
As usual, molecular biology overrules everything else
The scientists uncovered the pig domestication pattern using modern molecular biology tools. They relied on a genetic tool known as the mitochondrial clock. Mitochondria have their own DNA, which they use as the code for their own, specialized mitochondrial proteins. Because mitochondria are essential to cell function and survival, changes in DNA coding sequences are rare because selection pressures against them are strong. For this reason, any changes are usually random changes in noncoding regions, changes that accumulate slowly and at a fairly predictable rate over time. This rate of accumulation is the mitochondrial clock, which we use to tick off the amount of time that has passed between mutations.
Tick-tock, mitochondrial clock
Very closely related individuals will have almost identical mitochondrial sequences; for example, the mitochondria that you have are probably exactly alike in sequence to the mitochondria your mother has. You inherited those mitochondria only from your mother, whose egg provided these essential organelles to the zygote that ultimately became you. Were someone to sample the mitochondria from one of your relatives thousands of years from now, they would probably find only a few changes, but if they compared this sample to one from someone unrelated to you, they would find different changes and a different number of changes, indicating less of a relationship.
That’s how the researchers figured out the mystery of the pigs. They sampled wild boars from each of the areas and sampled domestic pigs from the same locales. After comparing the mitochondrial DNA sequences among these groups, they found that pigs in Italy had sequences very like those of wild boars in Italy, while pigs in India had sequences very like those of wild boars there.
Approachable teenage-like pigs
How did we domesticate the pigs? Researchers speculate that adult boars (males) who still behaved like teenagers were most likely to approach human settlements to forage. They were also less aggressive than males who behaved like full adults, and thus, easier to domesticate. They fed better on human food scraps than did their more-mature—and more-skittish—brethren, and enjoyed better survival and more opportunities to pass on their juvenile characteristics, which also included shorter snouts, smaller tusks, and squealing, to their offspring. Domestication was just the next step.