Tens of thousands of years ago, humans were wild animals. Our ancestors roamed the land in search of food by day, and huddled together for safety by night. But then something changed. We domesticated ourselves, and this process didn't just change us profoundly — it changed a lot of other life forms around us, too.
When did Homo sapiens become a "domestic animal"? There are many ways to answer this question, but most anthropologists would agree that the main difference between a wild-type human and a domestic one is agriculture. Humans began domesticating plants and animals between 10,000-8,000 years ago, and it changed us biologically and culturally. Our diets were transformed, and we abandoned the hunter-gatherer lifestyle for sedentary lives in villages (and, later, cities). Humans in agricultural settlements experienced a 5-fold increase in their populations. Evolutionarily speaking, this is a win, since descendants of farmers quickly outstripped hunters in terms of genetic presence. That means the genetic changes caused by domestication ripped through Homo sapiens populations like wildfire, transforming our species quite rapidly.
Agriculture allowed human populations to boom, but this also meant that there were enormous population crashes among domestic humans too. A study published last year shows that European agricultural communities often grew to large sizes, then abruptly dwindled to almost nothing, between 8,000 and 4,000 years ago. It's not clear what causes these population crashes among domestic humans. There's little evidence that it was climate and habitat change, so it's possible that pandemics were the culprit (domestication makes diseases deadlier, as you'll see in item 4), or overfarming and poor agricultural practices.
At first, domestic humans had a lot of problems with malnutrition because they were still mastering the art of farming — and they were eating new kinds of foods like seed crops. Periodic malnutrition during childhood affects bone growth, so many of these first farming communities contained people with skeletal and dental problems. But later, our jaws became wider and smaller, most likely because domestic humans had a softer diet with more processed food in it. Hunter gatherers have longer, narrower jaws because their diet causes more basic stress on the mouth. Interestingly, tooth size didn't evolve as quickly as our jaw sizes, which is why our teeth tend to become crowded in our jaws. This has led to a common practice among some humans in the modern world, who remove their wisdom teeth and straighten their crowded teeth with braces.
Not only did our bodies change with agriculture, our diseases did too. Tuberculosis has been present in Homo sapiens for a very long time, and underwent evolution during the Neolithic agricultural era to thrive in crowded situations which resulted from reliance on agriculture. In a 2010 study of pathology among domestic humans of the early agricultural era, anthropologists found an increase in bone lesions from infectious diseases, but skull injuries among males were lower. It would seem that diseases became a more common cause of (survivable) injury than human-on-human fights. If that's true, it fits with studies of domesticated dogs and rats, which also fight with each other less than their wild type counterparts.
Many studies have shown that humans and dogs experienced co-evolution for tens of thousands of years, and underwent domestication together. Dogs share very unique social and communication skills with humans, and their behaviors are more similar to human behaviors than even those of chimps and bonobos. This suggests that humans and dogs are both selected for the same sets of social behaviors. Domestic humans are therefore more doglike than any other animal. Or perhaps dogs are more human-like than any other animal?
"Paedomorphosis" is an evolutionary process that causes many domestic animals, including humans, to behave and appear more childlike. In humans, that change has meant a smaller, softer-looking face (including that smaller jaw we mentioned in 3), more closely-set eyes, and sparse body hair. The process of paedomorphosis is common in pigs, dogs, rats and cattle, too. So we didn't just infantilize ourselves. We turned many of our companion species into adult babies too.
Large parts of human populations in the west have a genetic mutation that allows them to digest milk the same way babies do. These people can break down lactose, a sugar found in milk. People who are lactose intolerant (this is very common among Asian populations) are closer to being wild-type humans, and they suffer stomach aches and other kinds of gastric distress from consuming dairy. The mutation for lactose tolerance is believed to have originated about 20,000 years ago, in populations where an ability to eat milk products from dairy animals conferred a great survival advantage. As a result, the mutation spread very rapidly.