There was a genetic change in the humans eight or ten thousand years ago, the ability to digest animal milk. Where it occurred it the gene spread rapidly. Did milk-drinkers have much of an advantage over non-drinkers? You bet. John Hawks, a scientist who deals in such matters, explains.
I received a letter about lactase persistence that motivated me to a fairly long reply; I thought I would share the question and answer:
I read with interest your article in the current Scientific American (Still Evolving). I do have one serious puzzle: While not a biologist (or an anthropologist), my understanding of the logic of Darwinian selection is that mutations are random and that they only have a chance of sticking around if they give the species some benefit for survival in the context of the current environment; and that survival translates primarily into the capacity to produce enough healthy offspring to continue (and maybe sometimes produce more offspring than before).
Now, when it comes to protection, even moderate, against malaria, this makes sense. But I do not see how lactose tolerance would fall into that category. My impression is that humans who are lactose intolerant and drink milk do not appear to have a life-threatening reaction. They may be miserable at the extreme, but they live. Certainly, being lactose tolerant in a population that raised dairy cows (goats, etc.) would improve one’s quality of life, but I need some help in understanding how this would benefit our species survival ability.
This is a great question; I had a very prominent geneticist ask me the same thing some years ago.
It can be difficult for people in today's industrialized societies to perceive the depths of the nutritional challenges faced by most people throughout history -- sadly even though the same nutritional challenges are still faced many people living today. Nutritional shortfalls impede reproduction in many ways. Women on calorie-restricted diets have lower fertility, and take longer after the birth of a child to conceive again. Adolescent girls with restricted caloric intake take longer to reach sexual maturity.
The lactose in milk amounts to approximately a third of its caloric content. That means that a person with lactase persistence can get up to 50% additional energy from milk. Milk can be processed by microbes to reduce its lactase content -- many yogurts and cheeses can be consumed by adults who do not have a persistence of lactase. But these methods are time-consuming and reduce the amount of energy available from a given volume of milk.
In a milk-consuming population, otherwise nutritionally limited, lactase persistent women would have reproduced earlier and have had larger families. Our best estimate is that this advantage was on the order of 10% during the last few thousand years in northern Europe. This kind of advantage could accure by lactase-persistent women starting their reproductive lives around 2 years earlier, or spacing their children around three months closer together. That kind of change in birth spacing or maturation need not be genetic itself; it can emerge simply as a result of better nutrition.
Lactase persistence worked especially well in societies where it was part of a system. Milk is a very efficient use of domesticated animals; milking animals yields much more energy over the long run than raising them for meat. Some ancient populations relied extensively on the milk produced by their herds; this was a much more important component of the diet in areas that were too arid or too cold for the earliest domesticated grain crops. Those areas, like northern Europe, Arabia, and the Sahel, are where different lactase persistence mutations most took hold.
Once such a system got started, people who carried the original, non-digesting lactase allele would have been even worse off. Consuming moderate quantities of milk can lead to digestive distress in people who do not produce lactase. The resulting diarrhea reduces energy intake even more and can cause worse health problems. That side-effect of milk drinking might initially have deterred people from adopting milk in their diet. But for any group where milk drinking had been established already, the bad consequences of lactose intolerance would increase the selective pressure toward lactase persistence.
Scientists took a look at men's faces from when weren't even men, a few million years ago and discovered that certain areas of the skull thickened. And these areas just happened to be the places in the face where you're likely to get punched. And the thickening of the bone in these areas coincided with human hands' evolution. That is, as hominins went from monkey hands towards human hands men's skulls adapted to take a punch. And, as humans the lost upper body strength needed to swing through trees the facial bones began thinning back.
University of Utah biologist David Carrier and Michael H. Morgan, a University of Utah physician, contend that human faces -- especially those of our australopith ancestors -- evolved to minimize injury from punches to the face during fights between males.
The findings in the paper, titled "Protective buttressing of the hominin face," present an alternative to the previous long-held hypothesis that the evolution of the robust faces of our early ancestors resulted largely from the need to chew hard-to-crush foods such as nuts.
"The australopiths were characterized by a suite of traits that may have improved fighting ability, including hand proportions that allow formation of a fist; effectively turning the delicate musculoskeletal system of the hand into a club effective for striking," said Carrier, lead author of the study. "If indeed the evolution of our hand proportions were associated with selection for fighting behavior you might expect the primary target, the face, to have undergone evolution to better protect it from injury when punched."
The rationale for the research conclusions came from determining a number of different elements, said Carrier.
"When modern humans fight hand-to-hand the face is usually the primary target. What we found was that the bones that suffer the highest rates of fracture in fights are the same parts of the skull that exhibited the greatest increase in robusticity during the evolution of basal hominins. These bones are also the parts of the skull that show the greatest difference between males and females in both australopiths and humans. In other words, male and female faces are different because the parts of the skull that break in fights are bigger in males," said Carrier.
"Importantly, these facial features appear in the fossil record at approximately the same time that our ancestors evolved hand proportions that allow the formation of a fist. Together these observations suggest that many of the facial features that characterize early hominins may have evolved to protect the face from injury during fighting with fists," he said.
The latest study by Carrier and Morgan builds on their previous work, which indicate that violence played a greater role in human evolution than is generally accepted by many anthropologists.
In recent years, Carrier has investigated the short legs of great apes, the habitual bipedal posture of hominins, and the hand proportions of hominins. He's currently working on a study on foot posture of great apes that also relates to evolution and fighting ability.
Research on the evolution of creatures in the genus Australopithecus -- immediate predecessors of the human genus Homo -- remains relevant today as scientists continue to look for clues into how and why humans evolved into who they are now from predecessors who inhabited Earth about 4 to 5 million years ago.
Carrier said his newly published research in Biological Reviews both "provides an alternative explanation for the evolution of the hominin face" but also "addresses the debate over whether or not our distant past was violent."
"The debate over whether or not there is a dark side to human nature goes back to the French philosopher Rousseau who argued that before civilization humans were noble savages; that civilization actually corrupted humans and made us more violent. This idea remains strong in the social sciences and in recent decades has been supported by a handful of outspoken evolutionary biologists and anthropologists. Many other evolutionary biologists, however, find evidence that our distant past was not peaceful," said Carrier.
"The hypothesis that our early ancestors were aggressive could be falsified if we found that the anatomical characters that distinguish us from other primates did not improve fighting ability. What our research has been showing is that many of the anatomical characters of great apes and our ancestors, the early hominins (such as bipedal posture, the proportions of our hands and the shape of our faces) do, in fact, improve fighting performance," he said.
Morgan added the new study brings interesting elements to the ongoing conversation about the role of violence in evolution.
"I think our science is sound and fills some longstanding gaps in the existing theories of why the musculoskeletal structures of our faces developed the way they did," said Morgan. "Our research is about peace. We seek to explore, understand, and confront humankind's violent and aggressive tendencies. Peace begins with ourselves and is ultimately achieved through disciplined self-analysis and an understanding of where we've come from as a species. Through our research we hope to look ourselves in the mirror and begin the difficult work of changing ourselves for the better."