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“The ability to digest milk as adults, and as infants, actually, is due to the expression of an enzyme called lactase.” That’s the University of Pennsylvania’s Sarah Tishkoff at the AAAS conference in Washington, D.C., on February 20th, talking about the recent evolution of lactose tolerance in different human populations within the past 9,000 years. “Individuals who don’t express this enzyme can’t digest that complex sugar.”
Can we put a number on the evolutionary importance of lactose tolerance? “The selective pressure was quite remarkable. We actually estimated it to be about 10 percent. So you’re more likely to have 10 percent more offspring essentially.”
How strong is that? Look at population genetics to see how fast a trait will spread that develops in one individual in a population and that confers a 10 percent reproductive advantage. A mathematical analysis reveals that in just 100 generations, such a trait can be found in 95 percent of the individuals in the population.
That time period could be less than 2,000 years for humans. “I’m often asked the question, are humans still evolving? I would say the answer is absolutely yes.”
—Steve Mirsky
[The above text is an exact transcript of this podcast.]
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21 Comments
Add CommentI am assuming that by "milk" the article is talking about cow or goat milk. Human breast milk HAS to be digestible before 9,000 years ago, right?
Reply | Report Abuse | Link to thisHuman breast milk contains lactose just like breast milk from other mammals. Nursing infants express the lactase gene in order to digest lactose but levels of the enzyme decline after weaning. This makes sense since our ancestors did not consume milk as adults and there would be no need for the lactase enzyme in adults.
Reply | Report Abuse | Link to thisEnzymes in a food have absolutely nothing to do with the food's value as a nutrient. Heating milk denatures proteins in the milk. This has no effect on nutritive value. Heat also destroys vitamins. That reduces milk's value. Certain essential fatty acids may also be altered by heating. But enzymes - no value at all. They cannot be absorbed into the blood but if they somehow reached the blood, they would create havoc with the immune system.
Reply | Report Abuse | Link to this“The selective pressure was quite remarkable. We actually estimated it to be about 10 percent. So you’re more likely to have 10 percent more offspring essentially.”
Reply | Report Abuse | Link to thisI don't know, am I just being a stickler, but I feel they should have pointed out that selection pressure isn't just related to the number of offspring being produced, but also the "fitness" of these offspring. While it's "essentially" correct that a gene leading to a 10% increase in offspring (all else being equal) is likely to increase the frequency of that gene in the population, this isn't necessarily how this selection pressure would manifest.
But hey, I guess most Sci Am readers are perfectly aware of this fairly simple concept. And it's not like we would want any stray creationists wandering around the interwebs to accumulate some knowledge about how evolution functions, now would we?
@zstansfi, "I don't know, am I just being a stickler, but I feel they should have pointed out that selection pressure isn't just related to the number of offspring being produced, but also the "fitness" of these offspring." I don't think the point of the article was to teach the theory of evolution. It is a basic summary of the findings and therefore focused on those findings. I think that it is safe to assume that if it confers an advantage to one generation it doesn't likely kill the next. That would negate the advantage part. Also, if it is confering an advantage by increasing offspring I don't see how those offspring would then not also be more fit by carrying the same gene, all else remaining equal. Doesn't the idea of fitness imply an increase in reproductive success?
Reply | Report Abuse | Link to thisI appreciate you wanting to ensure the creationist don't misunderstand but face it, all the facts in the world will not change their minds. Your clarification is not going to turn any of them away from their dogma. And it is not the responsibility of every researcher to include in their findings a popup book version so society's intellectual light weights won't turn to the dark side.
@stew6302, "assuming prophesy holds out" you do realize this is a science site right? It isn't a place for delusional people to speak matter of factly about apocalypse myths. Unless it is in regards to studying primitive people and the quaint superstitions that they held. There are of course modern people that still believe those primitive stories but they are mostly intellectually challenged individuals gone off their meds.
Reply | Report Abuse | Link to this"Got milk?" "Milk is good for you." You all have heard and been brain washed by the commercials, haven't you. I can't stop wondering...if milk is so good for you, why does nature wane all mammals from it at such a young age?
Reply | Report Abuse | Link to thisThe only purpose of milk is to provide nutrition and immunity to the infant until its body is capable of handling heavier proteins and fighting off disease.
People who drink milk, I am not one of them, I cannot stand the taste of it, is more likely to develop type I and type II diabetes, arteritis and inflammation of the joints because of excess calcium build up. So, "Got milk?" Get rid of it and you will live a lot healthier.
You are right this is a science journal, and science has demonstrated that there is a part of the brain that makes a person feel connected to God or to the universe as a whole. This is there either because of evolution, or because God put it there. Assuming you believe in evolution, by attacking Stew for having religion, you are in effect attacking him for using a part of his brain that evolution favored through natural selection. To me, he seems to be the one who is optimally using what nature has provided.
Reply | Report Abuse | Link to thisThe "Joke" is going to be on you... when none of the Goofy Stuff you think never happens. You are wasting your time commenting here because most everyone understands that science has facts as a foundation. I know it's makes no difference to you, your path is fixed.
Reply | Report Abuse | Link to thisYour right about..
Reply | Report Abuse | Link to this"There are of course modern people that still believe those primitive stories but they are mostly intellectually challenged individuals gone off their meds."
And/Or recreational drug users getting religious - believing in and worshiping a superhuman controlling power or powers, example:. a personal God.
There is a difference in having a normal faith in God and going over the edge and dysfunctional.
Reply | Report Abuse | Link to thisI drink milk and my bones have been checked. The doctor said I have the bone density of someone 20 years younger. We eat good, cook with milk and enjoy drinking milk. Who ever said it wasn't for everyone... maybe you came to a false conclusion. It is good for more that just your bones.
Attributing the spread of lactase genes to an increased rate of offspring has been forced by the assumptions of Natural Selection, that change is based on internal qualities that favour individuals. That was based on Darwin's decision that external conditions had no direct effect on species change. Darwin's decision was wrong and few biologists today would agree with it, yet biology clings to Natural Selection. Even if that explanation was correct, why is the lactase gene concentrated around the Baltic? Why did the birth advantage not apply in other parts of Europe in which dairy products were consumed?
Reply | Report Abuse | Link to thisAnother approach is to reverse Darwin's decision, so the main driver of change is pressure from external conditions. That also reverses the rather silly rule of biology that there can be no purpose for a change. Of course, individuals cannot work out the changes they need, but natural variations in a population mean that if pressure from a change in external conditions eliminates individuals who are unable to survive, eventually the species will either become extinct, or some individuals will be found who behave in ways that assure their survival. In effect, external pressures give purpose to the change and the species becomes adapted to the conditions. That is not Natural Selection based on advantage, it is Natural Deselection based on disadvantage.
So why would some individuals be disadvantaged, specifically in the region around the Baltic? The answer could be vitamin D. Humans evolved in temperate zones in Africa, with year round sun exposure and continuous supply of vitamin D. The Baltic and relatively high populations south of the Baltic were both barriers to southern migrations during winter, as happened across the steppes. So humans around the Baltic were more dependent on dairy for vitamin D through winter. Some people may have become sick from consuming dairy and if they stopped eating dairy they may have become sick from vitamin D deficiency, so either way, the cause would not have been advantage from birth rates, but disadvantage from sickness.
The explosion of diagnosis of food allergies in recent times tells us that those allergies, often to dairy, grains and other foods introduced into the human diet relatively recently in evolutionary history, have existed undiagnosed, for a very long time, so some people around the Baltic may have continued eating dairy without realising it was making them sick.
Joke, perhaps the fact that we have been processing milk completely different than the way we use to just 40 years ago. This massive Homogination and Pasteuration is destroying our ability to digest milk and its sad.
Reply | Report Abuse | Link to thisIts about time people stop giving bad answers for problems and start looking at the concrete evidence that Homogenized milk hurts more than it helps. Bring back RAW and pay more people!
Joke, perhaps the fact that we have been processing milk completely different than the way we use to just 40 years ago. This massive Homogination and Pasteuration is destroying our ability to digest milk and its sad.
Reply | Report Abuse | Link to thisIts about time people stop giving bad answers for problems and start looking at the concrete evidence that Homogenized milk hurts more than it helps. Bring back RAW and pay more people!
Yeah humans are still evolving...lol what a joke... there is no scientific evidence of that, unless you want to try and make some type of correlation between human's shrinking brains and evolution - which I would argue against because of the poisonous in our environmental like Mercury in your vaccines through Thimerisol and the Mercury in your light bulbs, but hey why bother with science when we can just theorize through journalists.
Reply | Report Abuse | Link to thisTo get back to the point of my contention; this article is a joke, they lead the readers to believe that we are evolving but in all reality it’s not us that are changing, it’s our milk production that has changed.
We pasteurize and homogenize this milk to a point where all good bacteria and enzymes are DEAD and then we expect that this milk, produced through massive means and disgusting ones by that measure, is going to be healthy and good for us?
Lactate or lactose isn’t the problem here folks, it’s the processing of the milk that is the problem. GO BACK TO RAW MILK!
zstansfi, I haven't read the original paper, but I suspect that when Sarah Tishkoff talks about "10 percent more offspring essentially," she means that 10 percent more children survive to reproductive age. That result could be the result of higher fertility, lower child mortality or both—all of which could come from having more calories from milk in the diet. Her comment would therefore have covered both the possibility of more offspring and the possibility of higher fitness for them (because their environment, which now included digestible dairy, was slightly less hostile).
Reply | Report Abuse | Link to thiszstansfi, I haven't read the original paper, but I take Sarah Tishkoff's mention of "10 percent more offspring essentially" to mean that 10 percent more children would survive to reproductive age. That result could come from higher fecundity, lower child mortality or both. So her remarks would have covered both the possibility of more offspring and the possibility of their having higher effective fitness (because their environment, which now included calories from dairy, would be slightly less hostile).
Reply | Report Abuse | Link to thisNo joke. Evolution is a reality, produced by pressures from external conditions.
Reply | Report Abuse | Link to thisExisting species are adapted to their conditions, so they are not under pressure until there is a change in conditions which some individuals in the species are unable to survive. Their demise produces change in the species.
Pasteurisation and homogenisation of milk might have created a change in the food supply sufficient to result in the death of some individuals. However, those changes are not relevant to this study, which covered periods dating back centuries or millenia.
Another point. The main reason people now have difficulty digesting milk is not the processing, it is because there are two main types of cow milk. Humans do not have the same difficulties with milk from older A2 breeds. Their problems are with milk from newer A1 breeds. Breeding for increased yield or some other advantage inadvertently bred in a weaker chemical bond to a peptide BCM7. In milk from A1 cows, that peptide is free for release in the human gut. In some individuals it produces serious consequences.
If this milk was sold RAW it would still make people sick and some people very sick. The answer is to buy milk from A2 cows, or goats or sheep.
Humans evolved over millions of years in a hunter gatherer lifestyle. Only in the past 10,000 years have dairy and grains been major components of their food supply. Evolution of the lactase gene is evidence that the health of some individuals has been affected by those changes. There have been other reports that selective breeding of grains aimed at increasing yields, rust resistance etc. inadvertently bred in highly allergic proteins or peptides that cause autoimmune responses. There is a possibility that people have been made sick or have even died for a very long time from unrecognised allergies to these foods.
I point out that, as Charles Darwin observed, selective breeding uses the same processes that occur in nature to evolve species. Darwin's mistake was to apply the analogy from unnatural breeding to the natural world. The correct direction to apply the analogy was from a process that has been operating for billions of years to breeding that has happened for thousands of years. Darwin might then have drawn correct conclusion that selection is the result of unnatural intervention. In nature, the only mechanism of change is elimination of some individuals, better described as Natural Deselection.
And yes, contamination of the food may well be evolving humans by making some people sick, or dead.
I would think that it would be advantageous to our species if we did become lactose intolerant. Then newborns would not need to compete with older siblings for their mother's milk.
Reply | Report Abuse | Link to thisDrink it or not? good/not good? should I eat cheese? Fact of life #1 you drink milk you will live a long time and then die/ #2 you dont drink milk you will live a long time and then you will die. ITS YOUR CHOICE to do what you belive is best for you (unless you have a severe milk allergy like my daughter)!! NOONE knows its just like eggs eat them don't eat them.
Reply | Report Abuse | Link to thisbiodan23,
Reply | Report Abuse | Link to thisOld habits die hard. The perspective that leads to your comment comes from Darwin's explanation of the evolutionary mechanism. That perspective has kept the focus of biologists on the behaviours of individuals, because Darwin decided that change resulted from variations that gave some individuals superior internal qualities that favoured them in competitive struggle.
The real driver of change is the effect of factors external to the individual that determine the success of those behaviours.
The perspective predetermined by Darwin's explanation also has biologists looking for competition, which implies that individuals develop an intention to gain an advantage over others and take some actions to achieve that advantage.
The probabilities you describe may have evolved weaning of infants and a reduction of milk tolerance as they began to eat other foods. The probabilities of success of a mother's behaviour were worked out by external factors. A focus on the behaviours of individuals as the source leads to the idea of competition, when in reality, all individuals do what they do, and external conditions determine which of the variants of behaviour survive. Elimination of unsuccessful behaviours evolves the successful behaviours.
When Origin was published, Huxley wrote to Darwin and questioned why he gave so little importance to external conditions. Darwin replied "You have most cleverly hit on one point, which has greatly troubled me; if, as I must think, external conditions produce little direct effect, what the devil determines each particular variation?" Why MUST he think so? Darwin's decision to reject the effect of external conditions came from the beliefs of the society he grew up in, seen in the writings of Malthus, Spencer and Darwin's cousin Francis Galton, and repeated in Origin.
Why do biologists in the C21st perpetuate Darwin's errors? The answer to that is not in biology, it is in cognitive science. Attributing cause to the internal properties of protagonists and excluding the effects of external factors is a classic example of an Attribution Error, which cognitive scientists have found to be the most common error of human brains, particularly prevalent in individualistic societies. More details on my website:
www.ideasintuitionandthinking.com
Reverse Darwin's decision, replace Natural Selection with external conditions as the driver of change and biology can produce explanations that make sense.