Lactose Tolerance and Human Evolution
In the HHMI movie by Dr. Sarah A. Tishkoff, she explains how lactase persistence is an example of human adaption. Throughout her lecture she goes into detail about how lactose tolerance and lactose intolerance came to be, and how this adaption developed. In this paper I am going to address five specific questions that explain the adaption of lactose tolerance/intolerance in adult humans.
The proximate mechanism that provides lactose tolerance in adult human is the activation of the lactase enzyme in our bodies. As children our bodies produced this active enzyme lactase (found in the small intestine), which allows for the breakdown of the sugar lactose that is present in milk. As adults this enzyme is shut off, but due to a genetic mutation in some people they “”maintain the expression of this gene at high levels as adults”” (Tishkoff, Lecture 2). This leads them to be lactose tolerant as they are able to digest the sugar lactose. Mutation is the proximate mechanism that provides lactose tolerance in adult humans.
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How it works
Lactose tolerance changes during development, by a mutation adaptation. All mammals including humans are born with the active form of the enzyme lactase. As babies we can digest the sugar lactose in milk because the active enzyme can breakdown the sugar lactose and it can be digested without any problems. Where the problem arises is as you get older and become an adult this enzyme shuts off and people become lactose intolerant.
The people who can tolerate milk have been found to have a genetic mutation near the gene that produces lactase enzyme started becoming more frequent. If you have this genetic mutation than you will be able to digest the sugar lactose as an adult. However, according to Dr. Tishkoff, “”there is some evidence that as people get much, much older there can be a slight decrease in the prevalence of the gene expression activity, but it will not be not huge for that particular trait”” (Lecture 2). This development of lactose tolerant is recent trait that has only risen in the past few thousand years, which in evolution this is a very short time period for something to evolve.
The ultimate mechanisms that led to the evolution of lactose tolerances in adult humans were mutations, and natural selection. The populations in which the lactose tolerance trait thrives in play a large role in the evolution of the trait in humans. Ancestors in these populations who practiced dairying and drank milk are the ones that contributed greatly to the next generations due to genetic mutations. These populations such as northern Europe have a higher ability than other populations do to digest milk. This is important because most of the world cannot digest milk.
Natural selection is also an ultimate mechanism that led to the evolution of lactose tolerance in adult humans. Natural selection occurred because the mutation regulates gene expression, which will keep the lactase enzyme active, and it will increase the fitness of the people who have this active enzyme. It will increase the frequency of the gene in the population, it will also increase the frequency of the variants that are flanking it, and this is called positive selection.
Not all humans are lactose tolerant as adult humans because they do not have the active enzyme lactase. They do have this enzyme but as humans become adults this enzyme is essentially turned off, and if they do not have the genetic mutation this enzyme will not become active. It is important to note again that most people are not lactose tolerant, it is seen mostly in northern Europe where almost “”99% of the population is able to digest milk”” (Tishkoff, Lecture 2).
Most adult cats can drink milk with no problems; this is probably because they continued to drink milk even after they became adults. Even though it is the same in cats as in humans, I think that cats can become tolerant the same way humans did. Their ancestors probably started becoming domesticated years ago and the cats probably started drinking milk throughout their lives which could have led them to be able to digest milk also due to a genetic mutation.
This mutation could have then be passed onto generations of cats to come. In the movie Dr. Tishkoff explained that in Europe if you had the C nucleotide you could not digest milk but in Africa if you had the C nucleotide you could digest milk (Lecture 2). This could also be true for cats in different parts of the world they could have developed this way as humans did. With random mating other types of cats could have also developed the adaption to digest milk as adults.
In conclusion, lactose tolerance in adult humans is a recent development to our evolution as a whole population. Even though most of the world is not lactose tolerant it is still amazing to see how fast this trait has evolved over the past thousand years. Through mechanisms such as natural selection and mutations this trait was able to evolve and adult humans were able to adapt to be able to digest milk.
- Tishkoff, S. (n.d.). Genetics of Human Origins and Adaptation. Lecture. Retrieved June 22, 2018, from http://media.hhmi.org/hl/11Lect2.html