 Why do some countries drink lots of milk, yet others don't? Why can some people digest cow's milk with no problem, yet others can't? Estimates around only 30% of adults worldwide can digest lactose. They mean sugar and milk. In this video, I'm going to look at how the Indo-European migrations from the steppe area around Ukraine and Russia may have helped spread the ability to digest milk across Eurasia and the world. Now I know that a lot of people love a glass of milk, but milk consumption is not uniform across the globe. This table shows the per capita milk consumption from 2020 for each country around the world, and includes dairy products made from milk, apart from butter for some reason. As we can see, pretty much all of Europe consumes a lot of milk, with Finland being particularly high, as well as Britain and Ireland. Russia is also relatively high, as well as the likes of Kazakhstan. India and Pakistan consume less milk products, but still reasonable amounts. Contrast this with parts of Asia and Africa, which basically consume little to no milk. Now this is a map of lactose tolerance around the world. As you can see, the inability to digest milk is extremely low in Europe, most of North America and the likes of Russia, yet extremely high in East Asia, parts of Africa and South America. Even for countries that drink a lot of milk today, this wasn't always the case, just to clarify terms quickly. Lactase persistence is the continued activity of the lactase enzyme into adulthood, allowing the digestion of lactose and milk. In most mammals, the activity of the enzyme is dramatically reduced after weaning. Now nearly all hunter-gatherers were lactose intolerant, and lactase persistence only seems to have developed around 10,000 years ago, in conjunction with humans beginning to domesticate animals. Early abilities to digest milk seem quite isolated, however. So lactase persistence was limited or non-existent for thousands of years, but when did this change? While during the Bronze Age, a period from around 5,000 to 3,000 years ago is where everything seems to have changed, at least in Europe that is. As Professor Esk Willerslev from the University of Copenhagen told BBC News a few years ago, it seems like the Bronze Age is the period where the genetic diversity and distribution that we know today is basically formed. Professor Willerslev saved as a co-author and an interesting paper that looked at the population genomics of Bronze Age Eurasia, which gave interesting insights into lactase persistence. The authors begin by noting that although lactose tolerance is high in present-day Northern Europeans, we find it a low frequency in the Bronze Age, at only around 10% in Bronze Age Europeans, indicating a more recent onset of positive selection than previously estimated. In particular, the authors looked at the presence of a gene variation called RS4988235, which is associated with lactose tolerance. What the authors found was that this variation was most prominent in Bronze Age cultures from the step, notably the Yamnaya culture. Now I have made various other videos on the Yamnaya culture, a fascinating culture itself that was instrumental along with various other associated archaeological cultures, such as the Corded Ware culture, the Bell Beaker culture, and the spread of Indo-European limeges and other traits across Eurasia. But back to the study, as the study found, interestingly, the Bronze Age step cultures showed the highest derived allele frequency amongst ancient groups, in particular the Yamnaya, indicating a possible step origin of lactase tolerance. There are various other pieces of evidence, however, that connect the step to lactose tolerance. If we look at the Bell Beaker culture, for instance, which is a connected archaeological culture to the Yamnaya culture and also to the Corded Ware culture, there was very prominent in Western Europe and Central Europe and into Britain and Ireland. A study from last year looked at the genome-wide patterns of selection and 230 ancient Eurasians, and found as gene variation associated with lactose tolerance in a central European Bell Beaker sample that lived between approximately 2300 and 2200 BC. In Ireland as well, we see even more evidence of this connection between the step area and lactase persistence. A study from 2015 looked at three individuals from Rathland Island, which is a tiny island just north of Mainland Island, who lived somewhere around 4,000 years ago. The result showed substantial step genetic heritage, with one individual from the early Bronze Age, a male individual known as Rathland One, having the genetic coding to be able to tolerate drinking raw milk into adulthood. So it definitely seems that at least one vector in the spread of the ability to digest milk in Eurasia and other parts of the world came from this step area, and was spread within the European migrations and these various archaeological cultures from the Yamnaya culture to the Bell Beaker culture. Think about that, the next time you have a glass of milk, an ancient ancestor on horseback galloping across the Eurasian steppe. But who were the Yamnaya people? To find out, please click here. Thanks for watching, please subscribe and hit the bell, and check out my merch store as well to pick up this jumper and many other great products. Thanks again for watching and I'll see you next time.