The Science

ONCE UPON A TIME

The great human migrations

The modern human (Homo sapiens) originated in Africa, the continent where we have spent most of our existence, some 200 000300 000 years ago. It was onlyabout 60 00070 000 years ago that a small East African population migrated out-of-Africa, giving birth to all non-African populations. As genetic diversity continues to increase with time, so the greater genetic diversity within the much older African population can be better explained.

One group of the out-of-Africa migrants moved east, reaching India, Southeast Asia and finally Australia approximately 50 000 years ago.

Another group of out-of-Africa migrants moved to the Near East/Arabian Peninsula, and further split into two groups; one eventually reaching and settling in Europe about 45 000 years ago, and the other returning to the continent to settle in North Africa.

The populations that settled on the Asian continent eventually reached Siberia. Approximately 15 000 – 20 000 years ago they crossed the Bering Strait, reaching the American continent for the first time.

Ancient DNA studies showed that when the modern humans travelled out-of-Africa, they crossed paths with other human groups, namely Neanderthals and Denisovans. Hence, the genome of current European and Asian populations has approximately 2.3% of Neanderthal input; and the Southeast Asian population can have up to 4% of Denisovan input in their genomes. Interestingly, the Sub-Saharan African population does not have any input of either Neanderthal or Denisovan in their genomes, as these groups were never present in Africa.

Given the origin and migration of our species, human populations have primarily been organised into three large ancestral groups: African (Sub-Saharan Africa), Asian (more precisely East Asia) and Eurasian (not only Europe, but related by ancestry, Southwest Asia, North Africa and to some extent Central Asia).

There are gradients of genetic diversity between the human populations – some DNA variants are more frequent in certain regions than in others. Thus, as we analyse millions of DNA variants at the same time, we acquire an excellent resolution in identifying the origin of an individual based on their specific genetic profile.

How does genetic testing for ancestry work?

DNA is passed from parent to offspring, from generation to generation, and has been doing so for hundreds of thousands of years. Therefore, your DNA contains bits of information from all your ancestors, serving as a map that can be used to determine where your predecessors originated.

When investigating your genetic ancestry, we are interested in three types of your DNA:

The DNA stored inside your autosomal chromosomes (DNA)

The DNA stored inside your mitochondria (mtDNA)

The male Y chromosome

Humans have 23 pairs of chromosomes including the sex chromosomes which determine the male (XY) or female (XX) gender. The Y chromosome only exists in males, passed from father to son, generation after generation. For males, we can therefore analyse the genetic information in your Y chromosome to discover your paternal ancestry (from your father’s side of the family). Paternal ancestry is defined by a haplo group: a group of individuals who share the same genetic variation and have the same paternal ancestor.

Both males and females have mtDNA, but it is only passed on by female parents to offspring. A mother will pass her mtDNA on to all of her daughters and all of hersons. The mtDNA can therefore be analysed to find out your maternal ancestry(from your mother’s side of the family). Maternal ancestry is also defined by a haplo group, being a group of individuals who share the same genetic variation and have the same maternal ancestor.

The other 22 pairs of chromosomes (called autosomal chromosomes) are made up of a mixture of DNA from both your mother and father. Considering that each of your mother and father’s DNA contains a mixture of their parents DNA, and so on and so forth, these 22 pairs of chromosomes contain a sequence of DNA that allyour ancestors have contributed to for thousands of generations.

This autosomal DNA is used to find out your admixture of various possible ancestries. Simply put, we can calculate the proportion of your genome that has originated from different ancestral groups in Eurasia (North Europe, South Europe, Arabian Peninsula, Near East, North Africa), Asia (South Asia, East Asia, Native America), or Sub-Saharan Africa (West Africa, East Africa).

Some of these groupings may seem strange from a geographical point of view, but remember that they represent ancestral groups. North Africa falls within Eurasia, as the origin of North African populations was mainly from the Near East, where around 45 000 years ago a group of migrants split up, with one population moving north to settle in Europe and the other movingsouth to settle North Africa.

Native America is included in the Asian ancestral group, as the settlement of the American continent, some 15 00020 000 years ago, took place from migrations with origin in Siberia.

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