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Are we all related? In genealogy a common ancestor of any set of individuals is defined
as someone from whom all the people in the group are descended. For example, you and
your cousin are descendents of the same grandparents so your grandfather and his parents are the
common ancestors of you and your cousin. The most recent common ancestor or MRCA is the
common ancestor from where a set of individuals are directly descended. So in this example,
your grandfather will be the most recent common ancestor of you and your cousin. We can expand this scenario to include
your second cousins, third cousins and so on and you can see that with the addition
of every new branch your common ancestor moves back a generation.
So how far do we need to go back to find the most recent common ancestor of all human beings
who are presently alive?
Let's do some calculations.
Assume you are the first generation and you have two parents. In a random mating model, where
two people can mate randomly per generation, your parents parents combined will be four people
and their parents combined will be 8 people and so on.
If generations are discrete and non-overlapping then it means that 'n' generations back,
there will be 2^n people. Assuming a generation is at most 30 years then it means a thousand
years or roughly 30 generations ago, there will be 2^30 or over one billion people in
the world.
However, we know that this is not the case. Historians have put the population of the
world at that time to be around 300 million. In a pure random mating with 2 parents per
individual and a constant population size we can find the number of generations to the most recent common ancestor by taking a log2 of the population
size. The world's current population is 7 billion and by this
method the most recent common ancestor of present day humans lived only 30 generations
or roughly 700 years ago which is roughly 1300AD.
This doesn't sound right as the Americas and Australia were not even discovered at
that time. The problem with the above model is that the ancestry tree is not a binary
tree. Mating patterns are structured by geography, proximity, culture, language and social class.
This leads to pedigree collapse which is a phenomenon where reproduction between two
individuals who share an ancestor causes the number of distinct ancestors in the family
tree of their offspring to be smaller than it could otherwise be.
In 2004, Douglas L. T. Rohde, Steve Olson & Joseph T. Chang wrote a paper in the scientific
journal Nature where they used common models to indentify the most recent common ancestor
of present day living humans. Their models are centred around local mating as it is very
unlikely that parents would have children far away from where they live but at the same
time, their models allowed a small number of random mating.
They split the world into "continents", "countries" and "towns", which are not just geographic
positions but can be viewed as abstract pools from which one is more or less likely to choose
a mate. These pools can be geographic, religious or cultural. The model also accounted for
a limited amount of migration out of towns, countries and continents. The arrows in the
diagram show the number of persons leaving a population zone per generation.
According to their calculations, if 5% of individuals left their home town per generation,
0.05% migrated out of their home country then the MRCA of everyone who is presently alive
lived in 1,415 BC i.e. only 3400 years ago. However, these migration estimates are far
too conservative. For example, it assumes that only 50 people leave per country in the
densely populated Eurasia. If the migration among towns is increased to 20% and other
numbers are increased by a factor of 5 or 10 then our most recent common ancestor lived
around AD55 i.e. roughly 2000 years ago. They also discovered that it takes 1.7 times the total number of generations
from MRCA where every individual is either an ancestor of the whole world today, or else
is an ancestor of no one alive today.
This is quite fascinating. What this tells us is that:
Before 1000BC - Every single human is either an ancestor of no one alive today, or an ancestor
of everyone alive today. Between 1000BC and 55 AD - , every single
human is either an ancestor of no one alive today, an ancestor of everyone alive today,
or ancestor of some people alive today. After 55 AD - every single human is either
an ancestor of no one alive today, or an ancestor of some people alive today.
This means we are all descendents of one single human who lived around 55 AD and therefore,
we are all related - some more than others.
If you want to learn more about this topic please check the links in the description
below.
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