The Journey of Man
We can define race biogeographically as:
Caucasian (Western Eurasia)
Black Africans (Sub Saharan Africa)
Mongoloids (E. Asia)
S. Asian Aborigines (S India)
Amerinds (Native American)
Oceanics
Australian Aborigines
When human polymorphisms are analyzed (Many
thousands from all over the world) we find:
85% variability within populations
7% variability differentiated populations within a race
(e.g. English with French)
8% variability between races
The concept:
As organisms (humans) go from generation to
generation mutations occur and accumulate. They
are passed on from mothers and fathers to
daughters and sons. If we know the rate at which
mutations accumulate with enough samples and
study we can track these mutations back to a
common ancestor. How?
We need a genome that doesn’t recombine and
shuffle the mutations around.
Father to son is found in the Y chromosome;
Mother to daughter is found in the mitochondrial
DNA.
For example, an analogy:
Imagine a soup recipe that is passed on from mother
to daughter. Of course, each generation modifies
the recipe slightly, a clove of garlic here, a pinch of
thyme there. Over time, family groups move to other
areas where more modifications take place. This
leads to a great deal of culinary diversity and a set of
soups that, on the surface, might look quite different
but that have a common base recipe.
In this analogy the soup is equivalent to the human
race the ingredients are the DNA sequence of the
genome and the changes in the recipe are
equivalent to the mutations in the sequence that are
passed on.
To see how tracing humans back to a common
ancestor works we can now imagine an international
soup potluck:
Each soup is different but we can conclude that they
once came from the same recipe because they all
contain a common ingredient, say….Impala! Impala
is a rare and hard to obtain ingredient yet is the
cornerstone of all of the soups.
So if we assume that changes occur at regular
intervals (mutation rate) and that there are on
substitutions or subtractions after adding the
ingredients (no recombination) than we see that the
most common ingredient is the earliest and we can
date the soup recipe.
For example:
Impala, mustard, pepper, cheese, oregano
Impala, salt, loganberries, peanuts, chili peppers
Impala, pepper, mustard, clams, basil
Impala, pepper, crab, juniper berries
Impala, parsley, pork, thyme and salt
In this example we see that there is another
common division that must have occurred after
Impala was used and that is should salt or pepper
be used to season it. Only five recipes are sampled
here. Given several hundred more soups with other
ingredients we could eventually expect to be able to
map out how when and in what form the original
recipe occurred and how the different variations
evolved.
We must assume that it is very unlikely that chefs
from around the world all thought to add impala
independently at about the same time!.
We can also see that later addition of rare local
ingredients serve to distinguish local, more recent
versions of the recipe.
We (geneticists and molecular biologist) have RFLP
and SNP equivalents of impala, salt and pepper, and
rare local ingredients.
Since Impala is a rare African species we can
conclude that Africa is the origin of the human soup.
Other Studies:
Mitochondrial DNA is present in large (amplified)
quantities and can be fairly easily isolated from
mummies to create accurate family pedigrees (also
from frozen mammoth or human remains).
Claims of DNA from fossilized remains (Jurassic
Park) are unfounded. The DNA has long since
succumbed to degradation.
Recently DNA was extracted from 40,000 yo
Neanderthal bones. 327 bp of mitochondrial DNA
was sequenced. There were many “odd”
polymorphisms which led scientists to conclude that
Neanderthals are not the ancestors of modern
humans but that we share a common ancestor who
lived about 500,000 yo.