Out of Africa: Modern humans migrated
along with bacteria
When humans made their way out of Africa some 60,000 years ago to populate the
world, they were not alone: They was accompanied by the bacterium Helicobacter
pylori, which causes gastritis in many people today.
Together, humans and the
bacterium spread throughout the entire world. This is the conclusion reached by
an international team of scientists led by Mark Achtman from the Max Planck
Institute for Infection Biology in Berlin, Germany. The researchers also
discovered that differences developed in the genetic makeup of the bacteria
populations, just as it did in that of the various peoples of the world. This
has also given scientists new insight into the paths taken by humans as they
journeyed across the Earth (Nature February 2007).
More than half of all human beings are infected with Helicobacter pylori, a
bacterium that can cause stomach ulcers. Like humans, the bacteria are also
split up into numerous regional populations.
A team of scientists led by Mark Achtman at the Max Planck Institute for Infection Biology, François Balloux
at the University of Cambridge and Sebastian Suerbaum at Hanover Medical
University have found signs of the parallel evolution of humans and H. pylori.
Using mathematical simulations, the researchers demonstrated that H. pylori
must
have left East Africa at the same time as humans - around 60,000 years ago. This
astonishing conformity was uncovered by scientists when they compared the
nucleotide sequencing patterns in the DNA of human and H. pylori populations.
In order to characterise the individual populations, the scientists employed
the principle of isolation by distance. According to this principle, the genetic
distance between two populations has a linear correlation with the length of the
migration paths taken since they were separated. "It's actually quite
logical," explains Dr. Mark Achtman, "because in the time that elapses
after a population leaves its point of origin, the number of mutations in its
genetic makeup continually increases."
However, while humans were spreading throughout the world, human populations had
to repeatedly pass through what scientists call genetic bottlenecks: when a
population shrinks, the gene pool also becomes smaller. These losses in genetic
diversity linger, even when the population starts once again to increase in
number. Since the Homo sapiens populations usually had to pass through several
genetic bottlenecks on their way across the globe, their genetic diversity
declined the further they journeyed from their origin in East Africa.
The migration paths taken by modern humans as they colonized the world.
60,000 years ago, Homo sapiens left their original home in East Africa --
taking the bacterium Helicobacter pylori with him. The abbreviation
"kyears" stands for thousand years. Credit: Max Planck Institute for Infection Biology |
Scientists have now uncovered similar signs of historical population
migration in the genetic makeup of H. pylori. However, the genetic diversity of
the bacteria is larger than that of humans.
This paves the way for researchers to
use H. pylori data to work out the migratory movements of modern humans. "The
parallels between the spread of humans and of H. pylori are truly
astonishing," says Achtman. "This bacterium could help us attain
further information on aspects of human history that are still hotly disputed
today if we analyzed H. pylori in conjunction with human data."
For
example, after leaving East Africa, the H. pylori population spread through
limited localities in southern Africa, West Africa, Northeast Africa, India and
East Asia. The genes of bacteria isolated in Europe, for instance, reveal
influences from Central Asia - an indication that human immigrants came to
Europe from Asia. --
Max-Planck-Gesellschaft
More information:
This work was published in Nature online in February 2007.
Contact: Dr. Mark Achtman
achtman@mpiib-berlin.mpg.de
Max-Planck-Gesellschaft
|
