London, June 17 : Scientists have suggested that in the absence of geographical barriers such as mountains and oceans, parasite "wedges" can keep populations of the same species apart, providing the opportunity for populations and even new languages to evolve separately.
According to a report in New Scientist, this theory has been put forward by Corey Fincher and Randy Thornhill of the University of New Mexico in the US.
Fincher and Thornhill say their hypothesis explains the longstanding ecological debate about why it is that biological diversity decreases as you move away from the equator and towards the poles.
"Individuals must balance the benefits with the costs of contacting other members of the same species," said Fincher.
For humans interacting with each other, for example, benefits include the opportunity to mate and trade, but these come at a cost: the risk of contracting a parasite or disease.
"These costly interactions especially come from interacting with people who do not belong to your society or group, whose immune systems are adapted for a separate set of parasites than your own," added Fincher.
The notion was first suggested in the 1970s by researchers who noticed that baboon populations living in the African savannah typically carried similar populations of bacterial fauna and would frequently interact.
In the rainforests, however, where each population tended to carry its own set of bacteria, primates typically interacted far less.
In the parasite-rich forest populations, interacting with others came with a high chance of contracting a lethal illness, making parasites an evolutionary driving force.
According to Fincher and Thornhill, similar situations can be found in human populations.
Taking languages as their measure of diversity, the pair looked at the concentration of different languages within an area and the number of parasites in that same area.
They collected data on parasites and languages around the world and found that the diversity of indigenous human languages is correlated with the diversity of human disease-carrying parasites.
The correlation was true on every continent and independently of historical contexts such as colonialism.