The Molecule of More

THE MOLECULE OF MORE

killed off  a  large portion of humans, and  the  population dwindled to 
less than 20,000, representing a serious risk of extinction.
That near-extinction event illustrates why migration is so impor tant.
When a species is concentrated in a small area, there are many ways in
which the entire population can be wiped out. Drought, disease, and
other disasters can easily cause extinction. Dispersing throughout many
regions, on the other hand, is like an insurance policy. The destruction
of one population doesn’t result in total extinction.
Based on the appearance and frequency of genetic markers in
modern peoples, scientists estimate that early humans spread out across
Asia about 75,000 years ago. They reached Australia 46,000 years ago
and made it to Europe 43,000 years ago. Migration to North America
occurred later, sometime between 30,000 and 14,000 years ago. Today,
humans occupy nearly every corner of the globe—but not because
humans recognized the threat and dispersed.

THE ADVENTURE GENE

Research on mice has shown that drugs that boost dopamine also
increase exploratory behavior. Mice given these drugs move around their
cages more and are less timid about entering unfamiliar environments.
So could dopamine have helped propel early humans out of Africa and
across the globe? To answer this question, scientists from the University
of California compiled data from twelve studies that measured the fre-
quency of dopaminergic genes in different parts of the  world.
They focused on the gene that tells the body how to make the D4
dopamine receptor (DRD4). You may recall that dopamine receptors
are proteins that are attached to the outside of brain cells. A dopamine
receptor’s job is to wait for a dopamine molecule to come along and
bind to  it.  Binding sets  off  a  cascade of chemical reactions inside the 
cell that changes the way the cell behaves.
We encountered this gene before when we described the connection
between novelty-seeking and political ideology. Recall that genes come
in  different varieties called alleles. Alleles represent slight variations in