Toxins Drove Evolution Of Human Taste Sense
Edited By Herman Rosen, MD
Plant toxins in the diets of early humans drove the evolution
of a bitter taste receptor better able to detect them, suggests
new genetic research by scientists at University College London,
Duke University Medical Center, and the German Institute of
Human Nutrition.
The ability to discern bitter flavors likely offered a survival
advantage by protecting ancient people from poisons. Today,
however, the same sensory sensitivity may have adverse consequences
for human health, by causing an aversion to bitter-tasting
nutrients, some of which might lower the risk of cancer and
heart disease.
In their study, the researchers examined the sequence of
one gene encoding the bitter taste receptor TAS2R16 in 60 human
populations. By reconstructing the history of the gene, the
researchers found evidence of evolutionary selection. Specifically,
they found that particular derived variants of the taste receptor
rapidly rose to high frequency many thousands of years ago,
before the expansion of early humans out of Africa. Through
further analyses they showed that one of the selected gene
variants confers an increased sensitivity to particular toxins,
including five that release cyanide when digested. The receptor
variant also is more sensitive to certain beneficial compounds,
they showed.
The researchers included David Goldstein, Ph.D., of the Duke
Institute for Genome Sciences & Policy and Nicole Soranzo,
Ph.D., of the University College London. Human taste senses
are generally less sensitive than those of primates or other
mammals, Goldstein noted. However, the new evidence for positive
selection on the gene for the bitter taste receptor suggests
that the preservation of specific sensory abilities may have
been particularly important, at least in the earlier stages
of human evolution.
“Humans have devised a number of behavioral habits
to inactivate toxins in foods, such as soaking of seeds, baking
or cooking,” Soranzo said. “Because of these other
means of protection, it is generally thought that the ability
to recognize compounds through the sense of taste is less important
for people than it is for other animals. However, detecting
signatures of selection for a bitter taste receptor suggests
that sensory detection of dangerous foods played an important
role at certain times during the course of our evolution,” she
added.
In mammals, including humans, taste receptors on the tongue
can detect five primary flavors: bitter, sweet, sour, salty
and umami—a savory or meaty taste. Taste receptors are
protein switches that trigger signals to the brain’s
taste-processing centers in response to particular foods or
other chemicals. In humans, 25 genes are responsible for encoding
receptors that detect bitter flavors. The current study provides
the second report in humans that different variants of those
taste genes contribute to variation among people in their response
to bitter foods.
The researchers sequenced the bitter taste receptor gene
TAS2R16 that responds to toxic compounds which release cyanide
when digested. Such toxins, called glucopyranosides, comprise
a wide class of natural defense compounds synthesized by over
2,500 plant and insect species and are present in various foods,
including cassava, almonds, green tea and some beans. They
found that taste receptors carrying a particular variant exhibited
increased sensitivity for five different potentially harmful,
cyanide-releasing compounds. The receptor variant also showed
greater sensitivity to two other compounds, salicin and arbutin,
with known beneficial effects.
“Bitter compounds are a heterogeneous class, some of
which are toxic and some of which lower the risk of cancer
and heart disease,” Soranzo said. Owing to their bitter
taste, these compounds are routinely removed by the food industry
and represent a key limitation in increasing the nutrient content
of plant foods.
“While this gene variant may have been advantageous
in our past through avoidance of natural toxins, one might
speculate that it may now contribute to increasing disease
risk through lowered intake of such beneficial compounds.” However
the team reported, human populations in Africa have retained
high frequencies of a lower-sensitivity bitter taste receptor
variant, with a geographic distribution similar to malaria
resistance genes. Earlier work has linked chronic ingestion
of low levels of cyanide-releasing foods to protection against
the disease, suggesting that more limited sensitivity to bitter
flavors may have been advantageous in regions where malaria
was most prevalent.#
Dr. Herman Rosen is Clinical Professor of Medicine at Weill
Medical College of Cornell University.
