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The avocado is a
fruit of a different time. The plant hit its evolutionary prime during the
beginning of the Cenozoic era when megafauna, including mammoths, horses,
gomphotheres and giant ground sloths (some of them weighing more than a UPS
truck) roamed across North America, from Oregon to the panhandle of Florida.
The fruit attracted these very large animals (megafauna by definition weigh at
least 100 pounds) that would then eat it whole, travel far distances and
defecate, leaving the seed to grow in a new place. That’s the goal of all
botanical fruits, really: survival and growth via seed dispersal.
But the great
mammals disappeared forever about 13,000 years ago in the Western Hemisphere.
Around that time, North America lost 68 percent of its diverse Pleistocene
megafauna, and South America lost 80 percent, Connie Barlow, author of
"The Ghosts of Evolution: Nonsensical Fruit, Missing Partners And Other
Ecological Anachronisms" says. But even after this major shift in the land
mammal population, the wild avocado still requires the same method of seed
dispersal, which makes it somewhat of an evolutionary anachronism.
“After 13,000
years, the avocado is clueless that the great mammals are gone,” Barlow
explains. “Without larger mammals like the ground sloth to carry the seed far
distances, the avocado seeds would rot where they’ve fallen and must compete
with the parent tree for light and growth.”
A fruit with
smaller seeds, like a berry, for example, can be consumed whole and dispersed
by small mammals, making the chances of fruiting in a new place higher.
After the giant
mammals had died out, if an avocado tree was lucky, a jaguar might’ve found the
fruit attractive—the cat’s stomach is designed for digesting large hunks of
meat, leaving potential for swallowing the avocado whole, though there is no
evidence to support this idea. Rodents like squirrels and mice may have also
contributed, as they traveled and buried seeds in the ground rather than
letting it rot on the surface. Wild avocados were appealing to larger animals
because they had enough tasty flesh to lure the animals in and could be eaten
in one bite. The fruit had a larger pit and less flesh than today’s avocados,
but it really served as a quick snack for big mammals like the mammoth. Barlow
writes in “Haunting the Wild Avocado,” originally published in
"Biodversity":
The identities of the dispersers shifted every few million years, but
from an avocado’s perspective, a big mouth is a big mouth and a friendly gut is
a friendly gut. The passage of a trifling 13,000 years (since the Pleistocene
extinction) is too soon to exhaust the patience of genus Persea. The genes that
shape fruits ideal for megafauna retain a powerful memory of an extraordinary
mutualistic relationship.
How the avocado
still exists in the wild after surviving its evolutionary failures remains a
puzzle. But once Homo sapiens evolved to the point where they could cultivate
the species, the fruit had the chance to thrive anew. Back when the giant
beasts roamed the earth, the avocado would’ve been a large seed with a small
fleshy area—less attractive to smaller mammals such as ourselves. Through
cultivation, humans have bulked up avocados so there is more flesh for us to
eat.
The avocado has
been a staple food in Mexico, as well as Central and South America, since 500
B.C. Spanish conquistadors discovered the fruit from the Aztecs in the 16th
century, but the ahuacate, the Aztec word for “avocado,” wasn’t grown
commercially in the United States until the turn of the 20th century. By 1914,
the exotic fruit made an appearance on California soil. Roughly 90 percent of
today’s avocados are grown in California, according to NPR. But Barlow is quick
to point out the difference between a cultivated avocado and those found
naturally.
“The wild varieties
of avocados that are still somewhat available have a thin fleshy area around
the seed—it wouldn’t necessarily be something that we would recognize as
edible,” says Barlow. “When we go to the store and we see an avocado on sale,
it’s always a question of will this be one with a tiny seed or will it be a
batch where the seed takes up five-sixths of the space of the fruit?”
Ecologist Dan
Janzen conducted groundbreaking research on these and other “anachronistic
fruits” and found that the avocado isn’t alone in this regard. His research in
the late ’70s in the neotropics— an ecozone that includes both Americas and the
entire South American temperate zone—sparked a shift in ecological thinking
regarding these evolutionary-stunted fruits. Other examples include: papaya,
cherimoya, sapote and countless other fleshy fruits of the neotropics. Another
surprising “ghost” you may see everyday: honey locust pods scattered about your
driveway. All of these fruits are not considered edible by most native
mammalian standards today. Barlow continues:
“In 1977, however, was beginning to suspect that he—along with every
other ecologist working with large tropical fruits of the New World—had been
wrong in one very big way. They all had failed to see that some fruits are adapted
primarily for animals that have been extinct for 13,000 years.”
What makes the
avocado even stranger as an evolutionary dancer without a partner is that the
pit is actually toxic.
“We don’t have
the liver or the enzyme systems to detoxify our bodies from something like the
avocado seed,” Barlow says. “But at the same time, the rhino, which has been
around for ages, can eat all kinds of things that are toxic to everyone else.”
A South American
folk recipe for rat poison mixes avocado pits with cheese or lard to kill off
unwanted rodents. Whether or not humans are supposed to eat avocados from an
evolutionary standpoint, America produced 226,450 tons of the fruit and
consumed 4.5 pounds per capita in 2011. The avocado, a true “ghost of
evolution,” lives on.
More avocado
facts to drop at your next party:
The Aztec word
for avocado, ahuacatl means “testicle.” This is most likely because the
avocado, growing in pairs, resembled the body part. After the arrival of
Spanish conquistadors, Spanish speakers substituted the form avocado for the
Aztec (Nahuatl) word because ahuacatl sounded like the early Spanish word
avocado (now abogado), meaning “lawyer.” The Spanish-Mexican word “guacamole”
was derived from ahuacamolli, meaning “avocado soup or sauce,” made from mashed
avocados, chiles, onions and tomatoes.
Copyright © 2016 Scientific American, a division of Nature America, Inc. All
rights reserved.
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