Ten thousand years ago, there was no corn, just a grass, called Teosinte specifically:
Zea mays spp. parviglumis.
This grass looked little like modern corn, the plant is highly branched, the kernals
are small, few in number, not fused, and enclosed in a hard fruit case.
But somehow, early farmers turned this grass into corn, so how did they do it?
When early farmers grew their proto corn, each year they selected what they considered
the best individuals to take seeds from for the next season.
With each generation the plant was enriched for traits that made it more useful to humans.
Over time random genetic mutations that would improve the crop would occur,
and these would be breed into corn by this process.
And slowly the plant became closer to the corn we know today.
But that’s not the whole story.
Up to 12% of the genetic material in corn is derived from Zea mays ssp. Mexicana
and these genes are thought to have been introduced through introgressive hybridization.
By this process genes are introduced into one plant from a closely related plant.
It begins with the two lines being crossed
producing offspring where one allele of each gene comes from each parent.
Then by repeatedly backcrossing with one of the parents we can be left with only a few
genes derived from the second parent,
effectively introducing these genes into one plant from the other.
How intentionally this was done by our ancient Mesoamerican farmers we likely will never know,
but we do know that the end result was a humble grass became the power crop we know today
But how easy was it to change a wild grass into something similar to today’s corn?
Well in the 1930s Nobel Prize winning scientist George Beadle did an experiment.
He crossed the wild grass with the domesticated corn producing an F1 hybrid
containing one copy of each gene from each parent: one from the wild grass and the other from modern corn.
He then crossed the F1 hybrid with itself producing an F2 hybrid
and scored the phenotype of these plants.
He was looking for how many of these plants resembled modern corn and wild type grass,
and how many were somewhere in between.
If you remember your high school biology and your Punnett squares, you’ll know what to expect.
For each gene one quarter of the plants would carry two copies of the allele from corn and
another quarter two copies originally from the wild grass, with the other half of the
plants retaining one copy from each.
So if one gene was governing the entire phenotypic difference between modern corn and wild Teosinte
we would find one in four plants that look like corn.
If two genes are required only one in sixteen plants will receive two copies of both genes
from corn and another one in sixteen will receive two copies of both genes from the wild grass
We can generalize this to one in four to the power of n.
Where n is the number of genes involved in the phenotypic differences.
Beadle scored about fifty thousand F2 plants and found that one in 500 resembled corn
and another one in 500 resembled wild grass; putting the number of genes dictating the phenotypic
difference at just four or five!
Not too many for our ancient agriculturalists to breed in.
Of course, many other genes have refined corn to be the crop it is today, but fundamentally,
changing a few genes can turn a practically inedible grass into a useful crop.
How could so few genes have such a large effect?
Well the genes involved are regulatory genes whose role is to dictate the expression of
other genes.
In this way these few genes control the expression of dozens of other genes that ultimately lead
to drastic phenotypic change.
(STEP BACK) Well that answers how they changed the plant but leaves the question of why they
would even start cultivating Teosinte in the first place?
(SOLILOQUY) There is a theory for that too, and the key to this is the hard fruit cases.
With heat applied to the Teosinte kernals, water is vaporised inside the kernals
cooking the starch and building up pressure, until the hard fruit case can withstand no more
and it bursts, resulting in white fluffy popcorn.
Then it’s just a matter of growing enough plants to support the cities of the Mesoamerican civilisations.
(STEP BACK) And that was not as easy as it sounds, the Aztec’s solution was the Chinampa,
artificial islands that were created by building up extensions of soil into freshwater lakes.
The Chinampas were made from layering mud over tree trunks stuck in the lake until you
had a legitimate island.
With these islands, they could grow enough food for their massive capital of Tenochtitlan.
They transported the food from the chinampas back to the city using a series of canals.
That’s right, Tenochtitlan had canals, just like Venice.
The water the city sat on, however, was brackish, or semi-salt water, and not so good for farming with
To fix this problem, the Aztecs built a series of dams to keep out the salt water,
and control the levels of the lake.
These dams were even complete with a system of channels to prevent flooding.
You’d actually be surprised by just how many technological innovations pre columbian
indigenous peoples had built.
If you go on over to my channel, I released a video talking about
just a few of these amazing technologies.
(SOLILOQUY) Fantastic, I’ll grab the popcorn, or the Teosinte kernals, and let’s head
over to step back history and find out what else the Aztec’s invented.
You’ll find links to their video in all the usual places