After the Civil War battle of Shiloh, Tennessee, in 1862,
16,000 injured soldiers were strewn across the battlefield.
So great were their numbers that neither army was able to retrieve and treat them quickly.
Thousands were left lying in the mud, in some cases for two days and two nights.
Many died from their wounds and the consequent infections.
But at night, some of the injured men noticed a strange blue glow emanating from their wounds.
The light could be seen from a distance.
Field surgeons observed that the soldiers whose wounds emitted this blue light healed more quickly
and had a higher rate of survival than those who didn’t.
They called it the Angel’s Glow.
What was it?
At the time, people believed it was divine intervention:
these men must have been touched by the healing hands of angels.
It took almost 140 years for a scientific explanation to emerge.
A 17-year-old high school student called William Martin,
acting on a hunch, persuaded his friend Jonathan Curtis to help him investigate.
Their paper, which won a national science prize,
argued that the soldiers had been saved by nematodes.
Nematodes are tiny white worms, invisible to the naked eye, that live in the soil.
But how could they have saved the lives of the soldiers lying in the mud at Shiloh?
To see what might have happened,
first we need to understand something that at first sight seems just as miraculous.
Plants can talk.
In fact, their survival depends on talking.
Talking to other creatures in the soil.
Plants talk with chemicals.
They use extremely complex chemical compounds to communicate with other species.
Mostly, they talk to bacteria.
Soil is full of bacteria.
In fact, to a large extent, soil is built by bacteria.
They use glue made from carbon to stick together tiny particles,
building the catacombs that trap air and water.
These structures then become the building blocks for bigger labyrinths,
created by the animals that also live in the ground.
Soil, that we once saw as nothing but a lumpen mass,
turns out to be a biological structure,
like a wasps’ nest
or a beaver’s dam, but much more complex.
Most bacteria, most of the time, exist in limbo.
But when a plant root pushes into a lump of soil,
it starts to talk to them.
It sends the chemical signals that release them from their torpor.
The plant doesn’t want to wake up all the bacteria in the soil.
Some of them will harm it.
So it speaks a language that can be heard only by its friends.
A language that’s specific and precise.
The plant’s chemical messages trigger an explosion of activity.
As the bacteria responding to its call wake up, the plant feeds them with sugar.
They proliferate around the root to form some of the densest microbial communities on Earth.
The bacteria unlock from the soil the nutrients the plant needs.
They produce hormones and other chemicals that help the plant to grow,
and can fire up the plant’s immune system, helping it to fight off disease.
Take a step back and you will see something fascinating.
The relationship between plants and the bacteria in the soil
looks very much like the relationship between humans and the bacteria in our guts.
The soil surrounding a plant’s roots, that we call the rhizosphere,
is, in effect, the plant’s external gut.
But what has all this to do with the soldiers lying in the mud at Shiloh?
Well, plants also cry out for help from larger creatures.
When underground caterpillars attack a plant’s roots,
it can start talking in a different language altogether:
a language understood not by bacteria,
but by those tiny white worms called nematodes.
The nematodes responding to its call use their sharp beaks to pierce the caterpillar's skin.
Then they wriggle into the body cavity
and regurgitate the luminous bacteria, called Photorhabdus luminescens,
that live in their guts.
These luminous bacteria release an insecticide that kills the caterpillar.
They also produce antibiotics that wipe out the microbes that might compete with them.
Then they digest the caterpillar from the inside,
and the nematodes eat the proliferating luminous bacteria.
When they’ve finished,
the nematodes burst from the caterpillar’s corpse into the soil, seeking new prey.
These prey can be easy to find, because the luminous bacteria produce an enzyme called luciferase
that makes the caterpillars they infect glow blue.
The glow seems to attract other caterpillars that can then be attacked.
William Martin and Jonathan Curtis argued that the soil contaminating the soldiers’ wounds
must have contained these predatory nematodes.
Mistaking the soldiers’ flesh for caterpillars,
the nematodes regurgitated their bacteria.
The antibiotics the bacteria produced destroyed the other pathogens infecting the wounds.
Because the luminous bacteria have evolved to infect insects,
whose body temperature is lower than that of humans,
the students speculated that only soldiers with hypothermia could have been saved by them,
which could be why some glowed and others didn’t.
The soil, on which we rely for our survival, is scarcely known to us.
But it turns out to be an ecosystem as diverse and complex as a rainforest
or a coral reef,
an ecosystem built by the creatures that inhabit it,
an ecosystem whose wonders we are only beginning to understand.