RESEARCH : THE ZOMBIE ANT

ZOMBIE ant staggers up a tree. Then explodes.
Evolutionary biologists retrace the history of life in all its wondrous forms. Some search for the origins of our species. Others hunt for the origin of birds.

Last month, a team of researchers reported an important new insight into the origins of zombies - in this case, ants zombified by a fungus.

Here's how it works : Sometimes an ant marching about its business outdoors, will step on a fungal spore. It stick's to the ant's body and slips a fungal cell inside.

The fungus called, Ophiocordyceps, feeds on the ant from within and multiplies into new cells. But you wouldn't know it, because the ant goes on with its life, foraging for food to take back to the nest. All the while, the fungus keeps growing until it makes up nearly half of the ant's body mass.

When Ophiocordyceps is finished feeding on its host, the fungal cells gather inside the ant's body. They form a mat and push needlelike projections into the ant's muscle cells. The fungal cells also send chemical signal to the ant's brain,  causing the host to do something strange. The ant leaves the its nest and climbs a nearby plant.

In the tropics, where many species of Ophiocordyceps live, the fungus drives ants upward, to a leaf above the ground. The ant bites down, its jaws locking as it dies.

The fungus sends out sticky threads that glue the corpse to the leaf. And now its ready to take the next step in its life cycle : Out of the ant's head bursts a giant stalk, which showers spores onto trails below.

''The ants are walking over a minefield,'' said David Hughes, an expert on Ophiocordyceps well at Pennsylvania State University.

Naturalists published their first accounts of Ophiocordyceps well over a century ago. But only in recent years have researchers investigated how these fungi go about zombifying ants.

As it turns out, it is an exquisitely intricate process  that leaves researchers with many questions. Scientists don'y know what chemicals get into the host's brain and causes it to leave the nest and climb.

''We still haven't found the smoking gun,'' Dr. Hughes said.

In 2010, Dr. Hughes and his colleagues identified a  48-million-year old fossil of zombie and with a death grip on a leaf.

The fossil demonstrated that zombifying fungi have been around a long time. But it didn't offer any hints as to how the fungus evolved from its ordinary ancestors.

''You think, where the hell did that come from?'' Dr. Hughes said.

He suspected that the answer might be lurking in the diversity of living fungi. But before the age of  DNA sequencing, researchers struggled to classify Ophiocordyceps.

In 2013, one of Dr. Hughes's graduate students, Jolio Araujo, began sequencing the DNA of fungi in scientific collections. He also went on expeditions of his own, turning over leaves to find zombie ants.

If they were a species he hadn't seen before, Dr. Araujo photographed them and pried off the tiny bodies to bring home.

WHAT was once thought to be one species of fungus now turns out to be at least 28. Each zombifies a different species of ants or attacks other insects.

These species all belong to a much bigger group of fungi. Many of their relatives feed on dead plants, while some insects - mostly a group called hemipterans, which includes aphids and cicadas.

Dr. Araujo, now a research fellow at University of the Ryukyus in Japan, analyzed the DNA of more than 600 of these related species. Comparing the genetic sequences, he was able to draw a fungal family tree.

The honor and serving of the latest operational research on zombie ants, continues. The World Students Society thanks author Carl Zimmer.