Plains Indians depended on buffalo for everything: meat for food, hide for clothing, horns and bones for weapons and tools.

One of the ways they hunted them was by stampeding them over cliffs.

Scientists thought that humans practiced communal hunting drives like this as far back as 5,000 years ago. But a recent discovery has changed that.

When constructing a new airport north of Mexico City, workers unearthed enormous bones and called in scientists, who discovered two massive pits 80 feet in diameter.

They realized the 800 bones within the pits had come from woolly mammoths. There were herds that lived in the area long ago.

They were also able to determine, based on sediment layers deposited within the pits and tool marks on their walls, that humans had dug them 15,000 years ago, by hand.

It appeared that early tribes had driven the giant beasts into them, trapping them for slaughter.

Like the Plains Indians, these hunters were resourceful with the animals, turning bones into knives and scrapers, which they used in butchering. Some of these were found in the pits.

It also appeared that many generations of humans hunted mammoths here, using this site for more than 500 years.

When they finally abandoned the pits, they left the mammoth bones artfully arranged inside, with tusks and shoulder blades encircling skulls, perhaps in tribute to the animals that fed and clothed them for centuries.