Early humans were regularly using animal bones to make cutting tools 1.5 million years ago.

A newly discovered cache of 27 carved and sharpened bones from elephants and hippos found in Tanzania's Olduvai Gorge site pushes back the date for ancient bone tool use by around 1 million years. Researchers know that early people made simple tools from stones as early as 3.3 million years ago.

The new discovery, published Wednesday in Nature, reveals that ancient humans “had rather more complex tool kits than previously we thought,” incorporating a variety of materials, said William Harcourt-Smith, a paleoanthropologist at the American Museum of Natural History, who was not involved in the research.

The well-preserved bone tools, measuring up to around 16 inches (40 centimeters), were likely made by breaking off the thick ends of leg bones and using a stone to knock off flakes from the remaining bone shaft. This technique was used to create one sharpened edge and one pointed tip, said study co-author Ignacio de la Torre, a researcher at the Spanish National Research Council.

The bone tools were “probably used as a hand axe” – a handheld blade that’s not mounted on a stick – for butchering dead animals, he said.

Such a blade would be handy for removing meat from elephant and hippo carcasses, but not used as a spear or projectile point. “We don’t believe they were hunting these animals. They were probably scavenging,” he said.

Some of the artifacts show signs of having been struck to remove flakes more than a dozen times, revealing persistent craftsmanship.

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The uniform selection of the bones – large and heavy leg bones from specific animals – and the consistent pattern of alteration makes it clear that early humans deliberately chose and carved these bones, said Mírian Pacheco, a paleobiologist at the Federal University of Sao Carlos in Brazil, who was not involved in the study.

The bones show minimal signs of erosion, trampling or gnawing by other animals — ruling out the possibility that natural causes resulted in the tool shapes, she added.

The bone tools date from more than a million years before our species, Homo sapiens, arose around 300,000 years ago.

At the time the tools were made, three different species of human ancestors lived in the same region of East Africa, said Briana Pobiner, a paleoanthropologist at the Smithsonian’s Human Origins Program, who was not involved in the study.

The tools may have been made and used by Homo erectus, Homo habilis or Paranthropus boisei. “It could have been any of these three, but it’s almost impossible to know which one,” said Pobiner.