A discovery dating back to the Stone Age raises a mystery about the users of tools

At a site in Kenya, archaeologists have unearthed some of the oldest stone tools ever discovered, raising the mystery of who used them.

According to a study published Thursday, early humans used tools to cut up hippos for their meat around 2.9 million years ago.

Scientists previously assumed that our immediate ancestors made and used these tools. But teeth of an extinct human cousin were also found at the site, indicating that other hominin species may have used these tools as well.

The artifacts are possibly the earliest example of what is known as the Oldowan toolkit, which spread across Africa and beyond during the Stone Age.