The 1.7-million-year-old Acheulean hand axes were some of the first stone tools. Over the next million years, these chunky teardrop-shaped rocks became widely used before being replaced by finer, more precise flint tips. But how our ancestors' hands evolved into a shape that could make such tools is a bit of a mystery.

Before the hand axes appeared, our ancestors had primitive wrists: good for hanging from branches, but too weak to grasp and handle small objects with much force. And no hand bones had been found to fill the gap between 1.7 million years ago and 800,000 years ago – by which time humans had developed the hands we have today. Now, a new fossil is helping bridge that gap.

In 2010, a team led by Fredrick Kyalo Manthi of the National Museums of Kenya discovered an intriguing bone in the north of the country. Carol Ward of the University of Missouri and colleagues identified it as a third metacarpal, the long bone in the palm between the middle finger and the wrist.

Like modern human metacarpals, it has a small lump at its base – the styloid. This projection helps stabilise the wrist when the hand is gripping small objects between the thumb and fingers. Isotope dating revealed the bone to be about 1.4 million years old. It is likely to have belonged to Homo erectus.