Scientists say that the way our hands look today has been shaped by our behaviour over millions of years.
Around 1.7 million years ago, our ancestors' tools went from basic rocks banged together to chipped hand axes.
And the scientists, judging by a fossil that belongs to the oldest known anatomically modern hand, assumed that the strength and dexterity needed to make and use the latter quickly shaped our hands into what they are today.
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.
Hand bones of early Homo erectus are almost unknown, said Richard Potts of the Smithsonian Institution in Washington, DC.
"Having such a well-preserved specimen begins to answer questions about hand evolution," he noted.
Mary Marzke of Arizona State University in Tempe said that the finding shows that our ancestors' hands were already evolving into their modern form 1.4 million years ago.
The forceful, repetitive and sustained processes of tool use, such as digging with rocks, would have made stronger hands desirable, said Marzke.
This would have been particularly useful for knocking off flakes to form and sharpen hand axes, added Potts.
Because the fossil is younger than the first tools, Ward's team believe it is the first evidence of anatomy evolving to suit a new technology.
As stone tools became more widespread, those who had the wrist structure to use them would have had an evolutionary advantage over their weaker-wristed kin.
Ward presented the research at this week's meeting of the American Association of Physical Anthropologists in Knoxville, Tennessee.