The arrival of the opposable thumb was a big moment for humankind, and while its importance has long been agreed upon, questions have endured as to exactly when these beauties emerged. Now, new research published in the journal Current Biology has lifted the lid on these snazzy appendages, providing insights as to when thumbs became so dexterous, and how their emergence ties in with novel behaviors in humans.

The study used virtual muscle modeling to assess the biomechanics and efficiency of thumb morphologies across different species among our ancestors. The approach is the first time muscular as well as skeletal structure has been taken into account while examining thumb function. They then compared the ages of the thumb specimens and their dexterity to key steps in the development of human behavior to see what our ancestors were working with when they made significant leaps in their evolution.

"Our methodology integrates cutting-edge virtual muscle modeling with three-dimensional analysis of bone shape and size," said first author and hand biomechanics expert Alexandros Karakostis, in a statement. "This process includes the precise 3D study of the areas of the bones where muscles attach in life. Importantly, we were able to validate the predictions of our models by confirming that the differences observed between living taxa –chimpanzees and modern humans – reflect those reported from past experimental studies."

Their findings suggest that a key ingredient to the thumb’s success as an opposable digit first emerged around 2 million years ago. The remains of those given access to the dexterous upgrade lived in what we now call South Africa and likely belonged to either an early Homo species or the extinct hominin side branch Paranthropus robustus. Interestingly, the pivotal thumb adaptation wasn’t present in the hands of Australopithecines, according to the study, despite being alive around the same time as our opposable-thumb-wielding SA specimens.

Their new-and-improved thumb structure gave these early humans roughly the same dexterity as modern humans. This finding is of particular significance as the timeframe encompassed by the trait’s emergence includes some major steps forward in human behavior as they began their first forays into animal exploitation and became more reliant than ever before on stone tool technologies.

"Increased manual dexterity in the form of efficient thumb opposition was among the early defining characteristics of our lineage, providing a formidable adaptive advantage to our ancestors," said researcher Katerina Harvati of the Eberhard Karls University of Tübingen. "It is likely a crucial element underlying the development of complex culture over the last 2 million years, shaping our biocultural evolution… These consistently high dexterity levels in species of Homo are indicative of the great adaptive value of thumb opposition for human biocultural evolution,"

The team behind the research now want to take a closer look at specific groups including Neanderthals to paint a bigger picture of the range of manual dexterity in early humans and how they may differ from the dexterity we maintain today. It’s hoped further investigation will also reveal new insights into the emergence of systematic tool use among our distant relatives.