From using tools to texting, opposable thumbs have been a big hit with humanity, and new research may have uncovered just when these dexterous digits evolved. 

Researchers have analysed the biomechanics and efficiency of the thumb across different fossil human species using virtual muscle modeling to uncover new insight into when these abilities first arose and what they have done for the development of more complex human culture. 

The findings suggest that a fundamental aspect of human thumb opposition first appeared approximately 2 million years ago and was not found in the earliest proposed stone tool makers.

Essentially, the opposable thumb came first. 

“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 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.”

The researchers showed that thumb efficiency and dexterity had increased to a significant extent in hominins that lived 2 million years ago in South Africa. 

At the same time, they found that the degree of this dexterity was consistently lower in the earliest proposed tool-making species, the Australopithecines. That includes the species Australopithecus sediba, which is also dated to approximately 2 million years ago. 

It is a notable finding because researchers had previously suggested that the human-like thumb proportions of A. sediba reflected tool-making capabilities.

“One of the greatest surprises was to find that hominin hand fossils from the Swartkrans site in South Africa, which date to ca. 2 million years ago and are attributed to either early Homo or to the extinct hominin side branch Paranthropus robustus, could achieve a thumb-using dexterity similar to that of modern humans,” says first author and hand biomechanics expert Alexandros Karakostis.

The new findings further show that later-arising species, belonging to our own genus Homo (including Neanderthals as well as early and recent Homo sapiens) share similarly high degrees of manual dexterity. Those findings applied also to the small-brained species Homo naledi, despite the fact that this species has not yet been found in association with stone tools.

“These consistently high dexterity levels in species of Homo are indicative of the great adaptive value of thumb opposition for human biocultural evolution,” Dr Harvati says.

The most important implication of the new findings is that an early increase of thumb dexterity about 2 million years ago may have been a foundation for the gradual development of complex culture. 

The study highlights that this timeframe includes important biocultural developments such as the appearance of the large-brained Homo erectus lineage and its dispersal out of Africa. Around the same time, humans gradually began to exploit animal resources and to rely more heavily on stone tool technologies.

The study is accessible here.