Thumbs up (and over)

Dexterous thumbs – considered a hallmark of being human – were present two million years ago, according to a study published in the journal Current Biology.

This was around the time that more systematic tool production and complex cultural developments emerged and our large-brained Homo erectus ancestors appeared on the scene, highlighting the pivotal importance of thumb evolution.

“Increased thumb dexterity [offered] a crucial evolutionary advantage for the gradual development of sophisticated behaviour and culture,” says senior author Katerina Harvati, from Eberhard Karls University of Tübingen, Germany.

“Importantly, the fact that all later species of Homo maintained, or independently developed, increased levels of dexterity underlines the vital adaptive value of thumb opposition efficiency in human evolution.”

Until now, it wasn’t clear when efficient human-like manual dexterity appeared, and which hominin species first developed this ability. Previous efforts to study the evolution of thumb dexterity compared the skeletal anatomy of modern humans and earlier hominins.

Harvati’s team, including hand biomechanics expert and first author Alexandros Karakostis, pioneered a novel approach integrating virtual modelling of muscle tissue, which isn’t present in fossils, with skeletal morphology of thumbs.

“This process includes the precise 3D study of the areas of the bones where muscles attach in life,” explains Karakostis.

Notably, they validated the model’s predictions by applying them to modern humans and chimpanzees with known muscle parameters to confirm the differences.

Results showed that the earliest species thought to have made tools, the Australopithecines, had consistently lower efficiency than modern humans, similar to that of present-day chimpanzees.

“This was true also of the species Australopithecus sediba,” says Harvati, “which shows some modern human-like characteristics in its hand skeleton that were previously interpreted as reflecting tool-making capabilities.”

In contrast, more recent hominins showed greater levels of thumb dexterity similar to those of modern humans, including Neanderthals and Homo sapiens, as well as Homo naledi.

“It is worth noting that the enigmatic and small-brained species Homo naledi also showed similarly high levels of dexterity,” says Harvati, “despite the fact that its cultural remains are still to be discovered.”

But “such enhanced manual abilities in this small-brained species suggest a decoupling of the traditionally assumed correlation between brain size and tool-using skills in the fossil record”, the authors write.

This discovery was derived from two-million-year-old fossil hominin bones found at the Swartkrans site, near Johannesburg in South Africa.

This supplemental video shows the steps of the study’s novel and integrative analytical procedure. Credit: Professor Katerina Harvati Dr Alexandros Karakostis and Dr Daniel Haeufle.