Scientists may have solved the mystery of how human hands became nimble enough to make and manipulate stone tools.
The team reports in the journal Evolution that changes in our hands and fingers were a side-effect of changes in the shape of our feet.
This, they say, shows that the capacity to stand and walk on two feet is intrinsically linked to the emergence of stone tool technology.
The scientists used a mathematical model to simulate the changes.
Other researchers, though, have questioned this approach.
Campbell Rolian, a scientist from the University of Calgary in Canada who led the study, said: “This goes back to Darwin’s The Descent of Man.
“[Charles Darwin] was among the first to consider the relationship between stone tool technology and bipedalism.”
“His idea was that they were separate events and they happened sequentially – that bipedalism freed the hand to evolve for other purposes.”
“What we showed was that the changes in the hand and foot are similar developments… and changes in one would have side-effects manifesting in the other.”
To study this, Dr Rolian and his colleagues took measurements from the hands and feet of humans and of chimpanzees.
Their aim was to find out how the hands and feet of our more chimp-like ancestors would have evolved.
The researchers’ measurements showed a strong correlation between similar parts of the hand and foot. “So, if you have a long big toe, you tend have a long thumb,” Dr Rolian explained.
“One reason fingers and toes may be so strongly correlated is that they share a similar genetic and developmental ‘blueprint’, and small changes to this blueprint can affect the hand and foot in parallel,” he said.
With this anatomical data, the researchers were able to create their mathematical simulation of evolutionary change.
“We used the mathematical model to simulate the evolutionary pressures on the hands and feet,” Dr Rolian explained.
This model essentially adjusted the shape of the hands or the feet, recreating single, small evolutionary changes to see what effect they had.
By simulating this evolutionary shape-shifting, the team found that changes in the feet caused parallel changes in the hands, especially in the relative proportions of the fingers and toes.
These parallel changes or side-effects, said Dr Rolian, may have been an important evolutionary stem that allowed human ancestors, including Neanderthals, to develop the dexterity for stone tool technology.
Robin Crompton, professor of anatomy at the UK’s Liverpool University, said the study was very interesting but also raised some questions.
“I am not personally convinced that the foot and hand of chimpanzees are a good model [of human ancestors' hands and feet] – the foot of the lowland gorilla may be more interesting in this respect,” he told BBC News.
He pointed out that there was a lot more to the functional shape and biomechanics of the human foot than just its proportions.
Paul O’Higgins, professor of anatomy at the Hull York Medical School, UK, said: “The results are quite exciting and will doubtless spur further testing and additional work.”
Researchers presented earlier today new evidence that neanderthals were more competitive & promiscuous than we are today! Maybe more surprizing is the method which the researchers used to acquire their new findings: via finger length measurements!
The study, published in the Proceedings of the Royal Society B, draws upon a famous and controversial indicator of social behavior: the comparative length of the index finger and the ring finger, also known as the 2D:4D finger ratio. If the ring finger is longer than the index finger, that’s supposed to be correlated with higher prenatal exposure to androgens — resulting in a higher proclivity for aggressiveness and promiscuity.
Scientists, in collaboration with researchers at the universities of Southampton and Calgary, used finger ratios from fossilised skeletal remains of early apes and extinct hominins, as indicators of the levels of exposure species had to prenatal androgens – a group of hormones that is important in the development of masculine characteristics such as aggression and promiscuity.
It is thought that androgens, such as testosterone, affect finger length during development in the womb. High levels of the hormones increase the length of the fourth finger in comparison to the second finger, resulting in a low index to ring finger ratio (2D:4D digit ratio). Researchers analysed the fossil finger bone ratios of Neanderthals and early apes, as well as hominins, Ardipithecus ramidus and Australopithecus afarensis, to further understanding of their social behaviour.
The team found that the fossil finger ratios of Neanderthals, and early members of the human species, were lower than most living humans, which suggests that they had been exposed to high levels of prenatal androgens. This indicates that early humans were likely to be more competitive and promiscuous than people today.
The results also suggest that early hominin, Australopithecus – dating from approximately three to four million years ago – was likely to be monogamous, whereas the earlier Ardipithecus appears to have been highly promiscuous and more similar to living great apes. The research suggests that more fossils are needed to fully understand the social behaviour of these two groups.
Dr Susanne Shultz, from the Institute of Cognitive and Evolutionary Anthropology at the University of Oxford describes:
“Social behaviours are notoriously difficult to identify in the fossil record. Developing novel approaches, such as finger ratios, can help inform the current debate surrounding the social systems of the earliest human ancestors.”
And Dr Emma Nelson, an archaeologist from the University of Liverpool, argues that comparing the finger-length ratios of extinct and present-day species is a valid technique for making an indirect assessment of our long-gone ancestors’ social behavior. She said:
“It is believed that prenatal androgens (male sex hormones) affect the genes responsible for the development of the fingers, toes and the reproductive system. We have recently shown that promiscuous primate species have low index to ring finger ratios, while monogamous species have high ratios.”
“We used this information to estimate the social behaviour of extinct apes and hominins. Although the fossil record is limited for this period, and more fossils are needed to confirm our findings, this method could prove to be an exciting new way of understanding how our social behaviour has evolved.