Stone tools influenced hand evolution in human ancestors.

KENT, MARCH 2011 -  New research from anthropologists at the University of Kent has confirmed Charles Darwin’s speculation that the evolution of unique features in the human hand was influenced by increased tool use in our ancestors.

Research over the last century has certainly confirmed the existence of a suite of features in the bones and musculature of the human hand and wrist associated with specific gripping and manipulatory capabilities that are different from those of other extant great apes. These features have fuelled suggestions that, at some point since humans split from the last common ancestor of living apes, the human hand evolved away from features adapted for locomotion toward alternative functions.

Now, researchers Dr Stephen Lycett and Alastair Key have shown that the hands of our ancestors may have been subject to natural selection as a result of using simple cutting tools. In a series of experiments that used stone flakes similar to those known from Africa around 2.6 million years ago, they analysed whether variation in the hand size of individual tool users reflects differences that affect the efficiency of these simple tools to cut through a rope.

Their results, published in the Journal of Archaeological Science, show that ‘biometric’ variation did indeed result in a significant relationship with cutting efficiency in the experimental task.

Hands holding stones: dexterity of the wrist joints, of the hands and fingers, enabled early humans to create and use stone tools - which had an important rule in the evolution of human civilization.

Dr Lycett, Senior Lecturer in Human Evolution at the University’s School of Anthropology and Conservation, explained: ’140 years ago, writing from his home at Down House in Kent, Darwin proposed that the use of stone tools may have influenced the evolution of human hands.

‘Our research suggests that he was correct. From a very early stage in our evolution, the cultural behaviour of our ancestors was influencing biological evolution in specific ways.’

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 hand bones of an early hominid - fosile presented by Wesley Niewhoener.

The hand bones of an early hominid

 

The rigth hand of 'Neanderthal Qafzeh 9'.

The hand bones of a neanderthal

 

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.

READ MORE ABOUT EARLIER RESULTS:
• Primate finger length linked with social behavior
• Human finger length & sexual orientation
• Evolution of the human hand & palmar creases

The hand of a white faced Capuchin primate ape.

Finger length in primates linked with cooperative, competitive, and sexual behavior!

Research at the universities of Liverpool and Oxford into the finger length of various primate species has revealed that cooperative behavior is linked to exposure to hormone levels in the womb!

The sientists have used finger ratios as an indicator of the levels of exposure to the hormone and compared this data with social behaviour in primate groups.

The team found that Old World monkeys, such as baboons and rhesus macaques, have a longer fourth finger in comparison to the second finger, which suggests that they have been exposed to high levels of prenatal androgens. These species tend to be highly competitive and promiscuous, which suggests that exposure to a lot of androgens before birth could be linked to the expression of this behaviour.

Other species, such as gibbons and many New World apes, have digit ratios that suggest low levels of prenatal androgen exposure. These species were monogamous and less competitive than Old World monkeys.

The results show that Great Apes, such as orangutans and chimpanzees, expressed a different finger ratio. The analysis suggests that early androgen exposure is lower in this groups compared to Old World monkeys. Lower androgen levels could help explain why Great Apes show high levels of male cooperation and tolerance.

HOW CAN WE UNDERSTAND THESE RESULTS?

Emma Nelson

Primate finger length researcher Emma Nelson explains:

“It is thought that prenatal androgens affect the genes responsible for the development of fingers, toes and the reproductive system. High androgen levels from a foetus or mother during pregnancy, may alter gene function and lead to subtle changes in relative digit length and the functioning of the reproductive system. Finger ratios do not change very much after birth and appear to tell us something about how very early androgens affect adult behaviour, particularly behaviour linked to mating and reproduction.”

ILLUSTRATION: A comparison of the human hand with primate hands reveals that only the human hand is featured with a long opposable thumb!
Comparison of primate hands: only the human hand is featured with a long opposable thumb!

SUGGESTION FOR FURTHER READING:

Finger length linked with social behavior!
Understanding our past: “the primate hand vs. the human hand”!
More finger length reports!
Evolution of the human hand & the mystery of the 5 fingers!