Scientists reveal why humans have thumbs

Thumbs come in handy when hopping from channel to channel swissinfo.ch

Researchers in Geneva have found out why a human hand consists of four fingers and a thumb, rather than five fingers.

The discovery is an important step towards understanding how humans evolved and could help explain why some people are born with malformed digits.

The research, published in the current edition of “Science”, was carried out by three biologists from the University of Geneva.

Their aim was to find out why hands develop asymmetrically, with a thumb instead of a fifth finger.

“This is a very important asymmetry because it is thanks to this you can hold a telephone in your hands, that you can turn your hands in all directions and that you can hang from a tree,” said Professor Denis Duboule, who led the scientific team.

“If you had perfectly symmetrical hands you wouldn’t be able to do this, you would only be able to do one kind of movement,” he told swissinfo.

Duboule said that only humans and great apes had the advantage of having a thumb that could be opposed to the other fingers.

This gives the hands an extraordinary mobility, and gives humans in particular an evolutionary advantage. Without thumbs, human beings would not have developed tools.

Architect genes

Scientists have known since the 1980s that certain genes in the body – known as architect genes – are arranged in the same order as the body parts they help to produce.

By studying mice, which are similar in hand structure and development to humans, Duboule and his team could ascertain that thumbs grow in a special way.

“What happens is that very early on during development when the arms just start to grow, the architect genes function only in one side of the arm,” explained Duboule.

“So the asymmetry is created by the asymmetric function of a particular set of genes,” he added.

Duboule says that the discovery should help further understanding of how humans evolved and why and how they developed differently to other mammals or birds.

Deformation

But the professor says there is also practical use to the research. According to Duboule, almost one in 500 children are born with deformed hands, for example, with too many fingers or fused digits.

Understanding how fingers form and the difference between them may provide some clues as to what causes such deformations.

“It doesn’t mean we can cure them because they are genetic diseases, but certainly we can help a lot in designing very powerful diagnostic tools to look at very early foetuses,” Duboule told swissinfo.

“For example, in a family which carries one of these genes, you can make a diagnosis and prevent the disease from occurring in the children,” explained the professor.

A step further

The study was funded by Frontiers in Genetics in Geneva, one of Switzerland’s new national centres of competence in research, designed to promote scientific excellence.

And although the painstaking research took seven years to carry out, Duboule says that the study is not yet over.

“Now that we know what makes the asymmetry, we would like to know how these genes are selectively activated or selectively functional in only this part of the growing arm,” he said.

“That’s a very complex problem for a geneticist, but we think we are about a couple of years away,” added Duboule.

swissinfo, Isobel Leybold

Key facts

Switzerland currently has 14 National Centres of Competence in Research (NCCR).
The programme to foster excellence in science was launched in 2001.
Each centre is managed from a university or renowned research institution.
Some 25 NCCRs will eventually be created.