Scientists believe that studying the humble zebrafish may have helped solve the mystery of human skin colour.
Zebrafish genes are similar to humans'
A team at Penn State University has found just one tiny change in a key gene plays a major role in determining skin pigmentation.
The finding, published in Science, may help explain why people of European descent have lighter skin than those from Africa.
It is hoped the research may lead to new ways to treat skin cancer.
Potentially, it may also lead to the development of new ways to modify skin colour without damaging it by tanning or using harsh chemical lighteners.
The genetic determination of human skin colour is one of biology's enduring mysteries.
Alterations in some of these genes are associated with disorders such as albinism, which causes very light skin, but also vision problems.
However, most of the genes responsible for normal differences in skin pigmentation have remained unknown.
The gene identified by the Penn State team - called SLC24A5 - had not previously been suspected to be involved in pigmentation.
Zebrafish are ideal for research because they share many similar genes with humans.
They also have similar pigment cells, which, like humans, contain granules called melanosomes.
The researchers found a variant of the zebrafish, called golden, had fewer, smaller and less heavily pigmented melanosomes than normal fish.
They found the lighter pigmentation was caused by a mutation in the SLC24A5 gene which cuts production of a key protein.
Adding protein from the normal zebrafish resulted in fish with a darker colouring.
Next, the researchers analysed data from the human genome, and found a similar pattern.
Most human populations carried the same version of the SLC24A5 gene, but people with a European ancestry carried a variant with just one mutation.
This mutation appears, like the zebrafish, to result in fewer, smaller and lighter melanosomes.
Further analysis showed that among people with mixed European and West African ancestry, those carrying the European form of the gene tended to have lighter skin.
The findings suggest that this single gene controls up to 38% of the colour range in this mixed population.
Researcher Dr Keith Cheng said the importance of the work extended beyond pigmentation.
"We know so little about the genetic and evolutionary architecture of human traits.
"We can not expect to use human genetics to understand complex diseases most effectively without first working out how fundamental characteristics, such as eye, hair, and skin colour, are determined.
"Working out the details of pigmentation with help from model systems like zebrafish is a great paradigm for seeking understanding of other complex diseases."
Dr Emma Knight, of Cancer Research UK, said: "The results of this research are intriguing but we shouldn't jump the gun and speculate about their implications for skin cancer.
"Much more research is needed to work out why Europeans have evolved a different version of SLC24A5 and what function this serves."