During the height of the Atlantic slave trade in the 1600s and 1700s, vervet monkeys, also known as green monkeys, ended up on ships sailing from Africa to the Caribbean Islands of St. Kitts, Nevis, and Barbados. The monkeys have since thrived on the islands. In fact, there are so many that most people view them as pests. Some people have found a use for the monkeys in biomedical research – in one series of experiments the monkeys have human stem cells injected into their brains to study Parkinson's disease.
In humans, Parkinson's disease is a non-fatal degenerative disorder of the central nervous system affecting an individual's motor skills, speech, and other functions due to loss of the neurotransmitter dopamine. The ability of a person to perform everyday activities and to participate as an active member of society generally diminishes as the disease progresses. In addition, the average life expectancy of a person afflicted with Parkinson's disease is commonly lower than for people who do not have the disease, because late-stage Parkinson's may cause complications such as choking, pneumonia, and falls that can lead to death. The number of people suffering from Parkinson's is increasing. A study of the five largest countries in Europe and the ten most populous countries in the world reports that there will be 8.7 million people with Parkinson's disease by the year 2030, a doubling in the number of those currently afflicted.