Where are the three mains regions of rainforest in the world?

answer:   migrations were important in the neolithic period. "two major migrations passing through southeastern europe were confirmed by the means of archaeo-genetic studies."

the first migration was the early neolithic period -- 6,000 before common era (bce) -- when the first farmers, from anatolia -- asia minor -- spread through europe. the second migration occurred during the early bronze age (3,000-2,500 bce) when the so-called "steppe population," from the eurasian steppe, replaced much of northern europe's previous population.

the first farmers of northern and western europe passed through southeastern europe with limited hunter-gatherer genetic admixture, which occurs when two or more previously isolated populations begin interbreeding. however, some groups that remained mixed extensively -- without the male-biased, hunter-gatherer admixture that prevailed later in the north and west, according to latest discoveries. southeastern europe continued to be a nexus between east and west, with intermittent genetic contact with the steppe people up to 2,000 years before the migrations that replaced much of northern europe's population.

"in some places, hunter-gatherers and incoming farmers seem to have mixed very quickly," says iain mathieson, a geneticist at the university of pennsylvania, who was first author of the paper. "but, mostly, the two groups remained isolated, at least for the first few hundred years. these hunter-gatherers had been living there for thousands of years, and it must have been quite a shock to have these new people show up -- with a completely different lifestyle and appearance."

the columbian exchange, also known as the columbian interchange, named for christopher columbus, was the widespread transfer of plants, animals, culture, human populations, technology, and ideas between the americas, west africa, and the old world in the 15th and 16th centuries, related to european colonization and trade following christopher columbus's 1492 voyage. invasive species, including communicable diseases, were a byproduct of the exchange. the changes in agriculture significantly altered and changed global populations. the most significant immediate impact of the columbian exchange was the cultural exchanges and the transfer of people between continents.

the new contact between the global population circulated a wide variety of crops and livestock, which supported increases in population in both hemispheres, although diseases initially caused precipitous declines in the numbers of indigenous peoples of the americas. traders returned to europe with maize, potatoes, and tomatoes, which became very important crops in europe by the 18th century.

the term was first used in 1972 by american historian alfred w. crosby in his environmental history book the columbian exchange. it was rapidly adopted by other historians and journalists and has become widely known.