The origins of the Caribbean's first islanders can be traced to two distinct groups, thousands of years apart, suggesting the archipelago was settled by highly mobile people, with distant relatives often living on different islands, a study published Thursday by Nature found.

The islands' first inhabitants, a group of stone tool-users, boated to Cuba about 6,000 years ago, gradually expanding eastward to other islands during the region's Archaic Age, the researchers said.

Where they came from remains unclear, as their genetics do not match any particular Indigenous group, according to the researchers.

However, similar artifacts found in Belize and Cuba may suggest a Central American origin, study co-author William Keegan said in a press release.

Following that first wave, about 2,500-3,000 years ago, farmers and potters related to the Arawak-speakers of northeast South America established a second pathway into the Caribbean from present-day Venezuela, the researchers said.

This group settled initially in Puerto Rico before moving westward and ushering in the region's Ceramic Age, marked by agriculture and the widespread production and use of pottery, according to the researchers.

This "moves our understanding of the Caribbean forward dramatically in one fell swoop," said Keegan, curator at the Florida Museum of Natural History.

"The methods [used in the study] helped address questions I didn't even know we could address," he said.

In the researchers describe as the largest study of ancient human DNA in the Americas to date, David Reich of Harvard Medical School and his team developed a new genetic technique for estimating past population size.

After analyzing the genomes of 263 individuals, researchers found that the number of people living in the Caribbean when Europeans arrived was far smaller than previously thought -- likely in the tens of thousands, rather than in the one million or more range reported by Columbus and others, the researchers said.

The findings are based on an analysis of DNA extracted from 174 people who lived in the Caribbean and Venezuela between 400 and 3,100 years ago, which was compared to genetic data from 89 previously sequenced individuals, they said.

Many present-day Cubans, Dominicans and Puerto Ricans are the descendants of Ceramic Age people, as well as European immigrants and enslaved Africans, with only marginal evidence of Archaic Age ancestry, according to researchers.

Highlighting the region's inter-connectivity, a study of male X chromosomes uncovered 19 pairs of "genetic cousins" living on different islands -- people who share the same amount of DNA as biological cousins but may be separated by generations, the researchers said.

For example, one man was buried in the Bahamas while his relative was laid to rest about 600 miles away in the Dominican Republic, they said.

"Showing relationships across different islands is really an amazing step forward," Keegan said.

Uncovering such a high proportion of genetic cousins in a sample of fewer than 100 men is another indicator that the region's total population size was small, according to Reich, a professor of human evolutionary biology at Harvard.

"One of the most significant outcomes of this study is that it demonstrates just how important culture is in understanding human societies," Keegan said.

"Genes may be discrete, measurable units, but the human genome is culturally created," he said.

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