But, the researchers found something strange in the data. Between 1800 and 1850 the distance couples traveled to marry each other doubled -- probably because rapid transportation made railroad travel possible in most of Europe and the United States. However, that increase in distance traveled to marry someone was accompanied by an increase in genetic relatedness between marriage partners.
In other words, during this 50-year period, people traveled farther to marry closer relations.
"Families dispersed, and people started taking the train to marry their cousin," Erlich said.
This observation implies that it was changing social norms, rather than access to rapid transit, that was the primary trigger for people to search genetically further afield than fourth cousins when it came to finding a spouse, Erlich said.
The authors also addressed an ongoing debate about the inheritability of longevity. According to their data set, previous studies have probably overestimated the heritability of this particular trait.
"We should lower our expectations about our ability to predict longevity from genomic data," they wrote.
But Erlich said these investigations just scratch the surface of what we can learn from a massive family tree.
"There are many questions we want to ask," he said.
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