Family tree spanning 13 million people reveals a lot of married cousins


02 March 2018 | Thomas McMullan | Alphr

“Geneticists have leveraged crowdsourced data from a geneology site to create a family tree that encompasses a staggering 13 million people. In the arboretum of human lives, this is the biggest family canopy ever to be amassed.

The outcome throws a light on the cultural and genetic movements of families across around 11 generations over a period of 350 years. Gleaning information from this mammoth tangle of lives, loves and children, researchers have been able to chart migration and marriage habits across Europe and North America.

They found shifting travel patterns, heredity influence, as well as a tendency for people in the 19th century to marry their cousins.

To create the tree, a team led by New York Genome Center geneticist Joanna Kaplanis collected 86 million records from genealogy website, a database of genealogical information maintained by enthusiasts.

The site compares and contrasts users’ families trees, which was useful for the researchers’ aim of analysing and combining as many of these trees as possible.

In result, published in Science, is a transformation of those records into 5.3 million family trees. The biggest of these is a tree that connects 13 million people, covering a timespan of 350 years between 1650 and 2000.

Mobility and marriage

Poring through the multiplicity of connections, the researchers were able to make a few interesting judgements about how human lives have changed over 11 generations. One thing they noticed was a tendency over the three centuries for women to move more often than men. On the other hand, when men did move, they tended to do so over larger distances.

“One potential explanation is that males tend to stay in their home town due to better economic opportunities: maybe a shop that they inherited or land. This creates pressure, or a social norm, for females to migrate closer to the home town of their husband,” study co-author Yaniv Erlich, a geneticist at Columbia University, told Ars Technica.

Erlich added that the larger distances travelled by men may be partly connected to the effects of war, with soldiers crossing continents before settling down with a wife in another country.

(A 70,000-person family tree, of individuals connected through marriage (in red) and shared ancestors. Credit: Columbia University)

This relationship between gender and migration remains consistent up to late 20th century, but one thing that has changed substantially is the patterns between marriage and close relatives.

From 1650 to the early nineteenth century, the average married couple in Europe and North America would have been born within 8km of each other, and would have been fourth cousins.

With the development of the railway and steamships, people tended to marry people that had been born further away. You would assume that this meant that individuals living in the early 19th century would have been less likely to marry their cousins.

Nope. People born between 1800 and 1850 actually showed a greater tendency to marry relatives, and those relatives tended to be closer in terms of family distance.

This shifted for people born after 1850, who were a much less likely to marry cousins than earlier generations. Still, right at the boom of modern transport, there seemed to be something of a cousin-marrying-bubble. The researchers think this might have something with changing taboos around kissing cousins during the opening decades of the 19th century.

Longevity in your genes

Another interesting insight from the family tree is about the effect of genetics on how long people tend to live. Previous studies have tended to claim genes account for around 25% of a reason for someone’s longevity, but this new study suggests that this figure may be closer to 16%.

Not everyone agrees with this finding. Paola Sebastiani, professor of biostatistics at the Boston University School of Public Health, told National Geographic that the results should be approached with caution, and that longevity is not best studied with big data. Sebastiani nevertheless applauded the team efforts to pull together masses of complex hereditary information.

Perhaps one of the most interesting aspects of the study is its use of crowdsourced data on a massive scale. This type of genealogical research has the potential to pull on heredity information in a way that would have been impossible only a decade ago, and could signal a new way of thinking about the study of human lineage. The authors of the study have also made the dataset available for further academic research.

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