Lactose tolerance may not be such an old trait in humans, as new analysis found it was rare as recently as the Bronze Age.

According to BBC News, authors of a study published in the journal Nature examined the genomes of 101 ancient adults. The humans who lived 3,000 to 5,000 years ago apparently had not yet widely developed the gene necessary for digesting milk's lactose sugar.

"It seems like the Bronze Age is the period where the genetic diversity and distribution that we know today is basically formed," study co-author Eske Willerslev, from the University of Copenhagen in Denmark, told BBC News.

Now highly common among Europeans, researchers initially believed the lactose digestion gene developed in the Neolithic age as a result of cattle domestication.

"At the end of the Bronze Age, this genetic ability is still extremely rare," Willerslev said. "We see a slight increased frequency not in Europe but to the east, on the steppe. One possibility is that after the Bronze Age, this genetic ability was brought into Europe and is then selected for."

The research was also a big-picture analysis of the Bronze Age affected migrations and lifestyles that would influence what modern Europe would eventually look like. For example, the Bronze Age was when Europeans transitioned from hunter-gatherers to farmers. The researchers also found the Bronze Age was a period where genes linked to light skin and eye color became more prevalent.

"Our study is the first to apply these recent methods on that large of a scale, so it will provide the basis for many future studies to unravel some of the more detailed questions," study co-author Martin Sikora, of the Center for GeoGenetics at the University of Copenhagen, told NBC News. "We have really only scratched the surface of what can be done with these data."