SAN FRANCISCO — The decade-plus evolution of YouTube from repository of cat videos and pirated content to potential TV replacement hit a road bump last year when marketers discovered their advertisements were showing up next to extremist videos and other unsavory content.

Ever since, YouTube has been scrambling to come up with new policies that give advertisers more control over where their ads go and provide additional assurance that the videos on the online service are better screened.

On Tuesday in a blog post, YouTube said it had altered the threshold for which videos can accept advertisements and pledged more human oversight of its top-tier videos. If that sounds familiar, that’s because YouTube has made similar promises in the past.

“There’s no denying 2017 was a difficult year, with several issues affecting our community and our advertising partners,” Paul Muret, a Google vice president, wrote in the blog post. “We are passionate about protecting our users, advertisers and creators and making sure YouTube is not a place that can be co-opted by bad actors.”