Today marks One Direction's fourth anniversary, and we've been celebrating all day. While #FourYearsAgoFiveStrangersBecameBrothers started trending on Twitter in honor of the 1D boys, it's actually true! The members of the band have given us evidence that they really are like brothers — and it is adorable.

Niall Horan was the first to chime in on their anniversary, tweeting about how he couldn't believe the band only got together four years ago. He finished off with a shocking photo from their X Factor days, saying that his 1D bandmates are "family" to him now.

But it wasn't just Niall who expressed his love. Harry Styles and Liam Payne also posted adorable statuses in honor of One Direction's fourth anniversary. Harry gushed about how "lucky" he feels to be a part of the band, and seriously thanked their fans.

harry styles 1d anniversary tweet

Meanwhile, we already know how much Liam loves his fellow 1D members, but we were still touched to hear that he appreciates the band and their Directioners. He echoed Niall and Harry, tweeting that he couldn't believe they had already been together for four years.

As if that wasn't heartwarming enough, Louis Tomlinson recently told Top of the Pops that the band has a little 1D family. "I think 'family' is probably the best way of describing us," he said. "It's different to friendship.”

Come on, how cute is that?

Do you believe the boys of One Direction really are like brothers? Sound off, and send your thoughts on their anniversary in the comments!