Why do Mezzo Sopranos sing so much lower in modern days?

In opera, Mezzo Sopranos used to never go below the mezzo soprano "tester" G3. They say that note is meant more for altos. There's no way. Are mezzo sopranos voices just getting more flexible as humans evolve because singers like beyonce and mariah carey can sing below low c. Same with altos...my mother is an alto and her speaking voice is about as low as my lowest notes (G 2 ish or lower, Tenor) In operatic music they generally never sang below G3 either. When I listened to leann rimes sing, her G3 sounds like is in the middle of her range...so much so that I could picture her going with ease down to D3 and probably even down further with some vocal exercise. Whenever I hear old ladies talk the talk with ease down to like A2 and notes like E3 sound almost as high as with men. They are altos...But Mariah carey talks that low too.