His work, How Men Age: What Evolution Reveals About Male Health and Mortality, looks at how natural selection has affected male aging. According to Inverse, he concludes that those with dad bods—pudgy men—have historically lived longer and had more babies.
One reason is pretty easy to understand. Our ancient human ancestors didn't have the most reliable access to food, so men who were able to eat and gain weight typically lived long enough to convince cavewomen to have their cavebabies.
According to the Telegraph, Bribiescas also claims that lower testosterone levels lead to "loss of muscle mass" and "increases in fat mass." Those are the same qualities we associate with that one photo of Leo DiCaprio, and they apparently lead to longer life and a stronger immune system.
Says Bribiescas: "This change in body composition not only causes men to shop for more comfortable trousers but also facilitates increased survivorship and, hypothetically, a hormonal milieu that would more effectively promote and support paternal investment."
Basically, our ancient dads got fat, got tired, and decided to take care of the kids.