Mammals evolved during the Triassic period. I believe the first mammal was like a tiny shrew-like animals. They called it the Eutherian. But the actually mammals were the dinosaurs.
This causes a net force on the object acting upwards. ... So, for objects that weigh less, per unit of volume than water, the buoyancy force will cause the object to float. For objects that weigh more, per unit of volume (i.e: are more dense) the buoyancy force can't quite win out and the object will sink.
The drift explanation states that the people with higher socio-economic status (SES) tend to have better lives. It further states that genetics and biology plays an important role in the socio-economic status so that healthier people can enjoy their SES for longer period of time.