Is it an either-or thing? The Nature link I responded to above proposes that human ancestors were coastal dwellers for a few 10's of thousands of years to survi

Is it an either-or thing? The Nature link I responded to above proposes that human ancestors were coastal dwellers for a few 10's of thousands of years to survive a climate catastrophe. How long would it take for this to start shaping physiology and behavior? This isn't my area of expertise, it's more fanciful fascination than opinion.