Shellfish are easier to harvest than most other sea meats. You can often find them laying on the rocks when tides are low. Those that are anchored like muscles can be found at the same place every time. Others need only a short dive. THeir only defense is the shell, which even not so brilliant animals can learn to open. Birds are known to fly high and drop shellfish so they will break on rocks. I am sure even a low IQ protohuman could have figured out how to smash them open with a rock. Or people with control of fire could just roast em right in the shell. They have plenty of protein and some good good nutrients in them. I'd say they would have been a very important and convenient food source for ocean dwellers and probably eaten often. But IMO, they would not have been as much relished as food sources that were rich in fat, unless of course those peoples had ready access to coconuts already. If you had coconuts and shellfish readily available, you'd be in pretty good shape for overall survival. YOu'd probably want to supplement with other things, but you could certainly get by a long time as long as you had protein and fat.