It is limiting the way researchers presume the burden of proof has to go the other way. The extreme case of this is with homo erectus being found on Crete, but we are not allowed to suggest that they knew how to build a raft. No, we have to assume that they swam to Crete, this is much more logical than assuming they knew how to build a raft.
"Crete has been surrounded by vast stretches of sea for some five million years. The discovery of the hand ax suggests that people besides technologically modern humans—possibly Homo heidelbergensis—island-hopped across the Mediterranean tens of thousands of millennia earlier than expected."
I'm getting really worn out by some commenters here. Assertions are made, not backed up, and often plain wrong. It's not what HN is supposed to be about.
For what it's worth I'm not sure that ocean travel is actually that common across human cultures. For instance, Madagascar was first inhabited by people who sailed from Indonesia ~1200-1500 yrs ago.
