I was interested to read Adam Benton's post The curious case of the people who forgot how to fish on the Evoanth blog. Basically some 30,000 years ago Tasmania was a peninsula off the coast of Australia and there is good evidence from cave sites about 20,000 years ago that Australians had established themselves on the peninsula. However about 12,000 years ago rising sea levels meant that Tasmania became an island and the population became isolated. Adam argues that the evidence points to them having abandoned fishing (common on the mainland) and lost the skills to make some tools.
Whether his interpretation of the facts is correct (and some of the comments suggest other interpretations) there may have been many comparatively small groups of early humans which became isolated for varying periods. This separation may have been physical, as in this case, or may have been due to different groups exploiting very different environments. Different groups could end up with distinctive tools and cultures and this account reminds me that skills obtained by one group in onne environment are not automatically passed on to later groups in different environments.