Let's now conclude our whirlwind tour around the globe by devoting five minutes to the last continent, Australia. Here we go again, for the last time.

In modern times, Australia was the sole continent still inhabited only by hunter/gatherers. That makes Australia a critical test of any theory about continental differences in the evolution of human societies. Native Australia had no farmers or herders, no writing, no metal tools, and no political organization beyond the level of the tribe or band. Those, of course, are the reasons why European guns and germs destroyed Aboriginal Australian society. But why had all Native Australians remained hunter/gatherers?

There are three obvious reasons. First, even to this day no native Australian animal species and only one plant species (the macadamia nut) have proved suitable for domestication. There still are no domestic kangaroos.

Second, Australia is the smallest continent, and most of it can support only small human populations because of low rainfall and productivity. Hence the total number of Australian hunter/gatherers was only about 300,000.

Finally, Australia is the most isolated continent. The sole outside contacts of Aboriginal Australians were tenuous overwater contacts with New Guineans and Indonesians.

To get an idea of the significance of that small population size and isolation for the pace of development in Australia, consider the Australian island of Tasmania, which had the most extraordinary human society in the modern world. Tasmania is just an island of modest size, but it was the most extreme outpost of the most extreme continent, and it illuminates a big issue in the evolution of all human societies. Tasmania lies 130 miles southeast of Australia. When it was first visited by Europeans in 1642, Tasmania was occupied by 4,000 hunter/gatherers related to mainland Australians, but with the simplest technology of any recent people on Earth. Unlike mainland Aboriginal Australians, Tasmanians couldn't start a fire; they had no boomerangs, spear throwers, or shields; they had no bone tools, no specialized stone tools, and no compound tools like an axe head mounted on a handle; they couldn't cut down a tree or hollow out a canoe; they lacked sewing to make sewn clothing, despite Tasmania's cold winter climate with snow; and, incredibly, though they lived mostly on the sea coast, the Tasmanians didn't catch or eat fish. How did those enormous gaps in Tasmanian material culture arise?

The answer stems from the fact that Tasmania used to be joined to the southern Australian mainland at Pleistocene times of low sea level, until that land bridge was severed by rising sea level 10,000 years ago. People walked out to Tasmania tens of thousands of years ago, when it was still part of Australia. Once that land bridge was severed, though, there was absolutely no further contact of Tasmanians with mainland Australians or with any other people on Earth until European arrival in 1642, because both Tasmanians and mainland Australians lacked watercraft capable of crossing those 130-mile straits between Tasmania and Australia. Tasmanian history is thus a study of human isolation unprecedented except in science fiction � namely, complete isolation from other humans for 10,000 years. Tasmania had the smallest and most isolated human population in the world. If population size and isolation have any effect on accumulation of inventions, we should expect to see that effect in Tasmania.

If all those technologies that I mentioned, absent from Tasmania but present on the opposite Australian mainland, were invented by Australians within the last 10,000 years, we can surely conclude at least that Tasmania's tiny population didn't invent them independently. Astonishingly, the archaeological record demonstrates something further: Tasmanians actually abandoned some technologies that they brought with them from Australia and that persisted on the Australian mainland. For example, bone tools and the practice of fishing were both present in Tasmania at the time that the land bridge was severed, and both disappeared from Tasmania by around 1500 B.C. That represents the loss of valuable technologies: fish could have been smoked to provide a winter food supply, and bone needles could have been used to sew warm clothes.

What sense can we make of these cultural losses?

The only interpretation that makes sense to me goes as follows. First, technology has to be invented or adopted. Human societies vary in lots of independent factors affecting their openness to innovation. Hence the higher the human population and the more societies there are on an island or continent, the greater the chance of any given invention being conceived and adopted somewhere there.

Second, for all human societies except those of totally-isolated Tasmania, most technological innovations diffuse in from the outside, instead of being invented locally, so one expects the evolution of technology to proceed most rapidly in societies most closely connected with outside societies.

Finally, technology not only has to be adopted; it also has to be maintained. All human societies go through fads in which they temporarily either adopt practices of little use or else abandon practices of considerable use. Whenever such economically senseless taboos arise in an area with many competing human societies, only some societies will adopt the taboo at a given time. Other societies will retain the useful practice, and will either outcompete the societies that lost it, or else will be there as a model for the societies with the taboos to repent their error and reacquire the practice. If Tasmanians had remained in contact with mainland Australians, they could have rediscovered the value and techniques of fishing and making bone tools that they had lost. But that couldn't happen in the complete isolation of Tasmania, where cultural losses became irreversible.

In short, the message of the differences between Tasmanian and mainland Australian societies seems to be the following. All other things being equal, the rate of human invention is faster, and the rate of cultural loss is slower, i n areas occupied by many competing societies with many individuals and in contact with societies elsewhere. If this interpretation is correct, then it's likely to be of much broader significance. It probably provides part of the explanation why native Australians, on the world's smallest and most isolated continent, remained Stone Age hunter/ gatherers, while people of other continents were adopting agriculture and metal. It's also likely to contribute to the differences that I already discussed between the farmers of sub-Saharan Africa, the farmers of the much larger Americas, and the farmers of the still larger Eurasia.