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Throughout history, humans wondered about the large, fossilized teeth and bones they would discover from time to time. In medieval times the fossils were attributed to fantastic beasts such as dragons and unicorns. During the Age of Exploration, while conquistadores went off in hot pursuit of gold, glory and pretty women, missionary priests had time for tamer endeavors. They would speculate about the large bones that could be found in the New World. Perhaps they had belonged to giant humans.

With modern hopefully more refined tools paleontologists have reconstructed a world of beasts as amazing as the dragons and unicorns of folklore, many larger versions of contemporary mammal, reptile and bird families. Pleistocene fauna, at the end of the last Ice Age. These included giant sloths (Megatheria) and giant armadillos (Glyptodonts) in South America, lions, saber-toothed cats, dire wolves, camels, horses, woolly rhinoceri in North America, and mammoth, Irish elk and rhinoceri in Eurasia.

Australia, then as now, was characterized by marsupials and monotremes, supported large carnivorous kangaroos, Diprotodon, a giant wombat, the Marsupial Lion (Thylacoleo carnifex), and the 3m "duck of doom", Dronornis, the 5 meter snake, Wonambi naracoortensis and the giant lizard, Megalania. Since humans coexisted with these animals, some researchers wonder if the beasts of the Australian Dreamtime legends refer to the megafaunal species. The mythical Rainbow Serpent, source of creation and destruction may be based on the Wonambi naracoortensis. The hairy manlike Bunyip may be the Diprotodon filtered through legend. Clearly it is difficult to translate the poetic vocabulary of myth into the technical language of paleontology. In its fossil form, Wonambi naracoortensis does not need the legend to deserve the name "Rainbow Serpent". Its fossils have opalized and now they shimmer in all colors of the spectrum.

Many islands had a unique megafauna that was driven to extinction on the arrival of man. These included giant bird forms in New Zealand including the moas, gorilla-sized lemurs, two species of hippo on Madagascar, and giant birds on New Caledonia and Hawaii.

Paleontologists are able to make inferences about the lifestyles of these prehistoric creatures by studying the mechanical qualities of the teeth. Grinding molars are useful for the lignins and cellulose of plants. Tearing, stabbing teeth are most efficient in slaughtering prey. In this way scientists can make educated guesses about the dietary habits of the animal.

The three major extinction theories can be remembered as "Kill, chill and ill". The "kill" theory is that Pleistocene humans triggered the megafaunal extinction. This theory has two variants, "blitzkrieg" and "sitzkrieg" or "slow burn". The blitzkrieg theory is that humans the hunted the megafauna to extinction in a short time. There is strong evidence of this in several forms. Firstly is the fossil evidence of megafauna found in conjunction with human remains, particularly with evidence of hunting, such as arrows in the bones, cut marks and cave paintings depicting hunting. Secondly is the biogeographical evidence; the areas of the world where humans evolved still have some megafauna (the elephants and rhinos of Asia and Africa) whereas the areas that didn't have early man, Australia, the New World, Madagascar and New Zealand, all lost their megafauna. It is theorised that the megafauna of Asia and Africa evolved with man, and learnt to be wary of them, whereas in other parts of the world the wildlife was ecologically naive and was easier to hunt. This is particularly true of the island faunas. Thirdly, the close correlation between humanity in an area and the extinction of the megafauna also provides weight to the human caused extinction. The "sitzkrieg" theory is that human encroachment gradually destroyed habitat and led to a die out. Evidence that human and megafauna co-exsted for a long time weakens the "blitzkrieg" explanation.

The "chill" theory explains the extinctions by climatic change following the last Ice Age. Since there were multiple Ice Ages, the climatic theory raises the question, why did the extinctions occur only after the Last Ice Age? One tentative answer is that a nearby supernova altered the galactic environment and as a consequence exaggerated the climatic perturbation. It will require data from space research to advance this claim beyond speculation.

The "ill" theory is that a pandemic or hyperdisease caused the megafaunal extinction. According to this theory, large mammals were particularly vulnerable because they were fewer in number than smaller species. Size and population size tend to be inversely related. Some skeptics argue that natural selection will eliminate an extremely virulent strain because it kills its host too quickly to propogate. However this caveat only applies to microbes that depend on a host to survive. Airborn and waterborn pathogens have no need to mind their manners. The researcher McPhee is searching DNA in mammoth fossils from Wrangel Island in Siberia. He hopes to find evidence of infection.

Some experts claim that various combinations of these factors are responsible for the extinctions. For now the disappearance of Pleistocene megafauna remains a mystery. However all researchers would agree that these long dead creatures continue to enrich our understanding of the world.

• Prehistoric mammals