The last chapter of the earth's history, the Pleistocene Period (1.8 million years to 10, 000 years before present) was significant for global cooling and giant mammals. Worldwide glaciations and extreme cold climate made this period notorious and it is also known as the Ice Age. During this period while the mainland and the sea of the northern latitudes were freezing, Indian sub-continent offered comparatively warmer and congenial climate. Animal and plant fossils recovered from the sediments of this period indicate a luxuriant growth in numbers too.
Plenty was the buzzword! Plenty of food both for the herbivores and carnivores aided in the growth, evolution and proliferation of fauna. Gigantism is commonly seen in the fossil mammals of the Pleistocene Period. Their sizes often make one wonder if the descriptions of the giants of the Indian mythology were derived from these animals! Compared to them, the mammals of the Holocene period (past 10,000 years or10k) are smaller in size.
What could be the reason for such an evolutionary trait that caused the animals to grow to gigantic sizes? Well Pleistocene gigantism is a global phenomenon says S. Biswas an eminent paleontologist of Geological Survey Of India (GSI) and it is related to Pleistocene freezing, a global event. Quoting Bergmann's rule he says that in a mammal body, the ratio of heat loss to the heat generated is determined by the ratio of the surface to the volume of the body. In an enlarged body the surface and volume both are increased by the square and the cube factor of the enlargement. Thus the ratio of heat loss to the heat generated is decreased by the enlargement of the body.
Individual mammalian species during the Pleistocene period thus acquired larger size. It is imperative that larger bodies need more food. Indian sub-continent offered ideal situation as far as food was concerned. Abundance of mammalian fossil giants in the Siwalik rocks (foot-hills from Jammu to U.P. - Nepal border), or in the river deposits throughout the peninsular India are a testimony for congenial environment for the organisms to thrive.
Holocene Period heralded the warming phase with frequent dry phases of climate. The changed climatic scenario demanded changes in the body structure and food habits of the animals. In order to survive the mammals of this period acquired adaptation and developed mechanical advantages. For example, pigs developed complex teeth enamel to facilitate chewing the hard grass and antelopes developed more spirals in the horns to tackle the predators in the newly emerged grasslands.
Detailed studies of fossils of the mammals of Pleistocene and Holocene periods has shown that these animals though highly developed and evolved were highly susceptible to climatic vicissitudes. Adaptations in their body sizes, shape, migration patterns, flourishing/extinction of species and sudden multiplication in numbers are all indicators of climate of the past 1.80 million years. None of the fossil mammal found in India represent extreme cold climate. Yes they do indicate wet and dry phases during the Pleistocene.
The great collision between the Indian and the Tibetan Plate had taken place around 20 million years ago. This gave birth to the youngest and the loftiest mountains of the world, the Himalayas. Rising Himalayas changed the climatic scenario drastically and the process must have generated severe earthquakes too. Such traumatic natural environs affected the mammals of the foothills of the Himalayas. Consequently some of the species started multiplying rapidly and developed multiple lines of evolution as a safe guard against the natural hazards. Fortunately our ancestors were not there! The species of the peninsular India on the other hand show a uniformity of pattern indicating a stable life and a stable environment.
It is interesting to note that the most prolific wildlife of present day in India is the deer family or the Cervidae. The fossil record does not show a proliferation of the family during the Pleistocene. Colder places like the Europe or Tundra have giants of Reindeers and other members of the family. Fossils of Reindeer have not yet been reported from India.
It indicates that the ancestors of deer originated in the cold climates of northern latitudes and later after evolution of species adapted to comparatively warmer climate migrated towards Indian sub-continent.
A close look at some of the mammalian fossils gives a comprehensive picture of the climate of the Pleistocene in India says Biswas. Mammals of the NW India, belonging to the families of bovines, pigs and hippopotamus indicate warm climate during the early Pleistocene. Absence of Giraffe and Camel fossils from the middle Pleistocene of Central India and the presence of Hippopotamus with eye orbits more raised than its ancestors indicate a humid phase. The latest Pleistocene of Peninsular India witnessed aridity indicated by the absence of water loving Hippos.
Warming of climate in the Holocene led to decrease in the gigantism. The era of the giants came to an end.