Oligocene epoch
36.6 to 23.7 million years ago

38 to 26 MILLION YEARS AGO

Around 35 million years ago, the continent of India meets up with the Euroasian continent. From 25 million years ago to the present, India will push north against the larger continent to form the Himalayan mountains and with it the highest mountain in the world — Mount Everest. The existence of these mountains would help to change the climate to the west in places like the Middle East and northern Africa to one which would receive less rain and possibly, with enough time, become a desert. (1)
As these geological events were happening, there came into existence two evolutionary branches from the Purgatorius primate. One species evolved into very primitive monkeys that were to lead to modern apes such as Chimpanzees, while the other became primitive ape-like primates living on open savanna country.
The reason for the ape-like primates moving out of the wetter tropical regions and into the drier, harsher and less-protected grasslands of the savannas is not fully understood. However, some scientists believe it may be because of the many individual primate groups of the forest competing for food (perhaps some primates had already gone back to the rainforests as the climate in some parts of the world began to dry up).
If this is true, it is quite conceivable that such an explorative and more curious group would have eventually expanded outwards in the hope of finding a better life if it wished to maximise its chances of survival.
Or perhaps the natural climate change was already forcing some primates to adapt to the savannah?
In the meantime, all the modern animals of the twentieth century had their origins nearly 36 million years ago. In other words, if you could travel back in time and look around on the great continents of the world, you will notice many familiar animals of today (give or take a few minor changes in appearance).

25 MILLION YEARS AGO

A number of mammals have grown in size to become the largest of its type in all the continents of the world (including Australia). They would never reach the size of those reptiles called the dinosaurs, but compared to the animals of our times, these were big creatures.
For example, the ancestors of whales were at one time no bigger than a small horse with sharp teeth, four small legs and a tail, and had a thin fur on their skin. But they later grew into the biggest warm-blooded predators in the oceans weighing up to 60 tonnes and 4 times longer than a fully grown adult great white shark of this era.
Actually the shark species nearly 25 million years ago were at least twice as large as the biggest modern day great white sharks. Evidence for this observation can be found in the massive fossilised shark tooth collected from the Tasman sea floor in 2003 by CSIRO marine research scientist Dr Alan Williams during the joint Australia-New Zealand NORFANZ research expedition (2).
However, as food supplies dwindled in the cooler oceans of the world and along the shoreline, these massive predatory whales would eventually die out, leaving behind a handful of other whale species with better survival rates because of their greater reliance on alternative food sources such as plankton.

NOTES

This process of drying up in the Middle East would be accelerated in more recent times by the presence of humans cutting trees and turning arable land into productive agriculture until conditions became too hot and dry.

The tooth was collected as part of a A$1.5 billion 10-year project started in 2000 by 53 countries to measure the number and type of species that exist in the oceans and on the ocean floor. Already after 3 years into the Census of Marine Life, over 100,000 known species (mostly crustaceans and small worms) have been identified out of a total 210,000 known species of marine animals and plants. Approximately 15,304 of the known animals and plants in the oceans are marine fish and 1,700 new marine animals and plants are catalogued and described at the Smithsonian Institution in Washington D.C. every year including 160 new marine fish species. There is an estimated 2,000,000 species of marine animals and plants in the oceans of the world. But the actual numbers of animals and plants in the oceans today and the rate in which the numbers are decreasing as the temperature and pollution levels of the oceans increase and as humans have an impact through widespread commercial fishing over the past 50 years are still unknown.
As scientific chairman of the census, Frederick Grassle, said:
'It [the census] begins the first systematic global effort to measure our oceans' vital signs and guide what must be done to reverse their decline.' (Darby, Andrew. Plenty more fish in sea and someone's got to count them: The Sydney Morning Herald. 25-26 October 2003, p.18.)

Part of the aim of this massive project is to learn more about the oceans and the creatures that live in them because so little is known about this environment especially at complete darkness depths of kilometres below the surface known as the Dark Zone compared to the creatures on land. It will also be part of the aim to make information about the census available to everyone on the Internet so that hopefully people can be self-educated and to make sound judgements on how to look after the natural resources available to everyone.
Or perhaps it will help future governments and businesses to decide what alternative food supplies they can sell to the overpopulated human race once we have exausted the animals on land?
The funding for this project was provided by the philanthropic Alfred P. Sloan Foundation in the United States.