6.3 An Age of Brain Growth¶
A number of types of mammal already appear in the Eocene. Some are differentiating in one direction, and some in another, some are perfecting themselves as herbivorous quadrupeds, some leap and climb among the trees, some turn back to the water to swim, but all types are unconsciously exploiting and developing the brain which is the instrument of this new power of acquisition and educability. In the Eocene rocks are found small early predecessors of the horse (Eohippus), tiny camels, pigs, early tapirs, early hedgehogs, monkeys and lemurs, opossums and carnivores. Now, all these were more or less ancestral to living forms, and all have brains relatively much smaller than their living representatives. There is, for instance, an early, rhinoceros-like beast, Titanotherium, with a brain not one-tenth the size of that of the existing rhinoceros. The latter is by no means a perfect type of the attentive and submissive student, but even so it is ten times more observant and teachable than its predecessor. This sort of thing is true of all the orders and families that survive until today. All the Cainozoic mammals were doing this one thing in common under the urgency of a common necessity; they were all growing brain. It was a parallel advance. In the same order or family today, the brain is usually from six to ten times what it was in the Eocene ancestor.
The Eocene period displayed a series of herbivorous brutes of which no representative survives today. Such were the Uintatheres and the Titanotheres. They were ousted by more specialized graminivorous forms as grass spread over the world. In pursuit of such beasts came great swarms of primitive dogs, some as big as bears, and the first cats, one in particular (Smilodon), a small fierce-looking creature with big knife-like canines, the first sabre-toothed tiger, which was to develop into greater things. American deposits in the Miocene display a great variety of camels, giraffe camels with long necks, gazelle camels, llamas, and true camels. North America, throughout most of the Cainozoic period, appears to have been in open and easy continuation with Asia, and when at last the glaciers of the Great Ice Age, and then the Bering Strait, came to separate the two great continental regions, the last camels were left in the old world and the llamas in the new.
In the Eocene the first ancestors of the elephants appear in northern Africa as snouted creatures; the elephant’s trunk dawned on the world in the Miocene.
One group of creatures is of peculiar interest in a history that is mainly to be the story of mankind. We find fossils in the Eocene of monkeys and lemurs, but of one particular creature we have as yet not a single bone. It must have been a creature half ape, half monkey; it clambered about the trees and ran, and probably ran well, on its bind-legs upon the ground. It was small-brained by our present standards, but it had clever hands with which it handled fruits and beat nuts upon the rocks and caught up sticks and stones to smite its fellows. Spite of the lack of material evidence, the facts of biological science almost compel us to believe that such a creature existed, the common ancestor of the anthropoid apes and the two species of men we will describe in the next chapter.