History of the Universe

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Cenozoic Era 66 mya to today


This era, whose name means "new life", is sometimes called the "Age of Mammals" since, following the extinction of the dinosaurs, mammals were able to diversify and fill all the niches now left vacant. The continents moved into the familiar places and the climate cooled, culminating in the current glacial period.

Continents 50 mya

Earth 50 mya. Click here for key. Previous image. Next image

As North America moved west the Rocky and Sierra Nevada mountains were formed. Similar westward movement of South America produced the Andes. India continued to move northwards towards Afghanistan (Afg).

Climate begins to cool

30 mya, Antarctica drifted over the South Pole and ice began to cover it, while India hit Asia to make the Himalayan Mountains.

Perhaps these were the events which started the climate on its long downward drift towards the age. We do not really know what causes an ice age.

By 25 mya much of the Earth was too cool and dry for trees and many forests were replaced by grassland.

Cows, sheep and goats evolved to live on grass and they spread. The horse family also evolved but with fewer types of animal.

Spread of mammals

Mammals could keep themselves warm. Perhaps that is why they survived the meteorite. They evolved quickly, filling most life-styles left empty by the dinosaurs.

Most mammal families we know today had appeared by 50 mya. Whales spread in the oceans while hoofed plant-eating animals, the horse family, became common on land and evolved into giants. Meat-eating and flying mammals also appeared at this time.

Horse Family

Around 50 mya a group of plant eating animals which were ancestors of the horse became common. They spread and evolved into giants, some eating leaves, other grass. The horses, tapirs and rhinoceroses are now the only representatives of this once common group. They have been replaced by the cow family, which have far more efficient stomachs.

This group is known as the even toed ungulates (Perissodactyls) because their feet have three or (in the case of horses) just one toe. The lower part of a horse's leg is actually a large toe.

Cow Family

About 40 mya another group of plant eaters, the cow family, evolved a special stomach in which bacteria grew. The bacteria broke down the hard cell walls of plants, letting the animal eat the cell juice. So the cow family could get much more goodness out of their food than the horse family and replaced them as the most common plant eating group. Pigs, hippopotamuses, camels, lamas, deer, giraffe, cows, sheep and goats all belong to this very successful group of mammals.

They are known as the even-toed ungulates (Artiodactyls) because they usually have two or four toes. Contrast this with the horse family.


The group of mammals to which humans belong, the primates, split off from the ancestral insectivore soon after it appeared around 50 mya. Early primates were probably tree-living, and this helped them develop agile bodies, grasping hands and feet, good vision and a well developed brain.

Primates evolved through several stages similar to the lemurs, tarsius, monkeys and apes.


Plants were also evolving quickly. Small fruiting plants called grasses, with wind-pollinated flowers and long thin leaves, began to spread. They could live in cooler drier places than other plants, and the climate just happened to be changing in that direction.

Grasses belong to a group of fruiting plants called monocotyledons, so called because they have only one seed leaf, or cotyledon, in the embryo contained in the seed. This group of plants includes lilies, orchids, palms and irises as well as the grasses. Contrast this with the other main group of fruiting plants - the dicotyledons.

They are typically herby types of plants. Their leaves usually have parallel veins.

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