The Face of Earth

The Earth’s surface is the scene of a constant battle between the upward forces of mountain building and the erosional forces of wind, water and waves, aided and abetted by gravity. In the midst of all this, Human too have made their mark, in their attempts to hold back the sea, concrete over the surface and reclaim land. By removing vegetation, humans aid erosion rather than prevent it.

The features of landscape can be classified according to the predominant forces at work and the timescale during which they have been at work. In Andes and Himalayas, mountain building still dominates over erosion: parts of Scotland and Canada where once the mountains were higher are now eroded into old age. The form erosion takes depends very much on the climate. Where temperatures frequently drop below freezing, ice can act like a wedge, chiseling great boulders from the mountains.

 Glaciers grind out broad valleys and transport the debris far away. Rivers cut into the hill sides and wash millions of tons of rock and soil away, depositing them on wide flood plains, in deltas and in deep sedimentary basins out to the sea. Wind scours deserts with blown sand and spreads dunes far and wide. Eventually all this material gets pressed into rock and pushed back into mountains.

Parallel evolution on different continents

The distribution of animal groups was influenced by land routes that were, in their turn, determined by continental drift. A land bridge between the Americas enabled more advanced mammals to invade the south while the armadillo and opossum moved north. Before the desert barrier was established in northern Africa animals now typical of the plains moved in from the north while African animals such as the elephant migrated north. In the east, some oriental and Australian species reached a transitional area between Asia and Australia, while others, such as the squirrel and the tree kangaroo, were unable to.

 The first mammals appeared 216 million years ago, although there was a setback in their evolution with resurgence 114 million years ago. Early mammals were small and probably laid eggs. Hoofed mammals, carnivorous bats and rodents had all diverged from the primate line before the Cretaceous catastrophe. After this period there was rapid development and diversification. Most modern mammals developed around 35 million years ago.  The ice age saw the emergence of man giant mammals, most now extinct. More extinction was to follow due to indiscriminate hunting by human.

Dinosaurs still roamed the Earth when the first primate like mammal appeared: a tree shrew called pergatorius. By 55 million years there were tarsier like primates with grasping hands and feet, binocular vision and relatively large brains. By 30 million years ago the hang nose Old World monkeys and the broad nose New World monkeys had split; ten million years later the ancestral apes split off. Eight million years old Sivapithecus, once thought to be ancestral to man, was probably closer to the orangutan. Molecular evidence suggests human ancestors split from those of chimpanzees 5 million years ago.

Hominid fossils are rare but the best candidate for our early ancestor is probably Australopithecus afarensis which lived in east Africa about 4 million years ago. It was small but had legs that could each be placed under its center of gravity, allowing it to walk upright. Foot prints in Tanzania suggest it did so. Homo habilis, the first member of our genus, made simple stone tools and had a bulge in its brain corresponding to the area we use for speech.  Walking upright may have been the key to human success, allowing the brain cavity to expand without obscuring vision, freeing the hands and putting a bend in the windpipe that we now employ in speech.

Mountain building and Dinosaur domination

Rocks on Earth go back 3800 million years the oldest are in Greenland, followed by ones in Australia, Canada and South Africa. Most present day continents formed 3000 to 2500 million years before and have broken apart and regrouped. Most great mountain ranges were formed by collisions of continents, and occurred in phases. The first was the Caledonian (460 million years old) as Europe collided with North America; next was the Appalachian uplift in the eastern USA; about 300 million years ago the collision between Europe, America and Gondwanaland saw the Hercynian phase; and in the last ten million years the Alps were formed by the collision of the Eurasian and African continental plates, and were then deformed by faulting and thrusting.

From their rise 235 million years ago, dinosaurs dominated the land. More than 800 species of dinosaurs have been identified. The biggest, Brachiosaurus, grew to the height of 28 m and may have weighed as much as 100 tones.  There were two main groups of dinosaur: the Saurischian, or lizard-hipped, and the ornithischian, or bird hipped.  Though the dinosaurs are long gone, their descendants, the birds, are still abundant.

Geological history is peppered with catastrophes that may have been the reason why many species suddenly became extinct. One such species was the dinosaur which disappeared 65 million years ago. The most popular theory is that a large asteroid collided with the Earth, throwing dust and steam onto the air, blotting out the sun and changing the climate. A large impact crater off the Yucatan Peninsula in Mexico is often cited as possible evidence of this.