Life on Earth - The Beginning
Earth is thought to be about 4.5 billion years old. The Earth has an atmoshere by virtue of its size - big enough for its gravitional field to hold on to atoms of oxygen, hydrogen and nitrogen. Oxygen, a chemical compound crucial to many forms of life on Earth, started on the Earth in miniscule quantities but began to grow as, for instance, it reacted with iron to produce rust particles. When there was no longer any iron to react with, oxygen began to build up in the atmosphere. The stage was set for the evolution of oxygen-breathing animals. The first animals large enough to be viewed without magnification appeared about 600 million years ago in the Ediacara Hills, South Australia.
 

Evolution of Life

 Image © http://www.dinosaur-world.com/milestones_of_evolution/images/evolution-bush.jpg
See http://www.dinosaur-world.com/milestones_of_evolution/milestones_of_evolution_history.htm for more images of the evolution of vertebrates, reptiles, amphibians etc.

Evolution of life
A simplified evolution of life on Earth is as follows: shellfish, fish, amphibians (both lives), reptiles, mammals, humans. Although the earth is 4.5bn years old the first life didn’t appear until about 3.5bn years ago. Single celled organisms began in the sea evolving slowly into the first worms and jellyfish 700mn years ago. Amphibians largely made way for reptiles because of the ability of the latter to lay eggs on dry land. Fish are the evolutionary ancestors of all amphibians, reptiles, birds and mammals. The first amphibians to inhabit the land - the tetrapods - arrived around 380mn years ago. They still lived near the sea as it was here they had to lay their eggs. The reptiles were able to live away from water because their eggs had hard shells.

The first plants, insects and worms moved onto earth 420mn years ago. The first amphibians walked on land 380mn years ago. Prior to this movement many groups of fish lived in the great seas but also in great warm lakes. From time to time these great lakes dried out and certain groups of fish evolved ways of surviving these new dry conditions, as they moved towards the nearest pond or lake.

The first dinosaurs arrived on land about 225mn years ago, and mammals first appeared at this time too, although their reign only really began at the extinction of the dinosaurs.

Birds and flight
Pterosaurs were among the first vertebrates in the air. Image from http://images.encarta.msn.com/xrefmedia/sharemed/targets/images/pho/t025/T025577A.jpg

It is now widely accepted that birds evolved from a line of dinosuars.

The origin of flight is still a mystery, but most likely it came about when insects ran, jumped and were borne aloft by air currents. Those insects with body parts that allowed for more efficient air travel survived and reproduced more often than their less endowed colleagues. Another theory is that early bird ancestors lived in trees and began flight by gliding.

The first bird flew about over Germany 150mn years ago. It seems likely birds evolved from dinosaurian ancestors around 160mn years ago.

Humans

The first humans evolved around 3m years ago, and the first fossils are the famous footprints in ash from 3.75mn years ago - a mother and her small child. Humans belong to the Order Primates ("one of the first"). The oldest known primate (Purgatorius) probably looked more like a squirrel or a rat than a monkey. Then monkeys, then apes, then Australopithecus the first human (because it could stand and walk fully upright as we do). Then homo erectus appeared 2 mn years ago and immigrated out of Africa into Asia. Approx 1mn years ago humans reached Europe. Tool use had by now reached a high level of organisation and evidence suggests that prior to the advent of homo sapiens, fire, a home base and shelter had developed within the human culture. The first fossil remains of man in Britain are thought to have been from 250,000 years ago.

Now us. Modern humans belong to the species Homo Sapiens (wise person) and stem from ancestors 100,000 years old. Neanderthal man existed in the cold ice ages of Europe from about 100,000-35,000 years ago. He first came to light in 1857 when a skeleton was found in the Neander Valley in Germany. Both species of human almost certainly overlapped.

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