Natural selection is the engine that drives evolution:
The survival and sustenance of life however depends upon the phenomenon– Natural selection. The organisms best suited to survive in their particular circumstances have a greater chance of passing their traits on to the next generation. Plants and animals interact in very complex ways with other organisms and their environment, to produce the amazingly diverse range of life forms present on Earth.
Darwin called this mechanism of evolutionary adaptations “natural selection” because the natural environment “selects” for the propagation of certain traits. We see the products of natural selection in the exquisite adaptations of organisms to the special circumstances of their way of life and their environment.
By understanding natural selection, we can learn why some plants produce cyanide, how animals first emerged from the ocean to live on land, and how some mammals eventually went back again. We can even learn about microscopic life, such as bacteria and viruses, or figure out how humans became humans.
Thus, the single–celled protobionts (prokaryotes) took the path in becoming everything from amoebas to zebras, and the path human beings continue to take today as they respond to environmental stimuli and steadily change into something more suitable for the world around us.
Thus, several lines of evidence indicate that life has been present for most of earth history. Prokaryotic ecosystems dominated the first two billion years of life on earth. Eukaryotes are thought to have arisen from prokaryotic ancestors through stages of endosymbiosis..
- Biology – Seventh edition: Campbell.Reece
- http://www.globalchange.umich.edu/globalchange1/current/ lectures/complex_life/complexlife.html