The Origin of Life on Earth
This painting shows the history of life on Earth, just as in slide #19. On the left is the earliest Earth, hot and with many volcanos. This ancient Earth already had oceans with the simplest molecules of life forming in them. Much of the water in the oceans and carbon for the life molecules may have come to Earth from comets impacting the planet. Time passes as we move to the right: The Earth cools down and the simplest molecules in the ocean organize into more complicated, twisting molecules (DNA, RNA, and proteins), and life itself starts. In the center of the mural, representing times from about 3.9 to 0.7 billion years ago, the only living things on Earth are algae, which grow in lumpy mounds near the ocean's shores. Further right, and closer to today, complicated multicellular life forms exist: jellyfish, snails, trilobites. Next come fish. Life begins to inhabit the dry land at about 400 million years (0.4 billion years) ago. From then to the present we see trees, dinosaurs, mammals, and finally human beings.
Mural at NASA Ames Research Center