Earth formed about 4.5 billion years ago. No one knows exactly when or how life began, and the final, most important events leading to the origin of life are perhaps the least understood chapters of the story.

Early Earth was dominated by volcanoes, a gray, lifeless ocean and a turbulent atmosphere. Vigorous chemical activity occurred in heavy clouds, which were fed by volcanoes and penetrated both by lightning discharges and solar radiation.

The ocean received organic matter from the land and the atmosphere, as well as from infalling meteorites and comets. Here, substances such as water, carbon dioxide, methane and hydrogen cyanide formed key molecules such as sugars, amino acids and nucleotides. Such molecules are the building blocks of proteins and nucleic acids, compounds ubiquitous to all living organisms.

A critical early triumph was the development of RNA and DNA molecules, which directed biological processes and preserved life's "operating instructions" for future generations. But the origin of life was triggered not only by special molecules such as RNA or DNA, but also by the chemical and physical properties of Earth's primitive environments.

Most of life's history involved the biochemical evolution of single-celled microorganisms. We find individual fossilized microbes in rocks 3.5 billion years old, yet we can conclusively identify multicelled fossils only in rocks younger than 1 billion years.

The oldest microbial communities often constructed layered mound-shaped deposits called stromatolites, whose structures suggest that those organisms sought light and were therefore photosynthetic. These early stromatolites grew along ancient seacoasts and endured harsh sunlight as well as episodic wetting and drying by tides. Thus it appears that, even as early as 3.5 billion years ago, microorganisms had become remarkably durable and sophisticated.

Many important events mark the interval between 1 billion and 3 billion years ago. Smaller strips of land dominated by volcanoes were joined by larger, more stable continents. Life learned how to extract oxygen from water, and living things populated the newly expanded continental shelf regions.

Finally, between 1 billion and 2 billion years ago, eukaryotic cells (those with a nucleus) developed, with complex systems of organelles and membranes. These organisms then began to experiment with multicelled body structures.

The evolution of the plants and animals most familiar to us occurred only in the last 550 million years. Marine invertebrates (such as shell-making ammonites) appeared first, then fish, amphibians, reptiles, birds, mammals and humanity. Land plant communities also evolved from relatively ancient clubmosses, horsetails and ferns, to the more recent gymnosperms (for example, conifers) and angiosperms (flowering plants).

SOURCE: NASA Ames Research Center

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