Event 7

3.8 billion years ago

Three-dimensional DNA molecule
This is a three-dimensional view of the double helix structure of the DNA molecule.
Credit: Image by brian0918™/Wikipedia.

The most remarkable event in the history of our Solar System was the emergence of Life. How self-replicating organisms emerged from an inorganic world is one of the great mysteries of science. All known modern life forms (about 1.75 million species) reproduce, grow, eat, excrete, breathe, and usually move. We humans are just one of these species of life, all of which are carbon-based.

Most scientists agree that the following conditions had to exist for life to originate and spread:
1) The presence of liquid water (much of the surface of the early earth, once it cooled, consisted of oceanic waters, in part from icy comets);
2) The presence of certain elements such as carbon, hydrogen, oxygen, and nitrogen that are the building blocks of life. Carbon, in particular, is an element that easily bonds with other, and is the basis for all organic life on the earth. These elements would have combined to form more complex molecules in the earth’s early seas, a so-called “primordial soup”;
3) A permeable outer layer or membrane that encloses the organism and allows for some chemicals to come in and others to go out;
4) Complex chains of organic molecules (DNA and RNA) that allows for self-replication and codes for growth, development, and metabolic processes;
5) A source of energy (e.g. sunlight, chemical reactions, electricity, volcanoes or hydrothermal vents, radiation) that drives the metabolic processes of an organism

Amino acid basic structure
Amino acids are critical for life and are considered to be one of the building blocks of life.
Credit: Image byYassineMrabet/Wikipedia.

Rocks from Greenland dated to 3.8 billion years ago contain hydrocarbons rich in carbon-13 (a rarer isotope of carbon than the carbon-12 on earth, but more common in life forms) that may have been produced by single-celled bacteria; if so, this is our earliest prehistoric evidence for life. Fossil stromatolites, structures produced by primitive single-celled bacteria (called cyanobacteria or “blue-green algae”) that trap grains of sediment on their mat surfaces and grow in incremental layers have been identified in deposits dating to approximately 3.5 billion years ago. Microscopic fossils from western Australia from this same time show the structure of individual bacteria, remarkably similar in size and shape to cyanobacteria today (just a few thousands of a millimeter wide). These fossils record the first definite emergence of photosynthetic single-celled cyanobacteria that lived in the shallow, warm waters along the coasts of the ancient earth and produced stromatolite structures.

These early organisms must not have required oxygen as part of their respiration, since free oxygen was not yet present in large quantities in the sea (this oxygen-poor condition is called anaerobic).

The earliest self-replicating organisms in the history of the earth were single-celled bacteria without a nucleus called prokaryotes. The next major step in organic evolution was the development of larger single-celled organisms with a nucleus (eukaryotes), which will be our next event.



Scientists have never been able to create life in the laboratory, although experiments have shown that many of the building blocks of life, such as many amino acids, can be produced by natural processes. In the 1950’s, scientists filled a glass container with gases thought to be present in the early atmosphere of the earth (methane, ammonia, hydrogen, and water vapor) and caused electrical sparks to be produced in the mixture to simulate lightning; all twenty of the most common amino acids found in modern life forms were produced in just a few days. It is now known that many amino acids have also formed naturally in space and are present in primitive meteorites called carbonaceous chondrites. So the building blocks for life could have come from terrestrial and/or extraterrestrial sources.

The fossil record shows the first good evidence of life in the form of microscopic bacteria and stromatolite structures dated by radiometric methods to 3.5 billion years ago.


As far as we know, life on earth happened only once (although it may have originated earlier only to go extinct from massive catastrophic extraterrestrial impacts and then developed again.) All life today, be it bacteria, fungus, tree, animal, or human, appears to have descended from some primordial, single-celled replicating creature some three-and-a half billion years ago. All life carries the same type of DNA that codes for reproduction, growth and development, and metabolism. Incredibly, we can analyze similarities and differences between the DNA of any two species in order to understand how closely related they are, and postulate how long ago they last shared a common ancestor.

Whether carbon-based life is present anywhere else in the solar system or the universe is still unknown, but many scientists believe that if other planetary bodies contain liquid water, the likelihood is high that life might arise, given hundreds of millions of years.


NASA’s web site for their Astrobiology Program, concerned with “the study of the origin, evolution, distribution, and future of life in the universe”:

The NASA-sponsored online popular magazine, Astrobiology, devoted to studies of life in the universe:

A short video clip from the Science Channel describing the experiment of young Ph.D. student, Stanley Miller, in 1953 to recreate the conditions for early life in the laboratory:

NOVA’s brief history of life gives an interactive slide show of easy-to-understand information from the beginnings of life to the present day:

A web site for the National Science Foundation-funded Exploring Origins Project, focusing on origins of life:

Wikipedia entry on the conditions, models, and hypotheses for the origins of life on earth:

A Duke University web site exploring conditions for life, not only on earth but also in outer space (“exobiology”), in easy-to-understand terms:

Web resources organized by the Astrobiology Web (“Your online guide to the living universe”) focused on life’s place in the universe:

The line-up of presentations at a symposium at Harvard in March of 2008, “Origins of Life: The Earth, the Solar System, and Beyond,” with streaming videos available of each presentation to play on your computer (free download of Realplayer is necessary). Co-sponsored by the Origins of Life Initiative of Harvard University):


Cockell, Charles et al. (Editors). An Introduction to the Earth-Life System. Cambridge: Cambridge University Press. 2008.

Hazen, Robert M. Genesis: The Scientific Quest for Life’s Origin. Washington, D.C.: George Henry Press. 2005.

Knoll, Andrew H. Life on a Young Planet: The First Three Billion Years of Evolution on Earth. Princeton: Princeton University Press. 2003.

Schopf, J. William. Cradle of Life: The Discovery of Earth’s Earliest Fossils. Princeton: Princeton University Press. 1999.

Schopf, J. William. Life’s Origin: The Beginnings of Biological Evolution. Berkeley: University of California Press. 2002.

Stinchcomb, Bruce L. World’s Oldest Fossils. Atglen, Pennsylvania: Schiffer Press. 2007.


Cohen, Richard. History of Life. Malden, Maine: Blackwell. 2005.

Dawkins, Richard. The Ancestor’s Tale: A Pilgrimage to the Dawn of Evolution. New York: Mariner Books. 2005.

Fortey, Richard. Life: A Natural History of the First Four Billion Years of Life on Earth. New York: Vintage Books. 1997.

Gould, Stephen J. The Book of Life: An Illustrated History of the Evolution of Life on Earth. New York: Norton. 2001.

Guerrero, Angeles et al. (Editors). Prehistoric Life: The Definitive Visual History of Life on Earth. London: DK Books. 2009.

Haines, Tim and Paul Chambers. The Complete Guide to Prehistoric Life. Buffalo: Firefly Books. 2006.

Lecointre, Guillaume and Hervé Le Guyader. The Tree of Life: A Phylogenetic Classification. Cambridge, Massachusetts: The Belknap Press. 2006.

Norman, David. Prehistoric Life: The Rise of the Vertebrates. London: Boxtree Ltd. 1994.

Palmer, Douglas. Evolution: The Story of Life. Berkeley: University of California Press. 2009.

Palmer, Douglas. Prehistoric Past Revealed: The Four Billion Year History of Life on Earth. Berkeley: University of California Press. 2007.

Palmer, Douglas (Editor). The Illustrated Encyclopedia of the Prehistoric World. Edison, New Jersey: Chartwell Books. 2006.

Prothero, Donald R. Evolution: What the Fossils Say and Why it Matters. New York: Columbia University Press. 2007.

Shubin, Neil. Your Inner Fish: A Journey into the 3.5-Billion-Year History of the Human Body. New York: Vintage Books. 2008.

Silvertown, Jonathan. 99% Ape: How Evolution Adds Up. Chicago: University of Chicago Press. 2008.

Southwood, Richard. The Story of Life. Oxford: Oxford University Press. 2003.


Miracle Planet. Ambrose DVD. Five part series, includes: 1. The Violent Past; 2. Snowball Earth; 3. New Frontiers; 4. Extinction and Rebirth; 5. Survival of the Fittest. Narrated by Christopher Plummer. 2005.

The Shape of Life. Four part series includes: 1. Origins and Life on the Move; 2. The Conquerors and Survival Game; 3. The First Hunter and Explosion of Life; 4. Ultimate Animal and Bones, Brawn and Brains. Produced by Sea Studios Foundation for National Geographic Television and Film in association with PBS. Narrated by Peter Coyote. 2001.


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