How did the first life forms on our planet evolve from a mere chemical broth? It is a matter of conjecture at what point a complex assemblage of molecules deserves to be called a life form. We do not know how and when those first primitive life forms emerged, but scientists have come up with a likely scenario. It is generally agreed that the appearance of life depended on the changing conditions on the early Earth. The Earth formed about 4.6 billion years ago, with a primitive atmosphere containing gases rich in carbon and nitrogen and deficient in hydrogen and oxygen. During its first 100 million years, the Earth was bombarded by many rocky fragments, some of which may have been as large as the Moon. Such impacts would have vaporized the oceans and a large amount of the Earth's crust. Material rich in hydrogen, oxygen, and complex molecules arrived somewhat later, in the form of comets and objects from the outer solar system. The early Earth was a strange and almost unrecognizable place, yet life first formed in this environment.
The next step after the first replicating molecules — aggregating complex organic molecules into cell-like structures — must have taken place in the 500 million years after the Earth formed. Although some of the processes can occur in dry environments, liquid water was probably critical to biochemical evolution because it provided a medium in which materials could move and stick together. One botanist has commented that "all cells, of all living organisms, are strictly aquatic creatures." Any land-based organism is merely a protective shell filled up with millions of aquatic cells.
Biological processes cannot develop unless they are set apart from the environment and protected from dilution. Some sort of compartment or membrane is required to form a cell. Florida biologist Sidney Fox has shown that simple heating of dry amino acids (as might happen on a dry planet) can create protein molecules. Once water is added, these proteins assume the shape of round, cell-like objects called proteinoids, which take in material from the surrrounding liquid, grow by attaching to each other, and divide. Though they are not considered living, they resemble bacteria so much that experts have trouble distinguishing them by appearance. Possibly related to proteinoids are objects discovered in the 1930s by Dutch chemist H.G. Bungenberg de Jong. When proteins are mixed in water solutions with other complex molecules, both sets of substances spontaneously accumulate into cell-sizes clusters called coacervates. The remaining fluid is almost entirely free of complex organic molecules.
The next step towards recognizable life is even more uncertain. If organic molecules or coacervates are present in a pool of water, they will be left in the pool as the water evaporates. In this way, evaporation of the water in tidewater pools provided high, localized concentrations of amino acids, proteins, and other molecules, allowing cell-like structures to form. The cell-like structures in the primeval pools of "organic broth" could have begun reacting with fluids in the pools and with each other, accumulating more molecules and growing more complex. This concept was first suggested by Charles Darwin, who speculated on "some warm little pond" where life might have begun. Eventually these early cells could have evolved into biochemical systems capable of reproducing and increasing in complexity. A cell is a sophisticated chemical factory. It is not surprising that we cannot duplicate this evolution in the laboratory, since it took half a billion years on Earth.
The earliest biological systems capable of independent life were bacteria. Bacterial cells are prokaryotes, cells without nuclei that contain a single long strand of DNA with several thousand genes. Indirect evidence of bacteria has been found in the Earth's oldest rocks. The evidence consists of carbon isotopes of possible biological origin found in a 3.8-billion-year-old rock from western Greenland. The earliest "probable" evidence for life is a colony of stromatolites — cabbage-like mats of sediment rimmed with bacteria and blue-green algae. These primitive life forms date from 3.5 to 3.6 billion years ago and have been found in Africa and Australia. Fossils of methane-producing bacteria have also been found in 3.4-billion-year-old rocks in South Africa.
The fact that the oldest fossils are younger than the oldest rocks may not be significant. Life may have originated considerably earlier. However, life probably could not have evolved much before 4.1 billion years ago because of the intense early meteoric bombardment and the possible ocean of liquid lava covering much of the Earth's crust. As paleontologist Stephen Jay Gould has observed, life arose "as soon as it could; perhaps it was as inevitable as quartz or feldspar." One set of calculations has been used to argue that life might have originated several times on the early Earth, and that all life today would have descended from just one of the origination events. It is also noteworthy that the early Australian stromatolites prospered in strange and hostile environments. Evidence suggests that these organisms lived near shallow hydro-thermal vents dominated by island volcanism. In an atmosphere with almost no oxygen, they metabolized the gas hydrogen sulfide (H2S), which is toxic to most modern life forms.
Using fossil and chemical evidence, and a little speculation, we can tell the story of life on Earth. For around 2 billion years, prokaryotes ruled the oceans of the Earth. Life remained in the oceans, where liquid water provided a supporting and protective environment. Organisms were mostly soft-bodied and rarely produced fossils, so their development is hard to trace. The land was barren. Some areas must have looked like today's deserts or like Mars. Some areas were moist and washed by rains, but instead of luxurious forests, there were only bare acres, eroded gullies, and grand canyons. Brown vistas stretched to the sea.
Gradually, life went through a remarkable transition. Early prokaryotes survived and evolved by using the organic compounds that were present in warm ponds and by using hydrogen sulfide as an energy source. However, as this source of food was extinguished by changes in the Earth's atmosphere, some prokaryotes invented photosynthesis. Photosynthesis allows the conversion of sunlight into stored chemical energy for future use. This fundamental process allowed the proliferation of advanced forms of life and allowed life to endure for a cosmic time scale. Life's destiny became coupled to the fusion energy source deep in the Sun's interior.
One result of the invention of photosynthesis was the release of oxygen into the Earth's atmosphere. Essentially all of the free oxygen on which modern life forms depend (including us!) was dumped into the atmosphere by microscopic organisms billions of years ago. At first this gas was nothing more than a waste product — oxygen was actually poisonous to the first organisms! Over time, organisms evolved that could use oxygen in their metabolism, and the oxygen content began to rise towards the modern value of 21%. The atmosphere therefore evolved from more reducing conditions (dominated by hydrogen compounds) towards oxidizing conditions (dominated by oxygen compounds). As evidence for this change, we find that oxidized sediments are rare before 2 billion years ago and common afterwards. Oxygen production modified the whole environment. Solar UV radiation broke down some O2molecules and the free oxygen atoms joined with other O2 molecules to make ozone (O3) molecules. The result was the formation of an ozone layer high in the Earth's atmosphere, which absorbs solar UV radiation and thus protects organisms on the surface.
About 1.4 billion years ago, life stepped upward in complexity. Cells called eukaryotes developed; a membrane at the center of these cells holds the DNA. Eukaryotes contain hundreds of times more genetic material than prokaryotes, with a corresponding increase in the complexity of cell function. New types of organisms appear in the fossil record: they are capable of oxygen metabolism, and, although less resistant to UV damage, they are able to flourish because of the new ozone layer. Earth then witnesses the expansion of life from the sea onto the land, a step as momentous as the contemplated colonization of other planets by us! As organisms continued to reproduce, some of them invented sex. Sexual reproduction allows an offspring to receive half of its genes from each parent. All eukaryotic cells can reproduce asexually, but sexual reproduction causes the gene combinations to change from generation to generation. Such new combinations in turn facilitate the experimentation and adaptation that allows organisms to survive in a hostile and changing environment.