History of Earth

Microscopic view of a zircon crystal determined to be the oldest known terrestrial material. It is 4.4 billion years old. Chemical analysis suggests that the Earth was cool enough to have liquid water,and, possibly, life much earlier than previously thought (Source: U. of Wisconsin)
From the point of view of Life on Earth, the surprising thing is that some of the oldest rocks found already have traces of living organisms (as chemical fossils), from a time shortly after the heavy bombardment of our planet ceased (around 3.8 billion years ago, the end of the "Hadean" eon). What this means is that Life arose in the earliest part of Earth's history, even as the planet was still growing by accretion. Thus, some 4 billion years ago, Earth must have been cool enough to allow living organisms to persist. Indeed, the stratification in sedimentary rocks 3.8 billion years old demonstrates the presence of large bodies of water. Of course, there had not been enough time yet to make continental crust, so there would not have been continents and ocean basins between them. Instead, there were large circular impact basins, like the mare (pronounced mah-ray) on the face of the Moon, but filled with water.

Lunar Mare Humorum. Mare are relics of the period of heavy bombardment in the early solar system. They were produced by the impacts of meteorites and asteroids. (Courtesy: ESO)
The lunar "mare" include the oldest landforms around, going back some 3 billion years. They are dry, despite their watery name. On Earth, such ancient features could not persist, because of mantle convection (there is evidence that mountain-building processes started even before 2.5 billion years ago, in the "Archean" eon) and because of erosion (through chemical weathering and running water). The Earth's face is young, because it changes all the time. In the memory systems of the ocean basins (the stacks of sediment at the bottom of the sea) we find nothing older than 180 million years, a time that represents less than 5 percent of the age of the oldest rocks found in the continents. So, most of what we know about the Earth's history comes from the record within the continents. The quality of that record is not particularly good.

Colorado River meanders through the Vishnu Schist at the bottom of the Grand Canyon. These are the oldest exposed rocks on the Earth, having formed during the Proterozoic period 1.6 billion years ago. (Courtesy: College of William & Mary)
In some places, the continental record has a great semblance of order, as in the stacks of layers visible in the Grand Canyon. However, invariably there are problems. At the bottom of the Canyon, for example, we find the "Vishnu Schist", ancient sediments that have been worked over and heated deep within the Earth, and have thereby lost much of their memory. The layers are not horizontal but show a steep angle, reflecting tectonic movement. This is only a mile down into the crust. If we were to drill deeper, we would find rocks that are ever more severely disturbed and have lost much of their memory. On top of the Vishnu Schist (which is Precambrian in age) there is a well-layered sequence of Paleozoic rocks, and we might expect to read the history of that time span from them with ease. However, the history is only incompletely represented. During large parts of the time span represented by the stack there was no deposition, or there was erosion. Or else the sediments are poor in fossils, so we can learn very little about Life during that time, in that place.

What we do know about Earth's history, then, is owed to the assembly of bits and pieces of evidence from all over. The further back we go, the hazier the view of what the world was like.

The Precambrian portion of Earth's history (in the Grand Canyon, whatever lies below the stack of layered rocks) takes up the first 4 billion years of history, about six parts out of seven. The last one seventh of geologic history is well represented by the fossil record; it is called the "Phanerozoic" (meaning "animals are apparent", in Greek). The earlier six seventh are divided into the "Hadean" (time of bombardment), the "Archean" (3.8 to 2.7 Ga) and the "Proterozoic" (2.7 to 0.6 Ga).

Life in the Archean was entirely dominated by procaryotic forms, that is, organisms with genetic material distributed throughout the cell. These were the archea and bacteria. Procaryotes began to make rocks in grand style in the Archean, with "banded iron formations." These are finely laminated deposits with alternating layers of iron oxides and quartz. It is thought that the iron was precipitated out of solution by photosynthesizing bacteria that produced oxygen during seasonal blooming. The point is that oxygen was normally absent, which allowed the dissolved iron content to reach high values. The quartz, presumably, originated from layers of opal (also precipitated with the help of organisms). Another type of layered rocks made with the help of cyanobacteria and found in Archean rocks, is represented by the "stromatolites", small bacterial reefs built by precipitation of carbonate in shallow water and by trapping mud within a bacterial film covering mounds rising from the sea floor. After some 2 billion years of evolution of bacteria and archea, we find a rich assemblage of highly diversified procaryotes within the cherts of the Gunflint Iron Formation.

A cell of Escherichia coli. E.coli is a prokaryote. Its cells have no nucleus. The light-colored mass at the center of the cell is the DNA. Prokaryotes are believed to be the first type of cells to have evolved on the Earth. (Courtesy: ThinkQuest)
The stage was set for the evolution of eucaryotes, organisms with genetic materials concentrated in a single nucleus. The eucaryotic cells differ from their procaryotic ancestors in having a number of internal structures called organelles. Apparently at least some of these organelles were acquired by a kind of contract with different procaryotes (a suggestion put forward by the American biologist Lynn Margulis). If so, the eucaryotic cell is the outgrowth of symbiosis, a compound organism to which two or more procaryote cells have contributed. Passing on the genetic information from such a complicated structure necessitated major innovations in the process of cell replication. Thus, the invention of the eucaryotic cell was one of the largest steps in the evolution of Life on Earth. After that, it took another billion years to make multi-celled organisms such as primitive types of coral, then trilobites and molluscs and vertebrates. Rock-building organisms proliferated and made limestone and chert. Thus, the continental crust everywhere bears the imprint of life processes.

Gunflint Chert Fossils from the Canadian Shield, the remains of bacteria that lived 2 billion years ago. These are the oldest fossils of microorganisms yet found. In addition to their age, they are remarkable for the diversity of organisms present. (Courtesy: Carlton College)
The path from fish to people is rather short, compared with all the major innovations that preceded this development in the last 0.4 Ga. Some fishes used part of their guts to breathe with, so they could survive out of water, some learned to walk and to bear young on land. From there to reptiles, birds and mammals took up the last six percent or so of Earth history. Intelligent life (depending on one's definition) starts perhaps 150 million years ago (if we are to believe the scenes in Spielberg's Jurassic Park): the last three percent of Earth history. People exist for about one percent of that time (depending on what one accepts as "people"). Thoroughly modern people, indistinguishable from our species, exist for the last 60,000 years: one eighty thousandth of Earth's history. Such is the abyss of geologic time.

A corn cell. Corn is a eukaryote. The nucleus is the large circle at the right. This is where the cell's genetic material is located. (Courtesy: ThinkQuest)