History of Life

Geologic Time Scale

The geologic time scale is a timeline that uses certain divisions to represent the history of the Earth.

In biology, the divisions are marked with major changes found in the fossil record. Times in years were assigned to these divisions:

1. Geologic time begins with Precambrian time and covers 88% of the Earth’s history; few fossils have been found in this range.

2. Eras: the largest divisions of geologic time; the time between Precambrian time and the present are divided into 3 eras:

Paleozoic Era (ancient time)–the many vertebrates and Invertebrates lived
Mesozoic Era (middle time)–the age of dinosaurs
Cenozoic Era (recent time)–the age of mammals

3. Periods: eras are subdivided into periods, which range in length from tens of millions of years to less than 2 million years.

Mass Extinctions

Huge numbers of species have disappeared over a relatively short period of time on at least 5 occasions in geologic history. Possible causes include asteroids, volcanic eruptions, continents changing positions, sea levels changing, or a combination of factors.

Earth’s Early History

Although the fossil record has missing pieces, paleontologists have assembled good evolutionary histories for many groups of organisms. The fossil record also indicates that major changes occurred in Earth’s climate, geography, and life forms. Several lines of evidence indicate that the earth is at least 4.5 billion years old.

Sometime during those 4.5 billion years, life began. It is not certain exactly when this event took place, but the fossilized remains of ancient plants and animals give us some idea of when the most primitive living organisms appeared.

The oldest fossils of bacteria and cyanobacteria (blue-green algae) have been found in rocks about 3.5 billion years old, during the Precambrian. Evidence indicates that these were the only forms of life that existed until about 1.7 billion years ago when a more complex type of cellular structure developed. Since that time a tremendous variety of organisms have inhabited the earth, most of which are now extinct.

How did life on Earth begin?

Our young planet was probably struck with many objects, creating enough heat to melt the planet.
Earth’s elements then rearranged themselves according to density, with the densest elements sinking to the core.
Less dense elements and compounds (HCN, CO₂, CO, N₂, H₂S, H₂O vapor) formed the early atmosphere. There was no oxygen gas at first.
Earth’s surface was violent with volcanic eruptions, comets, and asteroids.
About 3.8 billion years ago, the Earth’s surface cooled enough for thunderstorms to begin and the water vapor condensed to liquid water forming the Earth’s oceans.
Early oceans were brown due to huge amounts of iron deposits.
Much later, large organic molecules formed proteinoid microspheres, which acted like pre-cells. They had selectively permeable membranes and could store and release energy.
Small pieces of RNA could have formed and replicated. In ways that are not fully understood, these simple RNA pieces led to a DNA system that could direct protein synthesis. These first working cells were prokaryotes.
In the 1950s Stanley Miller and Harold Urey simulated the Earths early environment and were able to form several amino acids over a period of days.

Origin of Oxygen Gas

Microfossils (microscopic fossils of prokaryotes) show that the first forms of life were anaerobic.
As photosynthetic prokaryotes evolved, oxygen gas was released, accumulating in the atmosphere.
The presence of oxygen was poisonous to some of the early organisms (those that had evolved in, and required, anaerobic environments) and drove them to extinction. Organisms evolved that could use oxygen for respiration.

Origin of Eukaryotic Cells

Endosymbiotic Theory: smaller prokaryotic organisms entered a primitive ancestral cell and the two cells formed a symbiotic relationship. The prokaryote could have helped to form chloroplasts and/or mitochondria.
Sexual reproduction evolved next. This means of reproduction provided a mechanism for gene reshuffling and further increased the chances of evolutionary change in a species due to natural selection.
Multicellularity evolved later.