Life on Earth has evolved from simple prokaryotic organisms such as Archaea that appeared on Earth by 3.85 billion years ago. Life may have been here earlier, but the record is not preserved, and the method by which life first appeared is also unknown and the subject of much thought and research by scientists, philosophers, and religious scholars.
The ancient Archaea derived energy from breaking down chemical bonds of carbon dioxide, water, and nitrogen, and anaerobic archaeans have survived to this day in environments where they are not poisoned by oxygen. They presently live around hot vents around mid-ocean spreading centers, deep in the ground in pore spaces between soil and mineral grains, and in hot springs. The Archaea represent one of the three main branches of life; the other two branches are the Bacteria and the Eukarya. The plant and animal kingdoms are members of the domain Eukarya.
Prokaryotic bacteria (single-celled organisms lacking a cell nucleus) were involved in photosynthesis by 3.5 billion years ago, gradually transforming atmospheric carbon dioxide to oxygen and setting the stage for the evolution of simple eukaryotes (organisms containing a cell nucleus and membrane-bound organelles) in the Proterozoic. Two and half billion years later, by one billion years ago, cells began reproducing sexually. This long-awaited step allowed cells to exchange and share genetic material, speeding up evolutionary changes by orders of magnitude.
Oxygen continued to build in the atmosphere, and some of this oxygen was combined into ozone (o3). ozone forms a layer in the atmosphere that blocks ultraviolet rays of the Sun, forming an effective shield against this harmful radiation. When the ozone shield became thick enough to block a large portion of the ultraviolet radiation, life began to migrate out of the deep parts of the ocean and deep in land soils, into shallow water and places exposed to the Sun. Multicellular life evolved around 670 million years ago, around the same time that the supercontinent of Gondwana was forming near the equator. Most of the planet's landmasses were joined together for a short while, and then began splitting up and drifting apart again by 550 million years ago. This breakup of the supercontinent of Gondwana is associated with the most remarkable diversification of life in the history of the planet. In an incredibly short period of no longer than 40 million years, life developed complex forms with hard shells, and an incredible number of species appeared for the first time. This period of change marked the transition from the Precambrian era to the Cambrian period, marking the beginning of the Paleozoic era. The remarkable development of life in this period is known as the
Cambrian explosion. In the past 540 million years since the Cambrian explosion, life has continued to diversify with many new species appearing.
The evolution of life-forms is also punctuated with the disappearance or extinction of many species, some as isolated cases, and others that die off at the same time as many other species in the rock record. A number of distinct horizons represent times when hundreds, thousands, and even more species suddenly died, being abundant in the record immediately before the formation of one rock layer and absent immediately above that layer. Mass extinctions are typically followed, after several million years, by the appearance of many new species and the expansion and evolution of old species that did not go extinct. These rapid changes are probably a response to availability of environmental niches vacated by the extinct organisms. The new species rapidly populate these available spaces.
Mass extinction events are thought to represent major environmental catastrophes on a global scale. In some cases these mass extinction events can be tied to specific likely causes, such as meteorite impact or massive volcanism, but in others their cause is unknown. Understanding the triggers of mass extinctions has important and obvious implications for ensuring the survival of the human race.
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