History of Life on Earth
Life on Earth is extremely diverse. Up to 2,000,000 species are known to exist and many more have yet to be identified. Life has adapted to many environments on land, in rivers and lakes, in the air and in the seas and oceans. It can even be found in extreme environments previously thought to be uninhabitable, such as acidic volcanic crater lakes, oxygen-lacking hydrothermal vents at high pressures in the deep ocean, and in groundwater, deep below the surface of the earth.
Life has not always existed as the species we see today. We know from genetics and from evidence in the fossil record that life has dramatically changed over time, or evolved. The Theory of Evolution by Natural Selection, as written by Sir Charles Darwin, explains the mechanism of how organisms have evolved over time from a common ancestor through heritable variation to the diverse life forms that exist on Earth today. Species do not always evolve and adapt to survive the dramatic changes on Earth. Many have been wiped out and fossil records show that five mass extinctions have occurred since life first evolved on Earth, on average one every hundred million years. Those species that survive radiate and fill the vacated areas in the biosphere allowing the Earth to be dominated by different lineages over time.
The Earth itself has also changed over time. Movements of tectonic plates have changed the configurations of continents, caused volcanic activity and helped to drive climate change. Climates fluctuate and may go to the extremes of an icehouse or a greenhouse world. Ice core samples show the earth has gone through eight ice ages over the past 800,000 years. Each glacial period lasting about 80,000 years was followed by an inter-glacial periods lasting about 28,000 years. The present interglacial period began about 12,000 years ago and so the next major ice age is expected in about 16,000 years. It is these aspects of the changing Earth, together with meteorite impacts and organism interactions which help to drive the evolution of species.
The Earth is 4.8 billion years old but life has only been on earth for 3.5 billion years. The first life existed as single-celled organisms that did not have a nucleus. They were thought to have lived on sulphur or carbon, as oxygen only became prevalent in the atmosphere around 2.3 billion years ago. The first organisms that had nuclei, and therefore the potential to become multi-cellular organisms, appeared around 1.8 billion years ago.
Evidence of the first animals on Earth may be found in fossils around 650 million years old. During the Cambrian, an explosion of shelled, marine animals was followed by the first mass extinction. Sudden global cooling may have been the cause. About 25% of all species were lost.
The earliest fish appeared in the Ordovician but the end of this period saw the second mass extinction that wiped out trilobites. In the Silurian life moved from the oceans onto land for the first time in the form of plants, while in the seas the first coral reefs formed. The Age of the Fishes in the Devonian period was permitted by the dominance of coral reef environments in the oceans. Animals followed plants onto the land with the first amphibians appearing, together with the first spiders and woody stems in plants that gave rise to primitive trees.
The third mass extinction of marine animals occurred towards the end of the Devonian, about 370 million years ago. Gigantic primitive trees dominated the land forming coal swamps in the Carboniferous, permitting evolution of winged insects and the first reptiles. Sudden global climate change was probably the cause. About 19% of species were lost.
The Permian saw the rise of reptiles, amphibians and conifers and the first beetles but the largest extinction of all time, the fourth mass extinction at the end of the period, 250 million years ago, saw greater than 90% of all species on Earth become extinct. The extinction was probably caused by a combination of factors: a shift in the position of the Earths land masses (plate tectonics movement) and a giant meteorite impact off the coast of Western Australia both of which resulted in climate change.
Red star indicates possible meteorite impact site 251 million years ago.
The Age of Reptiles followed, with the evolution and diversification of dinosaurs and flying reptiles during the Triassic and Jurassic. The first modern corals arose, as did the earliest birds. In the Cretaceous period, we find evidence of the first flowering plants and the first mammals. Mammals lived along side the dinosaurs for 12 million years.
The fifth mass extinction at the end of the Cretaceous, 65 million years ago, was the most famous because it saw the extinction of 76% of all species, including the dinosaurs. An asteroid probably caused this extinction. Evidence of this is a crater approximately 200km in diameter preserved off the coast of Mexico.
Red star indicates Chixulub impact site 65 million years ago
The Tertiary (Palaeogene and Neogene) period heralded the Age of Mammals. An explosion in insects and the appearance of bats in the Palaeocene was followed by the evolution of primitive horses and cattle in the Eocene, during the last Greenhouse climate. Ice caps developed over the poles in the Oligocene and the first apes appeared. An explosion of flowering plants in the Miocene has led to the dominance of this plant lineage in the flora of the present, where 70% of all plant species are flowering plants.
The first human ancestors that walked on two legs branched off from other apes around 5 million years ago, in the Pliocene, and evolved at the time of a rapid diversification in mammals and birds. Early humans evolved in Africa during the Great Ice Ages of the Stone Age, during the Pleistocene. The last ice age occurred around 12,000 years ago.
The sixth mass extinction began at the end of the Pleistocene with the extinction of large mega-fauna, such as mammoths, sabre-tooth tigers and giant wombat-like Diprotodons, but it has continued to occur right up to the present day. Figures vary but it has been estimated that currently between 30,000 and 500,000 species go extinct annually.
Extinction removes groups of species and in the vacuum that is created new groups appear. The fifth extinction removed the dinosaurs and with the dominant predators gone, Mammals seized the opportunity and quickly spread over the planet, specializing, diversifying and growing in size. The conditions that followed from this mass extinction also provided the opportunity for the evolution of an intelligent species of Mammal - the Homo sapiens. Modern humans, evolved around 100,000 years ago in Eastern Africa, spread all over the Earth and have come to dominate the Earth over the most recent epoch, known as the Holocene.
Humans began disrupting the environment as soon as they appeared on Earth but increased pressure on the environment when they began migrating out of Africa and spreading throughout the world about 100,000 years ago. Fossil records clearly show that as soon as humans arrived in a new region native species became extinct. Species that had never experienced contact with humans before were probably hunted to extinction. Humans arrived in the Middle East about 90,000 years ago and in Europe 40,000 years ago. The Neanderthals who had been in Europe for about 160,000 years, lived along side early humans for about another 10,000 years, but then disappeared, either wiped out by warfare or by being unable to compete with humans for food and shelter. Humans arrived in North America about 12,500 years ago and wiped out the mammoths and mastodons. Wherever humans migrated species disappeared.
Massive pressure on the environment came about when humans invented agriculture about 10,000 years ago. Agriculture may have been invented in Middle East or in different places but over a short time it spread around the entire globe.
The use of agriculture accelerated the pace of the extinction. A natural system only allows a limited number of a given species to inhabit a given area. Agriculture enabled the human population size to expand because it allowed it to exceed the carrying capacity of the local ecosystem. Humans could produce food for immediate use, storage or trade and could overpopulate. By domesticating plants and animals they became the first species to live outside ecosystems. Traditional farming destroys ecosystems. It turns biodiversity into monoculture. It defines native plant and animal species as weeds and pests.
The current mass extinction is caused by humans through such activities as:
The rate of extinction increases as the effects of these causes interact with each other. Genetic pollution from genetically modified organisms and climate change from burning fossil fuels will have a greater influence on the rate of extinction in the future.
The sixth mass extinction is different from all previous extinctions because it has a biotic, rather than a physical, cause. Humans are causing vast physical changes on the planet. The rate of extinction has been increasing with time. The process started slowly 100,000 years ago but is has got faster. The process has been particularly rapid over the past 200 years since the industrial revolution.
The Earth has always recovered after mass extinctions given sufficient time (millions of years) and when the physical cause (ice from an ice age or debris from a meteorite impact) has gone. We are the cause of the sixth extinction. Unless we modify our behaviour the sixth extinction will continue and future generations will inherit a more impoverished and unhealthy planet. The Earth may no longer support us and we may become extinct.