The ongoing extinction, Holocene, is being caused by the activities of “One Weedy Species” – humans
The evolutionary history of life represents the current scientific theory outlining the major events during the development of life on planet Earth.
Biologically, evolution is important because it drives biodiversity: some traits will become predominant while others will become rare over the course of time.
<On Earth, life began about 3.5 billion years ago. The first life forms were single-celled organisms. The first multicellular organisms appeared about 610 million years ago (mya), which led to the evolution of many types of organisms during the next ten million years in an event called Cambrian Explosion. This sudden burst of evolution may have been caused due to environmental changes, which made Earth’s environ more suitable for a wider variety of life forms.
Plants and fungi appeared about 500 mya, followed by arthropods, amphibians (300 mya), mammals (200 mya) and birds (around 150 mya). Though large life forms have been very successful on Earth, most life forms today are prokaryotes — small, relatively simple single-celled organisms. More than 99% of all species, amounting to over five billion that ever lived on Earth, are estimated to be extinct. The current species are estimated to range from 10 million to 14 million, of which only 1.2 million have been documented.
Anatomist Georges Cuvier first proposed in 1796 in Paris that life on Earth was not static and that species could disappear due to comet or asteroid impact. The scientific community accepted his hypothesis in the early 1990s, with the further study of geological and fossil records that the Cretaceous-Paleogene (K-Pg) extinction was due to an extra-terrestrial impact.
Researchers now are of the opinion that K-Pg was just the latest of five major extinction events and we’re currently in the middle of a sixth event. Each event had a different impetus, some took place over the span of millions of years while others were extremely sudden, having in common though that they reshaped the face of life on Earth by wiping out a significant portion of it.
* Ordovician Extinction: This was about 440 mya, the earliest known mass extinction when most of the life lived in its seas. Its major casualties were marine invertebrates, including brachiopods, a group of shelled species, trilobites, marine arthropods, bivalves, large molluscs and corals. The main cause is thought to be due to the movement of Supercontinent Gondwana into Earth’s southern hemisphere, causing sea levels to rise and fall repeatedly over a period of millions of years.
* Late Devonian Extinction: About 370 mya, there was abrupt cooling with the development of glaciers and substantial lowering of sea levels. It caused an enormous loss to biodiversity, took place in a span of five lakh to 25 mya in which corals, trilobites and brachiopods vanished.
* Permian-Triassic Extinction: About 250 mya, trilobites were killed entirely, causing the disappearance of over 95% of marine life and 70% of land dwelling vertebrates, a huge loss to global biodiversity, and so named as Great Dying. The main cause is thought to be gradual change in climate, sudden catastrophe including volcanic eruptions, asteroid impacts and sudden release of greenhouse gases from the seafloor.
* Triassic-Jurassic Extinction: About 200 mya, it occurred before breakup of Supercontinent of Pangaea, assemblage of earlier continental units, possibly due to an asteroid impact and enormous volcanic eruptions, wiping out a third of marine species, large amphibians and crocodiles.
* Cretaceous-Paleogene Extinction: The most recent event occurred about 65 mya. It wiped out non-avian dinosaurs and extinguished many species of early mammals, amphibians, reptiles, birds and insects. Scientists estimate 75% of species living at the time of the K-Pg extinction were wiped out.
The ongoing extinction, termed as Holocene Extinction, started about 12,000 years ago. The present event may not have been the product of natural forces, as the previous five, but it is due to the increasing human interference in nature. Researchers have described it as Sixth Mass Extinction or the Anthropocene extinction. This is the ‘most serious environmental problem’/threat to the persistence of civilisation since the loss of species here will be permanent, according to the research published in the journal proceedings of the National Academy of Sciences of the United States of America.
To cite a few examples, the chemical analysis of the entire layer of soil in many parts of Australia shows it is charred and blackened, having been burned by fire on a large scale by ancient aborigines, causing the death of plants and animals. Archaeological evidence suggests that in whichever regions human arrived, the Neanderthals disappeared. But for humans, Neanderthals would not have gone extinct.
Indian Aurochs disappeared around 2000 BCE; Indian cheetah had gone extinct from India 60 years ago; Gigantopethecus, the largest known ape, is extinct; Sivatherium, a giraffe-like animal, is extinct. Silphium plant, used as a medicine and also as a contraceptive by ancient Greeks and Romans, is extinct.
The Center for Biological Diversity warns that the effect of Anthropocene extinction will worsen in the coming decades and the resulting genetic and cultural variability will change entire ecosystems. When the number of individuals in a population or species drops too low, its contribution to ecosystem functions and services become unimportant, its genetic variability and resilience is reduced.
According to a recent analysis, the sixth mass extinction of wildlife on Earth is accelerating. More than 500 species of land animals are on the brink of extinction and are likely to be lost within 20 years; the same number was lost over the whole of the last century. Scientists say without human destruction of nature, this rate would have taken thousands of years and warned that this might be a tipping point for the collapse of civilisation.
The Holocene Extinction is being caused not by natural catastrophe but by the activity of one, “One Weedy Species”, that is us — humans. Can ‘this species’ check extinction by taking well-known measures for the protection of biodiversity and, most importantly, for its own survival?
(The author is a retired IFS officer)
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