From the violent beginning of the earth, it took nearly 800 mega-annum (Ma) to cool down (Hadean Eon). Igneous and metamorphic rocks from 3800 Ma to 2500 Ma (Archaean Eon) were formed principally followed by sedimentary rocks from 2500 Ma to 570 Ma (Proterozoic Eon) and mixed rock types on land as well as in the oceans from 570 Ma to the present (Phanerozoic Eon). The understanding of the origin of life and its commencement during the Archaean Eon is still in its infancy as igneous and metamorphic rocks formed at high temperatures and pressure conditions did not preserve any fossils. However, it is conjectured that the atmospheric conditions during Hadean and to some extent Archaean Eons were not conducive for the advancement/evolution of life as the atmosphere was dominated by large quantities of water vapour, carbon dioxide, sulphur and nitrogen gases and perhaps acid. However, there are some evidences that life existed, for instance in the biogenic stromatolites dated older than 3500 Ma (mostly found in Precambrian rocks). The free diatomic oxygen necessary for evolution of life gradually built up in the atmosphere with the increase in the rate of photosynthesis by the cyanobacteria (blue green algae).
The life that prevailed during the Proterozoic was principally of unicellular organisms. At the beginning of Phanerozoic Eon a sudden evolution of life was recorded. In a little more than 500 Ma, most of the species including humans originated. Based on the biotic evolution, the Phanerozoic Eon can be classified into Palaeozoic Era (early life; 570-245 Ma), Mesozoic Era (middle life; 245-65 Ma) and Cenozoic Era (recent life; 65 Ma -today).
The Palaeozoic Era began with the origin of multi-cellular organisms – trilobites, jelly fish and worms (during the Cambrian period; 545-505 Ma) suggesting an abrupt evolution on earth. This was followed by snail, sponges during the Ordovician (505-438 Ma), corals and star fish during the Silurian (438- 408 Ma), fish, lungfish and fern during the Devonian (408 – 360 Ma), amphibians, insects and trees during Carboniferous (360-286 Ma), fin back reptiles and reptiles during the Permian periods (286-245 Ma).
The life that existed during Mesozoic era was different than the Paleozoic era in terms of the advanced reptiles, primitive mammals and gymnosperms. The Mesozoic era can be demarcated into three periods – Triassic (245-208 Ma) Jurassic (208-144 Ma) and Cretaceous (144-65 Ma). The life existed in the former period comprised of (i) the first appearance of dinosaurs and ammonites, followed by (ii) dominance of giant dinosaurs both in the oceans and on land, first appearance of mammals and conifers, and (iii) dominance of flying, carnivorous and horned dinosaurs, flowering plants, and insects during the later period, respectively. The Cenozoic era dominated by mammals began their appearance in terms of flowering plants, ancestral horse, insects, birds, whale, clawed mammals, camels dating back to 65 to 1.6 million years. The first appearance of human has been recorded in the Quaternary period (1.6 Ma) along with mammoth and the sabre toothed tiger.
However, the life that existed earlier either became greatly reduced or extinct in the later time span of the earth in a process termed as ‘mass extinction’. A mass extinction is usually observed during the end of a particular geological period. Mass extinction refers to the species diminishing or ending its existence in a particular biological world probably due to large-scale calamities or catastrophic processes such as earthquakes, tsunamis, volcanic eruptions and meteoritic impact. However, there could have also been certain climatic factors – famine, drought; and biological, genetic factors causing infertility; and human factors leading to deforestation and large-scale hunting of animals.
There are as many as six major mass extinctions recorded in geological time scale and a number of smaller ones that occurred in the Phanerozoic Eon.
i.About 52 per cent reduction in the families of trilobite, sponge and gasteropod occurred during the late Cambrian;
ii. Nearly 24 per cent reduction in the families of trilobite, brachiopod, crinoid and echinoid occurred during late Ordovician;
iii. More than 30 per cent reduction in the families of coral, stromatoporoid, trilobite, ammonoide bryozoan, brachiopod and fish occurred during late Devonian period;
iv. About 50 per cent reduction in the families of bryozoans and reptiles occurred during late Permian;
v. More than 35 per cent reduction in the families of brachiopod, ammonoite fish and reptile occurred during late Triassic; and,
vi. Nearly 26 per cent reduction in the families of beleminites, corals, echinoids, sponges and planktonic foraminifera occurred during the late Cretaceous period.
Among the above, the major extinction of rugose corals, trilobites, blastoids, inadunate, flexibiliate, camerate crinoids, productid brachiopods and fusulinid foraminifera occurred during the late Permian; conodonts during the late Triassic and ammonites, rudistid mollusks, dinosaurs and large marine reptiles towards the end of Cretaceous period. However, certain species, for instance sponges, brachiopods, bivalves and ostrocods continue to exist throughout the Phanerozoic Eon with variations in their populations – with the dominance of sponges, bryozoans, brachiopods and ostracods particularly during the transition of Palaeozoic to Mesozoic Eons. This suggests that the dominant species become extinct by the end of a particular geological period, thus offering space for the advent of newer and evolved life forms.
In most of the major mass extinctions the main causative factor was the trigger of large-scale volcanic eruption. The Antrim volcano that erupted in Ireland in about 511 Ma perhaps caused the mass extinction between Cambrian and Ordovician periods. The boundary between Permian and Triassic has been the most conspicuous by a massive and large-scale Siberian volcanic eruption (251-250 Ma ago), and the estimated volume of 1.0 to 4 million cu km covering an area of 2 million sq km.
The dinosaurs’ extinction towards the end of Cretaceous period is linked to Deccan volcanic eruption (India) where the volume of lava erupted has been estimated to 1.5 million cu km, covering an area of more than 500,000 sq km. Also, meteorites bombardment on Earth as well as cold periods appeared to have played a significant role in the mass extinction. It is clear from the recent studies that volcanic eruption can reduce the global temperature by 1 to 2oC because of the absorption of incoming solar radiation by volcanic materials. This gradually reduces photosynthesis leading to a considerable reduction in species as well as their population as a consequence of shortage of food. In addition, volcanoes emit a lot of oxides of carbon, sulphur and nitrogen leading to the acidification of the ecosphere. They may also trigger ice age – for instance, the boundary between Permian and Triassic, and Cretaceous and Tertiary periods were marked by cold (glacial) conditions.
Climate change has serious consequence on sustenance of life on earth. For instance, the severe drought that occurred over 4000 years ago collapsed the old world’s civilization – Akkadian, Mayan, Indus Valley and Chinese civilizations. To return then to the question of when the human race is going to be extinct – the answer is, not soon and certainly not now, because the catastrophic processes are not likely to occur simultaneously around the earth. Also human beings today can adapt better and ‘combat’ the risk of extinction. However, gradual processes, such as evolution can cause a considerable amount of species reduction and explosion of particular species due to scarcity natural resources. A catastrophic end of life on earth, however, marks the origin of new species – just as the extinction of the dinosaurs led to the rise of the mammals. The extinction of humans is thus inevitable.