Life on Earth

ORIGIN

Our Earth originated about 4.6 billion years ago by the accretion of dust and gases swirling around the sun in our solar system. The most characteristic feature of planet Earth is 'life' which evolved later around 3.5-3.6 billion years ago first, if 'life' evolved in a surface or subsurface aquatic environment. It has also been argued that if 'life' evolved around deep oceanic hydrothermal vents, its first evolution is even more ancient dating as far back as 3.77-4.29 billion years ago (Dodd et al. 2017). Milestones of evolutionary history- 'fossils', lead us only as far back as 3.5 billion years in the dark history period of Earth. At that time life existed in the form of small, simple filamentous microscopic organisms which resembled bacteria and thick mats of their bodies formed the fossils known as stromatolites. Life originated by 'abiogenesis' and gradually evolved into different types of organisms by organic evolution. 

Whatever living organism diversity exists on earth today is a small fraction of diversity which has existed on earth in different times. After the origin life diversified and this diversification was limited by extinction events from time to time in a long evolutionary history, some of the extinction events were so catastrophic that more than 75% of species existing before it were completely wiped out. This wrath of nature is now termed as 'mass extinction'. As many as five mass extinction events have been faced by life on earth, but after each one life diversified again. Species were not only made extinct by mass extinctions but there were numerous extinction events, though less severe than mass extinction, which ended a significant amount of species diversity on earth.

DIVERSITY

Today we estimate the diversity of life often by the number of species or 'species richness' which is embarrassingly diverse and not known completely to us. Mora et al. (2011) estimated the existence of about 8.7 million species of eukaryotic organisms on Earth's surface and its oceans. Our listing of eukaryotic species in Catalogue of Life (https://www.catalogueoflife.org/col/) shows 14,24,542 species of animals, 61,930 species of Chromista, 1,45,155 species of Fungi, 3,74,236 species of Plants, 2,592 species of Protists, together making this count of eukaryotes as 20,09,456 species which is mere 23% of the estimated species richness of eukaryotes. Taking it as a base, we have discovered only a smaller fraction of the species richness of the earth and a major fraction (77%) is still to be discovered.

The 'Pie of Eukaryotic Species' depicts our limitation in discovering the unknown species. Just a decade ago Mora et al. (2011) estimated that with the present rate of new species description, 3,03,000 taxonomists are required to discover remaining unknown species in 1,200 years by spending about US& 364 billion! This cost of a nearly unrealistic goal must have increased many folds in the past decade. It calls for an enormously larger task force of quality taxonomists to discover new species; another unrealistic expectation! Species are the babies of organic evolution and with slow rate of new species discoveries, dearth of quality taxonomists, lack of monetary resources for taxonomic research, ignorance of taxonomy in academic curricula,  vis-a-vis increased rate of species extinction are leading to the situation where we shall have no knowledge of these evolutionary babies and these will vanish without getting any name! We badly need good taxonomists!

CLASSIFICATION

Classification of life started with the 2-kingdom (Plantae and Animalia) classification of organisms proposed by Carl Linnaeus. It was upgraded to 3-kingdom (Plantae, Animalia, Protista) classification given by Antonie von Leeuwenhoek, 4-kingdom (Plantae, Animalia, Protista, Monera) classification by H.L. Copeland (1938), 5-kingdom (Plantae, Animalia, Monera, Protista, Fungi) classification proposed by R.H. Whittaker (1969), and 6-kingdom (Archaebacteria, Eubacteria, Protista, Monera, Fungi, Animalia, Plants) classification given by Carl Woese & G.E. Fox (1977) and later by Cavaliar-Smith (1998, 2004, 2010) with Bacteria, Protozoa, Chromista, Fungi, Animalia, Plantae as the kingdoms. In the last decade, Ruggiero et al. (2015) have proposed a detailed classification of life wherein they recognize 2-superkingdoms- Prokaryota and Eukaryota and 7-kingdoms. 2 of the kingdom (Bacteria, Archaea) constitute the superkingdom Prokaryota whereas rest 5 kingdoms (Protista, Chromista, Fungi, Animalia, Plantae) constitute superkingdom Eukaryota. The kingdom Plantae contains the group Gymnosperms, and Angiosperms and the members of these two traditional groups occurring in the Pantnagar area are displayed in eflorapantnagar.