We see the actual spirit of diversity in Nature.But have things always been like this, barring a few minor change-in-scenes, since times immemorial? If not, how did and how much has life forms changed? So, let’s find out from where all this started.

The Beginning: Life on Earth

After formation, Earth was not exactly suitable for housing or nurturing any kind of organisms. After undergoing drastic changes -including solidification of crust, formation and accumulation of various gases (like nitrogen,ammonia,methane,oxygen,hydrogen etc.) in the atmosphere, gradual development of ozone layer and formation of water molecules- Earth became close to be able to support even the most basic forms of ‘life’.

The earliest organic forms, though did not really satisfy our definition of life, but are considered to be the most basic forms of living cells.Experiments by Stanley Miller (1953), Sidney Fox and Oparin (1936), gave the world some theories about the formation of living organisms. Another mention worthy point is that the first life forms originated in water, as for the longest time, Earth was what we can imagine as a giant sea, with no signs of land whatsoever. These organic forms might have developed into self replicating units, then into primitive organisms and at later stages, resulted into some distinct type of chlorophyll synthesizing bacteria, some 2.5-2.3 billion years ago. This gradual process of arising of life from non-living matter (such as simple organic compounds) is termed as abiogenesis.

Evolution of chlorophyll occured around 600 million years ago. With that, the production of oxygen and ozone (after chemical conversion under suitable circumstances) sped up and gradually accumulated in the atmosphere. Ozone layering encased the surface of Earth and provided protection against UV rays, which is detrimental for living organisms. Before that, all organisms developed in deep water, which provided protection against the UV rays. Only after the formation of ozone layer did life flourished on land surface.

The Precambrian age, that last from 4600 to 542 million years old, has very few fossil records. Hence, such a large period of time is not subdivided into further categories in terms of biological evolution. The fossil of this period mainly of cellular organisms.The first living organisms are believed to have developed around 3.8-3.6 billion years ago, from the primitive organic soup (accumulation of large number of complex organic molecule assemblages). Around 3.5 billion years ago,first organisms with prokaryotic cells (cells without true nucleus and having a rudimentary cell structure) are believed to have developed. Those cells were at similar cell complexity level like the present day bacteria.Skip to 2.4 billion years later, the eukaryotic cells are supposed to have developed. For the longest period of time, only unicellular life forms flourished on Earth. Then,evidences indicated that, around 700 million years ago, the first signs of multicellular organisms were traced. Since then, there has been a steady flow of evolution. 

The ‘Complex’ Evolution

The Cambrian period (542- 488.3 million years) is considered to be a milestone in evolution. Even the fossils have records from around 600 million years ago. It is widely assumed that around 600 million years ago, organisms had developed to be able to leave fossilized records. The transition of life forms from water to land had supposedly begun around 500 million years, when plants started growing on land.Later different varieties of plants and animals populated the lands.

The Carboniferous period was an important landmark, which lasted from about 359 to 299 million years in the past, is an important landmark in the journey of evolutionary history. The environ domain was actively dynamic during that period. Humid and warm climate, swampy large areas, changes in sea levels, flood, mud and sand were deposited over the vegetations, etc.

Due to compaction and incomplete decomposition of  plant material in swamps, they converted into peat bogs, and then, into coal. Hence the name carboniferous. In the late Carboniferous period (about 300 million years) tropical rainforests were present in the equator region of Euramerica. Later, ferns replaced the forests, which are present even today.These vascular plants are without seeds as they do not produce flowers and fruits, and are generally found in temperate to tropical parts of Earth. These plants can be mainly divided into stem, leaves and true root; reproduction being facilitated through special structures, known as sporangia (singular:sporangium), that contain spores, which are dispersed into different mediums and germinates on coming in contact with the soil with suitable conditions.

Next period in the geological time scale in the Permian period, which started off around 299 million years ago. This period witnessed major altering activities in the continental lithosphere (which are broken into tectonic plates) starting from the joining of Euramerica, Asia and Gondwana led to the formation of a single landmass called Pangea (Greek translation: All lands), which is considered to be a supercontinent. In terms of ecology, this period was mainly dominated by forests of conifers and tree ferns,large amphibians and reptiles habituating them, and a huge variety of fish and shellfish in the ocean. But, at the end of Permian period, a mass extinction took place, commonly referred to as the ‘Great Dying’. It occured around around 252 million years ago, and wiped out about 90% of all the species present on Earth.Also, trilobites (meaning: three lobes), which a group of arthropod (a group of invertebrate animals with exoskeleton, segmented body, and appendages) and sea scorpions, which had survived the tides of oceans and time for hundreds of million of years, went extinct. 

Next in the line, the Triassic period had set off around 240 million years ago.It is believed that true mammals first evolved during this period, more precisely, during late Triassic period. Also, a specialized subgroup of  Therapsids, which is a group of reptile-like animals that included mammals and their ancestors, also evolved. Early Therapsids demonstrated traits of the present day mammals, such as erect posture. First flying vertebrate animals are supposed to have originated around 215 million years ago, and lasted for 150 million years, vanishing at the end of the Cretaceous period. Another mention worthy occurence in the Triassic period timeline is the splitting of Pangea into two separate land masses. The northern one was named Laurasia and the southern one, Gondwana. Primitive forms of dinosaurs are said to have emerged around 231.4 million years ago.

The Jurassic period is known as the ‘Age of Reptiles’, as they were present in abundance and had a diverse variation throughout this period. The land animals included Stegosaurus, Ceratosaurus, Brachiosaurus and Allosaurus. The oceans were traversed by Ichthyosaurus and Plesiosaurus and the skies are believed to be dominated by the likes of Rhamphorhynchus and Archeopteryx. This period coincided with the emergence of first mammals and birds.

 The Cretaceous period, which started around 145 million years ago, is geologically characterized by separation of India from Gondwanaland, formation of Andes and Rocky mountains, and extensive volcanic activities. Dinosaurs dominated the face of Earth and were of different types. They lived through a period of 135 million years, that is , throughout the Jurassic and Createceuos period. The Cretaceous-Paleogene mass extinction is considered to be the main cause of dinosaurs disappearing from Earth. An important claim by the scientists, on the basis of fossil records, is that the birds evolved from the Theropod dinosaurs, and the development took place in the Jurassic period. Some birds lived through the extinction and their descendents continue to live presently. This period also marks the appearance of angiosperms; plants that could produce flowers. 

Next chapter

The next major development in the course of evolution is considered to be the evolution of humans. But that’s a story for another day. As for our evolution is anything but simple and short. As Carlo Rovelli said in his book, Seven Brief Lesson of Physics: “We are a species which is naturally moved by curiosity, the only one left of a group of species (the genus Homo) made up of a dozen equally curious species. The other species in the group have already become extinct; some, like the Neanderthals, quite recently, roughly thirty thousand years ago. It is a group of species which evolved in Africa, akin to the hierarchical and quarrelsome chimpanzees — and even more closely akin to the bonobos, the small, peaceful, cheerfully egalitarian and promiscuous type of chimps. A group of species which repeatedly went out of Africa in order to explore new worlds, and went far: as far, eventually, as Patagonia — and as far, eventually, as the moon.

It is not against our nature to be curious: it is in our nature to be so.”

Website References

https://en.wikipedia.org/wiki/Abiogenesis

https://www.goodreads.com/quotes/9344668-we-are-a-species-which-is-naturally-moved-by-curiosity

Bibliography

Extinctions: No Comebacks by M.A. Haque