When we talk about the past and we make an assertion about something being really old, it usually means that the entity or occurrence is about 2000-5000 years old. The average life expectancy of the world in 2013 was 71 years. So, the really old incident was 80 generation ago if we assume life expectancy at birth to be constant. Although 80 generations don’t feel like a lot but we know how fast things can change for a species, we have seen it in our own lives. What if we desired to predict what life is going to be like 80 generations from now? It will be very challenging to make a meaningful prediction. And what about 8 million generations? Our brains have not evolved to comprehend time scales much longer than our life spans. It didn’t have any imaginable evolutionary advantage, at least before the information age.

Then we invented numbers, text and other cool stuff which helped to stockpile information in bodily form rather than some ethereal concept in our heads. We began noting down events in written language which can be preserved through time. It made it possible for history to exist in any physical form and get accumulated over time. Here is a chronological narrative of earth’s account with a description of key events that steered earth to the way that we see and feel today.

The earth as a planet has been orbiting the sun for approximately 4.5 billion years. That’s a very long time. A lot can happen in 4.5 billion years and in fact a lot has happened. Earth has changed from being a hot ball of fire to a gentle blue planet. It formed when a cloud of dust orbiting the newly formed sun coalesced into a spherical form under the sway of gravity. All the planets formed like this. In the present day, most of the dust has congregated to form the eight planets, the asteroid belt and everything else in our solar system.

 

The proto-planetary disk from which all planets are formed.

The proto-planetary disk from which all planets are formed.

 

The solar system was a violent place during its infancy. Huge rocks flew ubiquitously in emptiness in every possible course. That’s how we think we got water, from space. During the first 500 million years of earth’s existence, it was constantly being bombarded with space debris containing all sort of different compounds including water and possibly amino acids. These meteorites brought water with them in the form of ice which collected on Earth over time.

 

Before the earth was covered with water, a Mars-sized object crashed into it, untying a big chuck of mass that later formed the moon. The moon was previously much closer to the earth but with time, due to tidal friction, it has moved away and became tidally locked with earth.

 

moon-formation

 

To make it easier to record and study, the Timetree of earth has been separated into 4 eons – the Hadean, Archaen, Preterozoic and Phanerozoic eons, 4500-4000 million year ago, 4000-2500 mya, 2500-500 mya, 500-0 mya, respectively – which are further divided into various sub classes.
The oldest fossils were found about 3500 mya during the Paleoarchaen era of the Archaen eon. These fossils were called Stromatolites. These are rock like structures sculpted by singled-celled cyano-bacteria. These bacteria had no internal membrane bound organelles and performed photosynthesis in distinctive folds in their outer membranes.

 

The Stromatolites of Shark Bay

The Stromatolites of Shark Bay

 

The first living cells were prokaryotic cells which had no nucleus but diffused DNA and ribosomes inside the cell walls. About 2 billion years ago, the first eukaryotic cells were formed by fusion of two or more prokaryotic cells. Mitochondria are an example of a prokaryote which started living in other cells. Mitochondria have a separate DNA than the rest of the cell.

About 3500 mya, most of the world was concealed inside a thick layer of water. The temperature was abundantly high to prevent any ice from forming on the poles. Slowly, as the earth cooled down, ice started to colonize the poles and the water level throughout the world fell. During this time, several islands formed.

Throughout the history of the earth, numerous factors have led to increases and decreases in the ocean levels. And the landmasses have many times come together into a supercontinent and broken apart.

Photosynthesizing cyano-bacteria evolved in 3500 mya; they used water as a reducing agent, thereby producing oxygen as a waste product. The oxygen initially oxidized dissolved iron in the oceans, creating iron ore. The oxygen concentration in the atmosphere slowly rose, acting as a poison for many bacteria and eventually triggering the Great Oxygenation Event. Earth was continually wracked by hurricane-force winds. These extreme mixing influences are thought to have stimulated evolutionary processes.

A timeline of important events that marked the evolution of life on earth:

⦁ 800 mya – First multicellular organisms may have arisen.

⦁ 850–630 mya – A global glaciation may have occurred. Opinion is divided on whether it increased or decreased biodiversity or the rate of evolution.

⦁ 600 mya -The accumulation of atmospheric oxygen allows the formation of an ozone layer. Prior to this, land-based life would probably have required other chemicals to attenuate ultraviolet radiation enough to permit colonization of the land.

⦁ 580–542 mya – The first large, complex multicellular organisms evolved.

⦁ 580–500 mya – Most modern phyla of animals begin to appear in the fossil record during the Cambrian Explosion.

⦁ 535 mya – Major Diversification of living things in the oceans.

⦁ 530 mya – The first known footprints on land date to 530 mya, indicating that early animal explorations may have predated the development of terrestrial plants.

⦁ 251.4 mya – The Permian–Triassic extinction event eliminates over 90-95% of marine species. Terrestrial organisms were not as seriously affected as the marine biota. This “clearing of the slate” may have led to an ensuing diversification, but life on land took 30 million years to completely recover.

The Permian Extinction event marks the beginning of the Mesozoic era of the Phanerozoic eon.

 

Pangaea with modern borders.

Pangaea with modern borders.

 

About 300 mya, all the continents had assembled to form a supercontinent called Pangaea which was surrounded by a super ocean called Panthalassa. By the end of the Paleozoic era, Pangaea started to break into fragments.

First it broke into two parts, Laurasia and Gondwanaland. It is possibly the reason that fossils of lizard-like creatures were found on Laurasia whereas snakes and iguanas were found on Gondawana. These two species have a common ancestor and it is possible that they grew different morphological features since they got separated during the splitting of Pangaea.

Most dinosaur groups diversified during the Jurassic and the Cretaceous 200-66 mya, because this was the time when the continents were moving apart. This led to the availability of new niches for organisms that were geographically separated. And then, everything evolved separately, independent of each other but depending on the niches that they had occupied.

 

 

The splitting on Pangaea.

The splitting on Pangaea.


 

Anguidae is a diverse family of lizards native to the Northern Hemisphere. The group includes the slowworms, glass lizards, and other small lizards. Varanoid are the family of lizards which are much bigger in size like the monitor lizard or other marine reptiles like the Mosasaurs. The two groups diversified during the Cretaceous-Cenozoic boundary about 70 mya.

 

Anguis fragilis

Anguis fragilis

 

Varanus varius

Varanus varius

 

Current evidence strongly suggests that snakes evolved from an aquatic or burrowing varanoid ancestor, though much debate continues over precisely which lineage of varanoids.

One third of all the lizard families diverged during the Mesozoic-Cenozoic boundary possibly because of the Cretaceous-Paleogene extinction event as it put almost every species under a lot of evolutionary pressure.

Dibamidae are legless tropical lizards which diverged from Bifurcata, which is another family of lizards, which might or might not be legless, before the Splitting of Pangaea, therefore no geographical influences are seen in their Genome. Gekkons and Unidentatans, scinciformatans and episquamatans are similar examples of divergences before the splitting of Pangaea.

The evolution of mammals has passed through many junctures since the first appearance of their synapsid ancestors in the late Carboniferous period, 359.2 to 299 million years ago. Synapsids were mammal-like reptiles with characteristics similar to dinosaurs. Dimetrodon is a good example. Synapsids were the largest terrestrial vertebrates in the Permian period, 299 to 251 million years ago.

 

Dimetrodon

Dimetrodon

 

The ancestry leading to today’s mammals split up in the Jurassic. Even though these animals laid eggs rather than giving birth to young ones, they are more closely related to marsupials, like kangaroos, and other mammals which give birth rather than monotremes, which are egg laying mammals like the platypus.

 

Platypus

Platypus

 

Marsupials diverged from Eutherian (primates, carnivores, etc.) after the splitting of Pangaea into Laurasia and Gondwanaland, in the mid Jurassic, possibly because the splitting of the continents put them in different niches.
After the Cretaceous-Paleogene extinction event wiped out the non-avian dinosaurs (birds are generally regarded as the surviving dinosaurs) and several mammalian groups, placental and marsupial mammals diversified into many new forms and ecological niches throughout the Paleogene and Neogene, 66 to 2.58 million years ago, by the end of which all modern orders had appeared. The largest land mammal to have ever lived, called Paraceratherium, existed during the early to late Oligocene epoch, 33.9 million to 23 million years ago.

 

 

Paracerathrium

 

Modern humans are thought to have evolved in Central Africa about 200,000 years ago. By this time, the continents had taken their familiar shapes but sea levels were much lower.
We have a common ancestor with the chimpanzees about 6 mya. The australopithecines arose around 4 million years ago and diverged into several branches, one of which probably went on to become ancestors of the genus Homo.

 

Australopithecine

Australopithecine

 

The oldest member of the genus Homo is Homo habilis which evolved around 2.8 million years ago. It is the first species which used stone flint tools. The brains of these early hominids were about the same size as that of a chimpanzee, and their main adaptation was bipedalism as an adaptation to terrestrial living. The earliest fossils of anatomically modern humans are from the Middle Paleolithic, about 200,000 years ago such as the Omo remains of Ethiopia; later fossils from Es Skhul cave in Israel and Southern Europe begin around 90,000 years ago (0.09 million years ago).

 

Flint tools used by early humans

Flint tools used by early humans

 

The period between 2 mya and 11,000 years ago is called the Pleistocene. It was period of extensive glaciation and the earth went through several ice ages. The most recent one ended about 11,000 years ago with the end of the Pleistocene. A large amount of water from the ocean was trapped in the form of glaciers and ice sheets. Several land bridges existed during this period like the one joining eastern Russia with Alaska and the one joining the islands of south eastern Asia.

 

Earth during the Pleistocene

Earth during the Pleistocene

 

It made it possible for several species including humans to migrate to North America and from there to South America and so on.

 

Migration of hominids.

Migration of hominids.

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