The observable universe consists of over 200 billion galaxies that are home to trillions of star clusters. The probability of life existing in several stars is vast but rare; this is because several factors must be crossed off a long checklist in order for a planet to support life. The existence of microbial life in the universe has a higher probability compared to multicellular organisms. However, the existence of intelligent species in the universe has a much lesser probability due to the complexities in evolution regarding chromosomes and DNA.
Everything has a story to tell
Everything in this world, big or small, significant or insignificant, has a story to tell. Take, for instance, a pencil, right from the time it was manufactured until it withers away writing its last letter; it would have a wonderful story to share. If something as small as a pencil has such a beautiful story, just imagine what kind of a story would a 4.5-billion-year-old planet, our Earth, have to say. You may be wondering, who will be able to share the journey of our Earth? If you need to understand the history of our Earth and how life evolved, our genius Geologists are the ones who can explain it best.
Geologists, what would people who specialize in the study of lifeless rocks know about our Earth and the evolution of life? You may ask. Yes, it’s the layers of rock that have been sedimented over billions of years that tell us the history of our planet; it is the fossils of primitive animals like dinosaurs embedded under layers of rock that tell us about life and how it evolved. Geobiologists can tell us how life began, adapted, and evolved. Geochemists can tell us the inner workings of the Earth, the mechanisms behind the geological activity, and the constituents of minerals present in rocks.
History of our Earth
We all know that our Earth was a barren hell 4.6 million years ago, but don’t we want to know how a barren world began beaming with life? The process of the Earth cooling down took over a billion years. When our planet was first formed, it spewed out hot lava, making the surface thousands of degrees hot. Over a few million years, due to the lack of atmosphere and the vacuum of space, our planet cooled down. The hot lava that cooled down formed Basalt, the first rocks on our planet that added a layer of crust on our planet. Basalt contained some important ingredients necessary for life, minerals.
The Six Stages of formation
From an extensive study of our planet’s rocks, Geologists have theorized that our Earth has been through six stages in its formation from the beginning to the Earth we see today. These six stages are black, grey, blue, red, white, and green. The black stage was when the planet looked black due to the sedimentation of Basalt. Meteorites that formed the earth had about 250 minerals, a starter kit containing building blocks for life. Due to intense heat and pressure emitted during the planet’s early formation, many new minerals began to form, which turned the planet’s appearance from black to grey. In the Grey period, granite became the foundation of continents. One thing that baffles Geologists is that these six stages happened in quick succession over the course of 4.5 billion years, which is fast in a Geological time scale.
Water, the fundamental building block of life
The blue stage is the most important stage as this was when the first organisms began to form due to water and oxygen in the atmosphere. There has always been a never-ending debate on how water formed on Earth, with several theories floating about. Research has led experts to the discovery of hydrogen signatures in meteorites that matched the rocks found on the Earth’s mantle. With a lot of oxygen bound up with minerals, which could be liberated under certain circumstances and combined to form water. This process happens in magma, molten rock containing dissolved water that rises from the mantle to the surface in volcanos as a hot spring. As the pressure in the atmosphere fell, water vapours evaporated into the atmosphere as steam condenses and falls back on Earth as rain filling our oceans and rivers.
Geologists discovered Zircon, an essential mineral present 4.3 billion years ago, which could have formed only in the presence of liquid water. Experts debate whether microscopic life could have existed 4.3 billion years ago as the first forms of life were discovered in stromatolites, the earliest fossils of life that existed 3.5 billion years ago.
Proterozoic era, the formation of single-celled organisms
In the red stage of the Earth’s evolution, the atmosphere was rich in nitrogen and carbon dioxide but lacked oxygen. Microbes that were floating in water or stromatolites began changing everything, turning the Earth red. The first single-celled organisms were cyanobacteria that thrived in trillions. These microbes began living off the sun’s energy through photosynthesis, which led to a rise in oxygen levels in the atmosphere. Oxygen was a new gas to our planet back then; with the early oceans filled with dissolved iron, the new oxygen reacted with iron and began to rust, which gave Earth a red appearance.
With Earth’s new continents forming and breaking apart, there was dramatic extremes in the climate. Our planet froze into ice and snow, thereby making it appear white. In the white stage, most life was nearly wiped out with only a hand full of single-celled microbes living near dying volcanoes where there was warmth. These volcanoes spewed out carbon dioxide that heated up the Earth, and over the course of a few million years, ice began melting due to a natural greenhouse effect.
Paleozoic era, the formation of complex multi-cellular organisms
For nearly 2.5 to 3 billion years, single-celled organisms were the only complex life on Earth. Once all the ice melted, oxygen levels in the atmosphere skyrocketed and paved the way for cells to multiply and evolve to form complex life. Five hundred forty million years ago, our planet became a paradise beaming with life. The first multicellular organisms were invertebrates, algae, and small plants. Trilobites were one of the first invertebrate creatures that thrived in the seabed during the early Paleozoic era. They were arthropods, had hard shells, and survived on worms and small plants.
The Paleozoic era was known to be the kingdom of sea creatures, as primitive fish, corals, sea scorpions, amphibians, and aquatic plants evolved and thrived in this period. Around 359 to 299 million years ago, amphibians slowly began ruling the land and began evolving to adapt to living on land. This evolution happened throughout a few tens of millions of years. The end of the Paleozoic era was known as the Permian period when continents joined together to form a supercontinent known as Pangea. During this period, the first non-mammalian synapsids like Dimetrodon and Edaphosaurus and therapsids such as Gorgonopsids and Scutosaurus walked the Earth.
Pangea was relatively dry with harsh seasons as large water bodies did not regulate it. These conditions resulted in 95% of life on the planet becoming extinct. This event, known as the Permian extinction, occurred around 250 million years ago, which marked the end of the Paleozoic era. Although the adverse climate is one of the widely accepted theories that caused the Permian extension, other theories include extreme volcanic activity, hydrogen sulphide emissions, and perhaps even an asteroid impact.
Mesozoic era, the reign of the dinosaurs
As the famous saying by Ian Malcolm (Jeff Goldblum) in the Jurassic Park movie goes, “One thing the history of evolution has taught us is that life cannot be contained. It breaks free, expands to new territories, and crashes through barriers painfully, maybe even dangerously. Life will always find a way.”
After the great Permian extinction, the supercontinent Pangea began breaking free due to the movement of Earth’s tectonic plates and forming smaller continents. The movement of oceanic and land plates paved the way for rivers to form and diverse climatic conditions favourable for life.
The Mesozoic era is the period where Dinosaurs and small mammals thrived. This period of 190 million years is further divided into Triassic, Jurassic, and cretaceous periods respectively. Dinosaurs, birds, and small mammals co-existed in this era along with gymnosperms, angiosperms, and flowering plants. Sauropods, Theropods, and Ornithischians were the three main species of dinosaurs that existed with flying and aquatic dinosaurs. Sauropod dinosaurs had long necks, long tails, small heads and walked on all fours. They were herbivores and existed during the Jurassic period measuring nearly 25 to 30 meters in length. Some Sauropod dinosaurs are Brachiosaurus and Apatosaurus.
Theropod dinosaurs were bipedal, had three-toed limbs, and were either carnivorous or omnivorous. They existed during all three periods and hunted in packs. The infamous Tyrannosaurus Rex (T-rex), Carnotaurus, Allosaurus, and Spinosaurus are theropods. Ornithischian dinosaurs were herbivorous dinosaurs with pelvic structures similar to birds. They were quadrupedal, with some of them being omnivores too. They existed during the Jurassic and Cretaceous periods. Triceratops, Ornithopoda, Thyreophora, and Stegosauria are some examples of Ornithischians.
Dinosaurs coexisted with small mammals in the Mesozoic era that were quite small and weighed less than 15 kg. Mammals back then were cynodonts, with some fossil records stating that they lived during the late Permian period. Believe it or not, these small mammals were our ancestors, as they survived the great extinction that occurred when a large meteor hit the earth. A large meteor hit Earth during the end of the cretaceous period 66 million years ago, ending 75% of life. This mass extinction event marked the end of the Mesozoic era. Evidence of this meteor strike can be found in the Chicxulub crater impact crater buried underneath the Yucatán Peninsula in Mexico.
Cenozoic era, the reign of mammals
After the giant meteor hit Earth and wiped out the dinosaurs 65 million years ago, mammals, certain birds, and some sea creatures were left alive. Most mammals could survive the meteor impact as they were small, agile, and could easily dig deep holes and go into hibernation. The extinction of dinosaurs is one of the main reasons why mammals could evolve and thrive on Earth. If the dinosaurs continued to exist, most of the mammal species would have been wiped out as they were easy prey and might not have adapted well with the dinosaurs in the long run as it would have prevented them from evolving.
Mammals like rhinoceros, cats, dogs, hippos, gorillas, mammoths and other creatures evolved to adapt to the new climate. Prosimians, the ancestors of the first apes, existed over 50-55 million years ago. With millions of years of evolution, they became monkeys, which resulted in many prosimian species becoming extinct. During the Miocene geological period, 20-5 million years ago, apes evolved from monkeys and were displaced from many environments. Their evolutionary line led to hominins, chimpanzees who were connected to homo-habilis and Neanderthals. However, it took another few million years for homo-habilis to evolve to homo-sapiens, modern humans, whose fossil records date back to 100,000 years. Humans have also been through their fair share of challenges and survived an ice age that occurred 13,000 years ago.
The history of how life evolved on our planet is truly fascinating. Regardless of two major extinctions that wiped out nearly all life on Earth, there is always a way for survival if creatures can adapt and evolve to thrive in different environments. As Charles Darwin said, “it is not the strongest of species that survives nor the most intelligent that survives. It is the one that is most adaptable to change.”
Space Age 