HOW LIFE ORIGINATED ON EARTH
EVOLUTION
INTRODUCTION
Around 4 billion years back, life started on Earth. After a really long time, many different kinds of plants and animals came into being through changes over the years. All the trees, plants, animals, and creatures we see now are here because of this process called evolution. One of these groups is 'Homo sapiens' – that's us, humans!
Many of us think that humans came directly from monkeys, but that's not exactly what the theory of evolution says. According to this idea, chimpanzees, gorillas, monkeys, and humans all share a common ancestor, a kind of animal that doesn't exist anymore.
Back in 1859, a biologist, Charles Darwin wrote a book called "On The Origin of Species." In this book, he talked about 'Natural Selection.' This is about how animals, trees and other creatures reproduce, and when they do, their genes pass down with a little bit of change, known as mutation. These changes make differences in how they look or act. These differences are passed down to their kids too. Sometimes these differences are good, helping the animals do well in their surroundings. But sometimes they're not so good and can cause problems. This means that the ones who are the best fit for their environment are the ones most likely to survive. This whole process is called Natural Selection.
How Evolution Happened
Long ago, around 4 billion years back, Earth looked really different. It was mostly covered in water, and this is where life started. Now, let's ask a big question: what was the first living organism? Scientists called it the "First Universal Common Ancestor," or FUCA for short. Back then, there were no genes or DNA like we have now. Instead, there were tiny building blocks called nucleotides floating around. These little parts came together to make something called RNA. The very first ancestor for all living things wasn't even a cell at that time.
About 3.8 billion years ago, the FUCA changed and became "Large Universal Common Ancestor" or LUCA. LUCA was a tiny, one-celled creature. Every living thing we know today comes from this same ancestor, LUCA. Scientists figured out that LUCA had around 355 special instructions, called genes. These genes are like the recipe for life. What's interesting is that these same genes are still in all living things today, including us. In July 2016, a scientist named William Martin made this cool discovery. He found out that even though LUCA was just a single cell, it had these special genes. Here's another interesting thing: viruses, those tiny things that can make us sick, might have been around before LUCA or were changing at the same time. Viruses have been important since the very start because they've helped living things change and get better over time. Nowadays, even for survival, lots of things need a bit of virus help.
After a long time, millions of years later, the LUCA split into two groups: Bacteria and Archaea. These are kinds of tiny life forms, a bit different from each other in how their outer parts are made.
A lot of time passed, and something really special happened. A special type of bacteria called Cyanobacteria appeared because of changes over time. These bacteria were the first ones that could use sunlight to make food, like plants do. When many of these cyanobacteria lived in the ocean and did this sunlight trick, they released a lot of oxygen into the air. This big event, called the Great Oxygenation Event, happened around 2.2 billion years back. It filled the air with a lot of oxygen.
But evolution didn't stop there. It had more changes in store. It made the DNA, which holds our instructions for life, stronger and better protected. This led to the creation of something called the Nucleus inside the DNA.
After a long, long time, changes happened outside the nucleus too, in the jelly-like stuff inside cells called cytoplasm. Some of those cyanobacteria changed into parts called chloroplasts. This was like the very first step in creating plants.
On the other side, since there was now plenty of oxygen in the air, an ancient bacteria started using it. This was the start of something called Aerobic respiration, like the way we breathe and get energy from our food. This bacteria evolved even more and ended up living inside another type of Archaea to become Mitochondria, which are like the powerhouses of cells.
As time went on, these tiny life forms with all these changes split into three main groups: Animals (that's us!), Fungi (like mushrooms), and Algae (which are like plant-like things that live in water).
At the beginning, life had three main groups: Bacteria, Eukaryotes, and Archaea. Eukaryotes later split into plants, animals, fungi, and algae – these are like the bigger family of life.
But here's something scientists are still thinking about: Did the eukaryotes come from Archaea, or are Archaea and eukaryotes different groups that still had a common ancestor? It's a bit like trying to solve a puzzle that's not completely clear yet.
Inside our bodies, only 43% of the cells are actually human cells. The rest are things like bacteria, viruses, fungi, and RK I cells. Now, let's go back in time about 900 million years ago. That's when life started to have more than one cell working together, which is called multicellular life. This helped creatures survive better.
Around 555 million years ago, something interesting happened. There was a tiny creature called Ikaria Wariootia, and it's like the grandparent of all animals. Imagine something smaller than a grain of sand! It was a worm with two sides that looked the same, one in the front and one in the back. This was a helpful setup for moving around and doing things. There were a few exceptions, like starfish, which had a different way of arranging their parts. But most animals, including humans, have this bilateral setup, which is kind of like a mirror image on each side.
Scientists discovered really old fossils in South Australia. When they checked how old they were, they found out these fossils are from a creature that lived 555 million years ago. This creature had two sides that looked the same, kind of like a mirror. It's super old and is thought to be the very first ancestor of all animals.
Then, the idea of having a spine started to show up. Animals with spines are called vertebrates. This includes fishes, reptiles, birds, and mammals. The very first creature with a spine was a fish called Arandaspis, around 480 million years ago.
After this, some of these creatures started changing and got four legs. This happened in the water and they were called tetrapods. Around 375 million years ago, there was a fish named Eusthenopteron. It was big, about 6 feet long, and had strong jaws and sharp teeth. This fish is like a bridge between fish and land animals with spines.
Here's something interesting: Some creatures tried living on land but went back to the water. They did this because they couldn't handle the land conditions. Dolphins and whales are good examples. They're mammals like us but chose to live in the sea. This means that dolphins and whales must have had ancestors that stayed on land, while some went back to the water.
Mammals evolved from reptiles around 225 million years ago. The oldest known mammal is Brasilodon quadrangularis, which was about 20 centimeters long and looked like a mouse. It laid eggs, unlike most mammals. Today, there are five species of mammals that lay eggs, called monotremes. One of them is the platypus, which has been around for at least 120 million years.
Mammals are classified into three groups: monotremes, placentals, and marsupials. Monotremes lay eggs and nurse their young with milk. The platypus and the echidna are the only two living monotremes. Placental mammals have a placenta, which is an organ that connects the mother to the fetus and allows for the exchange of nutrients and oxygen. Humans, dogs, cats, and whales are all placental mammals. Marsupials have a pouch in which their young develop after they are born. Kangaroos, koalas, and wombats are all marsupials. The different ways that mammals mature their young reflect their different evolutionary histories and adaptations to their environment. For example, monotremes lay eggs because they live in Australia, where the climate is relatively cool and dry. Placental mammals have a placenta because it allows for the efficient transfer of nutrients and oxygen to the fetus, which is important for the development of large brains. Marsupials have a pouch because it allows their young to develop in a safe and warm environment, even though they are born at a relatively early stage of development.
At this time, the dinosaurs had already been extinct. The asteroid had killed all the dinosaurs. There were many empty spot in earth's ecosystem, where mammals could fit in.
A study found that placental mammals started to diversify and become more widespread only after the dinosaurs went extinct. If the dinosaurs hadn't gone extinct, there wouldn't have been as much diversity in placental mammals, and humans might not have existed.
40 million years ago, the Indian plate collided with Asia, forming the Himalayas. This event caused a lot of changes in the landscape and the environment, and it also had a big impact on the evolution of mammals. One of the branches that emerged from this split is the ungulates, or hoofed mammals. Ungulates include a wide variety of animals, such as cows, buffaloes, pigs, goats, and horses. They are characterized by their hooves, which help them to walk and run on a variety of surfaces. The ungulates were further divided into two groups: the even-toed ungulates and the odd-toed ungulates. Even-toed ungulates have hooves with an even number of toes, such as cows and pigs. Odd-toed ungulates have hooves with an odd number of toes, such as horses and rhinoceroses.
In addition to the ungulates, another branch of placental mammals formed is the carnivora order, which includes carnivorous mammals such as dogs, tigers, lions, and cats. Whales and dolphins are classified as cetaceans, which are a group of marine mammals that also includes porpoises and manatees. Cetaceans are closely related to ungulates, which means that whales and dolphins are actually more closely related to cows and buffaloes than they are to other marine mammals such as seals and sea lions.
Afrotheria is a group of mammals that includes elephants, hyraxes, aardvarks, and tenrecs. These mammals are all found in Africa, and they share a common ancestor that lived on the continent about 100 million years ago.
There are 4 reasons behind evolution that are known to us:
- Genetic mutation
- natural selection
- Genetic drifts
- Gene flow
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