Evolution of Life on Earth

Billions of years ago on the planet earth simple organic compounds assembled into a more complex coalition that could grow and reproduce. They were the very first life on earth, and they gave rise to every one of the billions of species that have inhabited the planet since. At the time earth was completely devoid of what we would recognize as a suitable environment for living things. 

Considering the young planet it had a widespread volcanic activity and with an atmosphere as such that created hostile conditions. So the question arises where on earth could the living being. In order to benign the search for the cradle of life, it is important to first understand the basic necessities for any form of life. Elements and compounds essential to life include hydrogen, methane, nitrogen, carbon dioxide, phosphates, and ammonia. In order for these ingredients to co-mingle and to react with each other, they need a liquid solvent that is the water. And in order to grow and reproduce, all life needs a source of energy. Life forms are divided into 2 camps autotrophs like plants, that generate their own energy and heterotrophs like animals, that consume other organisms for energy. 

We have to know that the first life form wouldn’t have had other organisms to consume of course. So it must have been autotroph, generating energy either from the sun or from chemical gradients. So first we need to understand what location meets these criteria, places on land, or close to the surface of the ocean have the advantage to access the sunlight. But at the time when life began, the UV radiation on the earth’s surface was likely too harsh for life to survive there. One of the settings offers protection from this radiation and an alternative energy source, the hydrothermal vents that wind across the ocean floor covered by kilometers of seawater and bathed in complete darkness. If we observer a hydrothermal vent which tends to be a fissure in the earth’s crust, where we will observe the seawater seep into magma chambers and will be later will be ejected back out with very high temperatures, this comes along with a rich slurry of minerals and simple chemical compounds. 

The energy will be particularly concentrated at the steep chemical gradients of the hydrothermal vents. There is another line of evidence that points to the hydrothermal vents, which is the Last Universal Common Ancestor of life or LUCA for short. LUCA wasn’t the first life form, but as far back that we can trace. Even so, we don’t actually know what LUCA looked like, there is no LUCA fossil, no modern-day LUCA still around. Instead, scientists identified genes that are commonly found in species across all 3 domains of life that exist today. Since these genes are shared across the species and domains, they must have been inherited from a common ancestor. These shared genes tell us that LUCA lived in a hot oxygen-free place and harvested energy from a chemical gradient, like the ones at hydrothermal vents. 

There are 2 kinds of hydrothermal vents, the black smokers and white smokers. Black smokers release acidic carbon-dioxide rich water heated to hundreds of degrees celsius and packed with elements like sulfur, iron, copper, and other metals essential for life. But scientists now believe that black smokers were too hot for LUCA. So now the top candidates for the cradle of life are white smokers. Among the white smokers, a field of hydrothermal vents on the Mid Atlantic Ridge called Lost City has become the most favored candidate for the cradle of light. The seawater expelled here is highly alkaline and lacks carbon dioxide, but is rich in methane and offers more hospitable temperatures. Adjacent black smokers may have contributed the carbon dioxide necessary for life to evolve at Lost City giving it all the components to support the first organisms that radiated into the most incredible diversity of life on earth which we see today. With an estimated 3.8 billion years of existence in evolution on earth, it is an odd thing indeed. The story of evolution begins with the rise of the first simple single-cell organism. However, these organisms are destined to spend quite some time alone. Later a single cell engulfs another creating a symbiotic relationship and the first eukaryotic cells or the cells with internal organs. 

Moving on to the next stage of evolution these cells begin to form colonies and thus the first multicellular life develops. A teaspoon full of soil contains more organisms than all the humans. As life begins to prosper on earth as we see the first instance of sexual reproduction. Seaweed has started growing and oxygen is now spread across the atmosphere.

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Origin of Life

Consider the earth 4 billion years ago, there is no oxygen in the air. Molten lava flowed into an already poisonous sea, although it is hard to imagine this harsh environment was perfect for nurturing a miracle. Rain washed the necessary chemicals from the air; the lightning and UV radiations cooked these chemicals into an organic soup. And somehow when the smoke cleared, there was something new, something amazing, a very special molecule with a graceful spiral shape. It had the talent it could make copies of itself faster then it could be destroyed. This was the origin of life on earth and when this molecule learned to protect itself inside the cell walls life began to transform the planet. Three-and-half billion years ago, cells learned how to directly use the sun’s energy in the process of photosynthesis and life grew exponentially. 

The ocean’s team with cyanobacteria and layers of these microorganisms mixed with sediments became the world’s first living structures. Stromatolites for more than a billion years these 3 foot high mounds mark the limits of life’s progress. But more important than the mounds themselves was their waste oxygen. These new gases at first toxic to all life gave rise to the ozone layer. Shielded for the first time from the sun's damaging UV rays life became unstoppable. One-and-half billion years ago the cell developed an additional membrane to protect its genes. Now life was so abundant that every drop of water teamed with organisms. 800 million years ago the first multicellular organisms began to appear. For a while, multicellular animals just meant a collection of identical cells. But gradually these colonies began to have cells that are specialized for different purposes. 

The first multicellular animals that had specialized cells were sponges. Some of the cells pumped water and some filtered out tiny bits of food. Enemies and their relatives had muscle cells in nerve cells. This enabled them to bend stretch and flex, but none of their great cell diversity enabled them to move. Staying put was a common trait back then, 600 million years ago an ancient worm was the first animal to develop a centralized nervous system. It had nerve cells that ran the length of its body and the concentration of these cells at one end formed the first primitive brain. In fact, this was the first animal with a head and light-sensitive cells in that head were the world’s first eyes. It could recognize both the direction and intensity of light. 

Since it can both see and move this worm interacted with the world in a very different way. 400 million years ago sponges, enemies, and flatworms dominated the oceans, then later in 30 million years a huge variety of creatures appear. This event is called the Cambrian explosion. Let us take an example of the stunning developed marine worm, in the course of Cambrian explosion competition for food cause both predator and prey to become extremely sophisticated. This is the top hunter of that period, a terrifically capable hunter the Cambrian was a very dangerous time. Only slightly lower in the food chain another deadly predator proudly ocean depth. This one had 5 eyes on the top of the head and a single extendable call is used for hunting. 

Every animal group alive today had its origin in the Cambrian. The first fish appeared more than 500 million years ago a predecessor to mankind’s own group the chordates. Fish quickly became a success story, getting faster and slicker and a far more numerous. They developed bony spines and crucially jaws with teeth. 400 million years ago much of the earth was already covered in green. Plants had colonized freshwaters and spread on to the land. Once they were established animals soon followed. Centipedes were the first among the land creatures, they developed simple lungs and skin that retain vital water. 

Scropeans cousins of the horseshoe crab also made an early move on to the land. Their line of 8 legged predators has spread far and wide since. While the invertebrates are the first animals ashore others were not far behind. Fish penetrated the weed-choked lagoon using Lynn light fins and pushed their way through the tangle to stock their prey. Fins became more and more like legs, the vertebrates were on the verge of a breakthrough. The first amphibians emerged from the swamp some 370 million years ago, their soft moist skin absorbed oxygen and simple lungs allowed them to breathe air. The exertion of hauling themselves over land required plenty of fuel. But the land was now crawling with life. The first flightless insects made easy prey. Many amphibians came to live on and because the food was so plentiful. But they were forever confined to the damp places because of their delicate skins. 

Reptiles evolved from amphibians, but with tougher waterproof skins they were able to occupy entirely new habitat. Reptiles also pioneered another breakthrough, which is the waterproof eggs. Now they can breed in dry places, inside the eggs the developing young were housed in a miniature ocean. Reptiles were on the verge of global domination, but one thing held them back. With their skin now waterproof and airtight breathing became a problem. Lizards breathe by expanding their chest but because of how they walk they often have to hold their breath. Their waddling gait inherited from ancestral fish forces the chest to flex as they walk. Their lungs can expand to draw in air, and the lizards easily become breathless. Ancient relatives of crocodiles though found a solution by standing up on their legs. These reptiles are benign to walk tall and breathe easier. 270 million years ago it was hot, and these ancient reptiles need plenty of water. Spending time at the river makes the heard nervous, as they know this is a great place for an ambush. 

Fortunately, the top predator is not hunting if she/he has eaten and come to the river to drink. At that time the predators had developed to 18 feet long and had an armored back which makes them have a huge amount of food and therefore it needs to defend a very large territory.