History of life

Imagine you were outside the Earth but you were able to observe everything that has happened on it since its beginning.

At first, the Earth was a blazing hell, there wasn't even a crust and there was a constant shower of meteorites. When the Earth cooled down, it formed a crust and oceans and life appeared. When I say "life", I'm referring to very simple life forms, they were able just to replicate. When the right conditions occur, life arises easily, I say this because life appeared soon after conditions were met.

Life is just a natural phenomena, like rain and lightning and rainbows. Life has no equivalent really, although it has some similarities with fire, which also replicates, but without heredity.

There were lots of organic compounds on the Earth, in the oceans. Organisms are composed of organic compounds (carbon based).

The first life forms even replicated badly, which is the only thing they could do, really. They could not even move by themselves. The only thing that mattered was replicating before being destroyed. Some, of course, did better than others and soon the oceans were full of primitive life forms. When they used most of the organic compounds around, competition arose, very primitive competition, but competition nonetheless. They even started to "steal" organic compounds from each other.

Don't imagine that competition and "stealing" were the way modern organisms do. It's similar to how magnets attract iron. You don't need a good magnet to attract iron but a better magnet will attract more. Also, a good magnet can "steal" iron from a worse one. The better was a life form at attracting organic compounds and using them for replication, the better it did.

You can already see that life has a tendency to become better and better at what it does. Evolution is like a ladder, once you climbed the first step, you can climb the second one and so on, increasing in complexity.

An interesting episode in life's existence is the "oxygen catastrophe". One of the most important events in the history of life was the discovery of photosynthesis. It was also one of the deadliest, because it provoked the largest extinction in the history of life. Free oxygen (O₂) did not exist in the atmosphere in the first 2 billion years of Earth's existence. When it appeared, it was deadly for those anaerobic microorganisms. The "oxygen catastrophe" also provoked a "snowball Earth" event, called "the Huronian glaciation", which lasted 300 million years and the whole Earth was covered in ice.

I think we should learn something from this event, that is: life can destroy itself. Also, every end is a new beginning. Humans can provoke a worldwide catastrophe, just as cyanobacteria did back then. Cyanobacteria didn't know what they were doing, they were not aware of the consequences. We know what we are doing but we choose to ignore the consequences.

Most of history, life forms were very simple by our standards, but even the simplest bacteria is much more complex than the first life forms.

Life adapted itself to the oxygen in the atmosphere and even started to use it, most animals need oxygen to survive (including humans).

Life forms were not very interesting (to us) until the "Cambrian explosion", around 530 million years ago. If life appeared almost 4 billion years ago, the first 3 billion years were not interesting, because there were no animals or plants. The Cambrian explosion was indeed an explosion on the timescale of the Earth, because it lasted only tens of millions of years. This may seem a lot to us, but comparing them to the 3 billion years when nothing interesting happened, it is an explosion.

The Cambrian explosion can be compared to the industrial revolution. Technological progress was very slow until then. We have invented in 200 years more than in all the previous millenniums, ever since the early stone tools, 2 million years ago. What happened in both situations? I believe that something extraordinary was discovered, like the steam engine in the case of the industrial revolution.

So, we have:

• 600 million years of simple animals
  
• 500 million years of fish
  
• 475 million years of land plants
  
• 400 million years of insects
  
• 360 million years of amphibians
  
• 300 million years of reptiles
  
• 200 million years of mammals
• 150 million years of birds
• 130 million years of flowers
• 200 thousand years since humans started to look like they do today

These are approximate dates and, of course, they all looked very different in the beginning. In evolution everything is continuous, it's like a flow. You can't really say: here is where mammals appeared. It goes like this: reptiles -> mammal-like reptiles -> reptile-like mammals -> mammals. Everything is like that but we like to classify things, we don't like continuous things.

So, as the observer of the Earth during this long history of 4,6 billion years, what would you see? You would first see simple replicating structures that by competing become better at what they do and acquire new functions, which help them in their survival and reproduction. At one point, you observe that life forms start to produce their own organic compounds from CO_2, water, and sunlight, you observe mass extinctions, most created by natural disasters, some created by life itself. After mass extinctions life starts again to grow and cover the Earth. This is what you would see, life becoming more and more complex, with some mass extinctions along the way but the game stays the same.

The constant is competition. Competition has driven progress in life forms from those simple beginnings to the modern complex organisms we see today. If resources were infinite, competition would not exist and evolution would not have taken place. We are competing for the same resources: organic compounds. Plants compete for light, which helps them create organic compounds from water and CO₂.

See this:
The Origin of Life
Timeline of Evolution