Evolution of Life on Earth

Introduction

Even today, scientists aren’t entirely sure how life began on earth. There are many possible theories that scientists have put forth such as – inanimate concoctions giving rise to self-replicating proteins and eventually to life. Other theories suggest that life on earth began deep in the oceans, near hydrothermal vents which acted as a nursery for harbouring the first forms of life. Then, there are some theories that sound far-fetched due to lack of evidence- such as an asteroid or a comet that contained elements necessary for life crashed into the earth, and thereby kick-starting evolution.

Meaning of Evolution

But before we delve into the origin of life on earth, let us understand the meaning of evolution. All organisms that we see on earth are believed to have “formed” or evolved from precursors of these organisms over a gradual period of time. Evidence of Evolution can be backed up by various fossils of extinct organisms that show features and characteristics present on their modern day relatives.

For instance, we now know that dinosaurs are extinct. But they do have descendants that survived the mass extinctions and evolved to become birds. Indeed, the chickens that you see in farms are the descendants of famous dinosaurs such as Tyrannosaurus Rex and Allosaurus. The humble chicken shows some features and characteristics such as air-filled sacs, wishbones, and eggs that are typically associated with dinosaurs, especially the theropods (a group of bipedal, meat-eating dinosaurs). Today, fossils evidence suggests that some dinosaurs even had feathers, which further supports the claim that birds are descendants of dinosaurs. So why did dinosaurs die out and the chickens survive?

65 million years ago, an asteroid impact event triggered an apocalyptic mass extinction and it resulted in nearly 80% of life being wiped out. The life that was not able to adapt to the new climate and the lack of food perished. Most dinosaurs that lived at or near the time of this mass extinction were very big, and food sources were abundant. But the asteroid impact blocked out most of the sunlight, thereby diminishing food sources. This along with the extremely hostile environment that formed post-impact acted as a catalyst for this mass extinction. Organisms that were able to adapt survived the ordeal, and the organisms that were not, perished.

Therefore, evolution can be understood as a gradual change in the characteristic features of a species, brought about by the need to adapt to situations and the environment through natural selection.

Before Life

The earth was formed more than 4.5 billion years ago from rotating dust and gas particles that coalesced in a rough sphere. For nearly a billion years, the earth was a very hostile place where life could not possibly survive. It was theorized that primitive earth had many active volcanoes, rifts and tectonic plates than it has now and this led average temperatures to soar over a blistering 200° C. Also, the sun was ⅓ dimmer than it is today. There were many small proto-continents as large continents were unable to form due to high geological activity and active plate-tectonics. Oxygen was also not freely present in the atmosphere during this period. Instead, it was trapped along with water vapour in rocks, which gradually escaped due to intense heat and pressure. 3.8 billion years ago, temperatures finally dropped below the boiling point of water and rain began pouring down on earth. And it rained for a very long time, probably thousands of years. Eventually, basins began to fill up and the world saw its very first oceans. But the oceans would remain empty for a million years more.

Speculations on Life’s Origin

3.5 billion years ago, the first undisputed evidence of life was found and these were cyanobacteria – a photosynthetic organism. Scientists have also evidence which supports that life began even earlier. But today, these organisms are credited with the creating the oxygen-rich atmosphere that we have today. Fast forward a few million years and the very first eukaryotic organisms appeared on the scene. The Last Common Universal Ancestor (or LUCA) is a speculated organism that serves as the ancestor of all organisms that have ever lived or living on earth. In other words, it could be called a “progenote” – the hypothetical ancestor to all life on earth.

Scientists still don’t really know how life began in the first place. One of the most popular theories on how life began is Abiogenesis. It states that life originated from non-living, inanimate materials through natural processes and occurrences. In 1953, Stanley Miller’s experiment reinforced this theory. He recreated conditions that were present on earth nearly 4 billion years ago in his experiment. He filled ammonia, methane and carbon monoxide-hydrogen and water in a container to represent the gaseous composition of early earth’s atmosphere. Then, he subjected the container to electrical sparks to simulate lightning strikes. This experiment was done for a week and by the end of it, Miller had noticed an accumulation of a red-brown substance on the walls of the container. When he analyzed the composition of this mysterious substance, he had found that it contained organic compounds and amino acids which are paramount for life. His experiment opened the gates for future scientists to delve deeper into Abiogenesis and today, this is one of the most credible contenders for theories on the origin of life.

Another alternative theory to Abiogenesis is the Panspermia Hypothesis. This theory states that life on earth came from meteors and comets and other stellar bodies. These may harbour chemicals required for life such as organic compounds and water. These interstellar objects travel vast distances, and one of them may have crashed into the earth, thereby kickstarting the mechanism of life. However, this is just a speculation and there isn’t enough evidence to back up this theory.

Evolution and Extinction

Extinctions are the result of organisms not able to adapt fast enough to meet the changing needs. It could be due to competition from newer, more efficient organisms, lack of food or resources, loss of habitat, adverse changes to an organism’s’ environment or a combination of all these factors.

Sometimes, catastrophic events might trigger mass extinctions which might give other species a clean slate. These can range from having an asteroid crash into earth, continental shifts, extreme volcanism, or other unknown events. Humans came to be the most dominant species thanks to the mass extinction which completely wiped out all the non-avian dinosaurs 65 million years ago. There were also other mass extinctions in the earth’s past that killed off nearly all life on earth such as the Permian-Triassic extinction. This mass extinction occurred around 250 million years ago and it enabled the dinosaurs to take over the planet as the most dominant species until their extinction 66 million years ago.

Further Reading

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Practise This Question

Lamarck's theory of evolution is commonly known as