Third Week in December
Life on Earth is astounding. From our viewpoint in the process, it is virtually impossible to grasp the evolution of life on Earth. However, attempting to grasp the magnitude of life on Earth—its history and its success—is worthwhile, because it is wonderful, awe-inspiring, and interesting.
When we refer to year 0 as the Big Bang, we can say that in the year 9 billion, 600 million Earth formed. Our current atmosphere formed in year 10 billion, 200 million. Life in its first form of single-cell prokaryotes emerged in year 10 billion, 100 million. Life emerged before our current atmosphere formed. In the year 10 billion, 300 million cyanobacteria (blue green algae) formed and through photosynthesis populated the atmosphere with oxygen. Cyanobacteria was so successful over a period of 200 million years, its impact on the Earth is referred to as The Great Oxygenation Event. As more oxygen was expelled into the atmosphere, it attached itself to other elements on Earth at the time. The elements of the periodic table emerged.
In the year 11 billion, 700 million eukaryotes formed, followed by multicellular life forms 1 billion years later. Fairly rapidly, 450 million years after that, bilaterians evolved. Bilaterians are organisms that have a distinct front (head) and back, as well as a top side and bottom side. At that time, Earth was becoming populated with a greater variety of life forms.
Evolution was happening at a far greater rate, too. More organisms means more possible diversity. In a matter of tens of millions of years fish and proto-amphibians evolved, followed by plants that live on land, then insects, then amphibians, followed by reptiles, then mammals, then birds, then flowers (after which the Cretaceous-Paleogene Extinction Event occurred, which wiped out three quarters of life on earth including dinosaurs), then primates, and then in year 13 billion, 699 million, 800 thousand, anatomically modern humans formed.
With just an overview of the history of life on Earth you can see how rugged life forms are and how sensitive. You can see how long it took life to take hold and to evolve. Life as a collection of organisms on the planet seems too vast and varied to comprehend. Life is a wonder about which we should always be interested in learning more.