Okay, here goes:
About 4 billion years ago, give or take a few hundred million years, the first life forms appeared on the Earth. These were far more primitive than even the simplest bacterium, consisting of little more than a self-replicating RNA strand inside a proteinoid or liposphere cocoon. Since some RNA strands could act as enzyme-like catalists – “ribozymes”, they’re called – the ability for an organism to make an exact duplicate of itself allowed those duplicates to inherit their own copies of those ribozymes. Competition for the resources on the early Earth meant that those organismic lineages (or “species”) that could make better copies of themselves would tend to survive, and those that made poor copies of themselves would die out.
Soon after, a ribozyme called a “ribosome” evolved, which allowed RNA strands to be “read” and decoded as proteins. Proteins make much better catalysts than ribozymes do, so those organisms that mastered protein synthesis competely displaced those organisms that relied on ribose to do their dirty work. Soon after that, a large-scale “back up storage device” for RNA evolved, which was called DNA. DNA allowed for much longer sequences of protein encodings to be preserved and copied than RNA. Before long, DNA-using bacteria had become the masters of the planet.
About 2.5 billion years ago, though, a catastrophe occured. A particular species of bacteria called cyanobacteria started producing oxygen via photosynthesis, and became so successful that it covered most of the Earth’s oceans. All that oxygen meant instant death to any bacteria it touched that wasn’t aerotolerant. It was the worst case of air pollution in the history of the world. The mass extinctions that occurred were horrendous. But out of this seething cauldron of oxygenated water emerged a new kind of single-celled organism. An organism with a motile, cell-wall-free exterior. An organism with its DNA organized into chromosome pairs safely ensconced inside an inner double-membrane called the “cell nucleus”. An organism that entered into a symbiotic relationship with air-breathing purple bacteria, thus giving it an unbeatable powerhouse of energy.
An organism called … a eukaryote.
About 1 billion years ago, give or take, eukaryotic cells began to cling together and act in concert as though they were a single large being. These so-called “multi-cellular organisms” began to grow larger. The DNA of these cell colonies started to acquire instructions which allowed individual cells within the colony to “differentiate” themselves from other cells within the same colony and thereby specialize at certain tasks. A multi-cellular organism could have one region of cells that swallowed nutrients from its surroundings, and another region of cells that processed those nutrients, and yet another region of cells that expelled the useless parts left over from processing the nutrients. These sponge-like creatures were the first animals.
Animals like the sea anemone came later. They did more than just sit there anchored to the ocean floor. They had muscles that could move, and a network of “nerves” that could relay signals to and from the muscles. The cells in one part of their body could react to changes in their environment detected from cells in another part. It was a small step from these critters to free-swimming beasts.
About 500 million years ago, a huge surge in the number of animal phyla, called the Cambrian explosion, occurred over a span of only a few million years. One of the new phyla to arise were called the chordates. Instead of just having a loose network of nerve fibers, these critters had a central nerve “bus”, called a notochord, running down the entire length of their back. They were also bilaterally symmetric, that is, the left side looked like an exact mirror-image of the right side.
In their attempt to outrun predators, some of these chordates hid themselves in the brackish waters where a river met the ocean. Now remember, up until this point every organism on the planet had lived exclusively in the briny seas. They were adapted to the level of salt and minerals in ocean water. These new, mineral-poor waters at the mouths of rivers required two innovations: First, the chordates had to evolve a kidney to pump excess water out of their system, and second – and most significantly – the chordates had to store minerals, such as calcium, in their bodies for later use. A perfect place to store calcium turned out to be around the notochord. Soon, these calcium storage rings surrounding the notochord became articulated and useful in their own right. The first vertebrates, of the class we now call “fish,” had evolved.
These calcium deposits, called “bones”, turned out to be eminently useful as structural support members. They could be used to create an endoskeleton, thereby giving these soft-skinned fishes the articulated limbs that their arthropod cousins posessed. These turned out to be very useful for pulling them through the shallow, obstacle-infested waters of swamps. Just by sheer luck, these bones proved so strong that they could actually support the creature’s weight entirely out of the water. Soon, creatures called amphibians had evolved which could spend part of their time on land, eating the land-dwelling plants that lived there, and then return to the water to swim around and lay their eggs.
Then came a wonderful innovation. Some amphibian somewhere along the line laid an egg with a hard shell, which didn’t need to be immersed in water. Its eggs were now safe from all those nasty water-dwelling egg predators. It didn’t take long for these critters to evolve an exclusively land-dwelling existence. They were called reptiles.
By 250 million years ago, some reptiles had evolved many of the features similar to those in a modern ecosystem, perhaps even including warm bloodedness. These creatures were called mammal-like reptiles. But 250 million years ago, another disaster struck. For reasons that are still unclear, almost all animal species on the planet went extinct. This tragedy, called the Permian mass extinction, paved the way for three new classes of critters to evolve from the few mammal-like reptiles that survived: the dinosaurs, the birds, and the mammals.
For millions of years, dinosaurs were the masters of the Earth. No mammal larger than a rat could gain a foothold. Then, 65 million years ago, another mass extinction called the K-T boundary event was brought on by a giant asteroid or comet smashing into the Yucatan peninsula. This wiped out the dinosaurs. With the dinos out of the way, mammals were free to thrive and prosper and evolve in every which-way. 40 million years ago, an order of mammals called the primates appeared, which had hands that could grasp tree branches. From them descended the Great Apes. About 5 million years ago, a group of African great apes resembling chimpanzees descended from the trees and journeyed out onto the African plains. They posessed an exquisite skill with making tools out of whatever crude objects they found lying around. Soon, they acquired neoteny (the retention of juvenile features into adulthood), which vastly increased their brain size. In one group, the Cro Magnon, these newer, larger bains eventually acquired the ability to create language and draw pictures, and to innovate new tools. They are the direct ancestors of all modern humans.
And Batman is a modern human.