Four billion years ago, approximately a hundred million years after the emergence of solid land, life arose on Earth. We are not sure which of several hypothesized mechanisms accidentally created the first self-replicating molecule, most likely ribonucleic acid (RNA). It could have arisen from deep-ocean hydrothermal vents, from pools of organic “soup,” from alkaline seepages along the ocean floor, or from several other processes. However it happened, once this molecule was formed, it copied itself repeatedly. But not all copies were perfect, either through copying errors, or through environmental attacks on the molecules. The mini-environment in which the molecule first arose was soon filled with a number of slightly different versions of the original.

       And evolution began. The four keys to evolution, as enumerated by Charles Darwin, are variation in a population, inheritance of traits from a parent, reproduction, and limited resources. This mini-environment of self-copying molecules had all the ingredients necessary. Once the environment was filled, offspring could only be made by those who could compete better for the scarce resources, or who could survive and copy at the fringe or outside the environment. Molecules with beneficial variations survived and reproduced in greater numbers. Those without did not. And the cycle repeated: variation due to copying errors or the environment, better survival and reproduction by those with certain beneficial traits, a new generation with variation, a new wave of survival and reproduction. Individual molecules usually did not change, but the population changed generation by generation, one tiny modification at a time.

       One of the reasons it is sometimes hard to fathom how evolution works is that it is excruciatingly slow. One form does not change into another over one generation, or over a dozen. The time for the evolutionary step from the first living, replicating molecules to the first organisms that had cellular walls may have been on the order of a hundred million years. That means billions of generations of trial and error passed, tiny step by tiny step, until one-celled organisms emerged on the Earth. We can hypothesize about possible steps along the way: the accidental changes that allowed a self-replicating molecule to make proteins which attached to itself, thus either shielding it from harm or helping draw in building blocks for new copies or energy for the steps necessary to make a copy; the growth, generation by generation, of such shielding until it curved around upon itself and created a bubble; the advantages that the organism would have if the key molecular strand were on the inside of the bubble and the bubble allowed only nutrients through; the additional nutrients it would receive if the bubble folded back upon itself to increase surface area; the extra layer of protection the key strand would have if it pinched its own fold off from the main bubble in a tiny central bubble. Whatever the exact steps, by about 3.9 billion years ago single-celled organisms had developed, using deoxyribonucleic acid (DNA) as the key molecular strand, RNA as a messenger, and proteins built in the folds of the DNA and RNA as the building blocks of the other components of the cell.

       Eventually, some cells began to use forms of photosynthesis to generate energy for the cellular chemical reactions. After more evolution, cyanobacteria evolved that produced oxygen as a waste product. Again, the timeframes are enormous; we have traveled close to a billion years from the first one-celled organisms to get to cyanobacteria, which arrived around 3 billion years ago. The waste oxygen that they produced increased in the atmosphere to a much higher concentration than had ever been on the earth. Oxygen was toxic to most of the one-celled life at the time, and killed off the vast majority of it. However, some of the cells had various random mutations that allowed them to tolerate higher concentrations of oxygen, and in maybe a half a billion more years, evolved the ability to use oxygen as a way to extract energy from food.

       Over a billion years after that, around 1.2 billion years ago, some of these cells evolved a novel strategy to increase the probability of successful offspring: the ability to share DNA with another similar cell. The advantage this conferred was that beneficial mutations could be shared within a population and would evolve to the higher percentages in the population more quickly. For example, if one cell had a mutation that made it use food more efficiently, and another cell had a mutation that made it produce more offspring, some of the offspring of the shared DNA between these two might have both mutations, and would thus have a much greater advantage over its peers. This advent of what we call sexual reproduction greatly sped up the evolutionary process.

       Another 200 million years later or so, the first multi-cellular organisms arrived on the scene. For organisms, the ability to sense and move toward food nearby spurred the evolution of nerves, sensory organs and eventually a brain. Such organisms are called animals, and the first rudimentary animals in the ocean, such as jellyfish and sponges, arrived about 600 million years ago. Animals continued to achieve evolutionary success through a variety of means. Some became predators eating other animals and gained food through size or speed or enhanced sensory ability. Some ate only plants and succeeded where there was an abundance of plants available to eat. Some reproduced in such large numbers that even though most of their offspring might starve or be eaten, some would survive. Some evolved the ability to eat many different plants and animals, ensuring that food would always be available. Some developed armor-like structures or speed to either defend against or run away from predators. Most animals evolved based upon a combination of these various strategies for reproductive success. This great explosion of evolutionary changes and strategies among animals occurred relatively quickly on the evolutionary timeline, and animals soon filled the oceans. This event is called the Cambrian explosion, and occurred roughly from 565 to 525 million years ago.

       In the next 200 or so million years, plants and animals finally evolve to colonize the land. The animals included insects, arachnids, and some that resembled amphibians and reptiles. The Earth was now teeming with life. But about 250 million years ago, there was a massive die-off, killing maybe 95% of all animal species. Scientists are still debating possible causes of this mass extinction (called the Permian-Triassic extinction event, or P-T event), and have several different theories that alone or in combination could have caused it. However it happened, it set the stage for the emergence of mammals and birds from early reptilian ancestors. Also appearing on Earth were the first giant reptiles, the dinosaurs. In the 185 million years following the P-T event, plants evolved flowers, birds evolved flight, and mammals evolved placentas. Mouse-sized placental mammals from that time are the ancestors of all placental mammals on Earth today, including humans. Man’s last common ancestor with rodents was about 100 million years ago.

       But about 65 and a half million years ago, a meteor struck the Earth at what is now the Yucatan peninsula in Mexico, and the ensuing environmental catastrophe wiped out about half the animal species. This event is known as the Cretaceous-Tertiary extinction event (the K-T event), and the change in temperature wiped out virtually all of the cold-blooded dinosaurs. Without the threat of large predators, mammals diversified, and larger mammals appeared. Many species of mammals, with a wide range of adaptations, arrived. The world had begun to take on a familiar appearance, with a high degree of variety in flora and fauna, and mammals dominating the land.

Theory & Evidence