Bridging the Gap: Major Steps in Evolutionary History
Roughly 3.8 billion years ago on our planet, something incredible happened. From the humble beginnings of a frothy, chemical soup, something emerged that copied itself. Then, the copies made copies. Some of these structures were able to copy themselves better, while some were not able to copy themselves at all, and those structures that were able to leave more copies became more prevalent while those that didn’t went extinct. This should be a familiar story by now; it is the story of life and evolution on Earth as we currently understand it. But how do you get from a self-replicating structure made of a few molecules to the complex workings of the human brain?
Step 1: It’s Alive!
The first step is the most mysterious step in the process that eventually produced all the organisms we see around us today. Screened from our view by the shroud of time, the exact origin of life remains an enigma. There are several factors at play that make the clues to this event incredibly rare. The first is the huge, 3.8 billion year gulf between then and now. Fossils and other traces of early organisms are obliterated over time by the slow grinding of geology; the cycling of the Earth’s crust. The tiny, fragile fossils left by early life stand almost no chance of surviving into our present period. However, there are a few, tantalizing clues left in geologically calm areas around the world. Tiny mounds, called “stromatolites,” are the fossil remnants of extremely old life forms. Resembling similar structures we see today, stromatolites are formed by colonies of single-celled organisms as their grains of silt cling to their cell membranes, piling up over the generations.
The second barrier that limits our understanding of this event is life itself. Once they were established, the first organisms began to evolve rapidly, and out-competed the earlier, simpler forms. The more simple forms went extinct as their progeny adapted and spread, leaving us with very little idea of what these first organisms were like. The simplest one-celled organism today is the product of 3.8 billion years of evolution, just as we are, and as a result is no where near the simplicity enjoyed by the first organisms.
These two factors make our understanding of the origin of life murky indeed. However, we know that life did originate, so the only question is how. Miller and Urey showed in their famous experiment in the 50’s that organic compounds could form on Earth naturally under the right conditions. Interestingly, it seems that they can also form in outer space. The early Earth was a chemical soup, and bombarded by comets that potentially carried the seeds of life. Out of this bio-chemical froth, only one simple structure capable of self-replication was needed. Once it had a foothold, life would be almost impossible to dislodge.
Step 2: Going Green
The Earth is green. Nearly everywhere you look (in nature) the world is painted with chlorophyll. This is a result of a struggle that occurred roughly 2.4 billion years ago when the first cyanobacteria evolved. Prior to the emergence of cyanobacteria, there was almost no free oxygen in the earth’s atmosphere, and there were almost no organisms that exploited it, as we do today. Once the first photosynthesizing organisms came into being, they began pumping out vast amount oxygen. At first, this wasn’t a problem, as the oxygen became bound up with iron and organic matter. However, once these two oxygen-sinks became saturated, free O2 began to accumulate in the atmosphere, and effectively poisoned most of the life in existence at the time. Additionally, when the free O2 in the atmosphere combined with methane, it may have lead to the longest and most severe episode of glaciation in the planet’s history.
For our ancestors, this was an incredibly important event. Somehow, they adapted to exploit all of this free O2 as it was poisoning their neighbors. Not only did it grant them the aerobic energy pathway; it killed off nearly all of their competitors as well. If this event had not occurred, life on Earth would be very different.
Step 3: It’s Business Time
Sexuality is ingrained into us at a cellular level. As we all know, the only hope our genes have of living on into future generations is if we successfully mate with a member of the opposite sex and leave reproductively viable offspring. However, this wasn’t always the case, and is still not the case for the majority of organisms alive today. Bacteria and archaea reproduce by mitosis, essentially splitting themselves into two exact copies of the parent cell. This was the only method of reproduction until our one-celled eukaryot ancestors stumbled upon this method of reproduction. At first, it seems like a disadvantageous strategy; finding a mate and mixing genes is an energy-expensive process, and since two individuals are required to reproduce rather than just one, the number of descendants is reduced by half. On the other hand, there are many compensations that make up for these drawbacks. The most important of these is genetic variation. Asexually reproducing organisms essentially make a copy of themselves that is nearly genetically identical, like twins separated by a generation. Sexually reproducing organisms, on the other hand, produce a novel suite of genetic traits with each generation. Two sets of genes come together to produce a third that is distinct from either parent. This genetic variation allows for rapid evolution and adaptation, and provides resistance to parasites and viruses that may specialize on a particular genome. Additionally, whenever a mutation occurs that is unfavorable, this mutation can be edited out by recombining with a healthy genome.
Step 4: Getting Clumpy
Now the fun begins. Until this point in time, life had been little more than a film of slime covering the oceans and rocks of the early Earth. But it was not a boring place if you were a single-celled organism; they competed with each other for space and resources as fiercely as modern organisms do today. There were predators, parasites, and viruses that plagued these early life forms, and various strategies were devised to deal with them. If you look at modern organisms, one of the most widely used and effective strategies against predation is to stay in groups. Nearly all animals that are food for something else do this, from colonies of ants to herds of elephants. The reason is obvious: it’s safer in numbers. This same strategy was used long ago by our ancestors, clumping together for mutual protection. Those clumps that were more integrated and able to fend off predators more successfully survived and passed on their genes more than those that did not. Once these clumps of cells were established they began competing with each other, and the more cohesive, large clumps out-competed the smaller, more diffuse ones. This process is eventually what led to the specialization of different types of cells in multi-cellular organisms. There are several different theories about how this may have occurred. The Symbiotic Theory is the idea that different groups of unrelated cells came together for mutual protection. These groups of cells became interdependent, and unable to survive without each other. Eventually, their various genomes were incorporated into one single genome, however there is currently no explanation how this may have happened. The Cellularization Theory states that one single cell developed multiple nuclei with partitions between them, essentially seeming like a multi-cellular organism without actually being one. The Colonial Theory is similar to the Symbiotic Theory, but starts with a colony of single-cell organisms all of the same species that gradually diversify over time. This last theory is the most widely accepted, and has been seen to occur independently in several lineages.
Step 5: An Explosive Case of Life
This famous event in evolutionary history was one of Darwin’s main issue with the theory of evolution. Almost overnight, it seems, multi-cellular life diversified from simple cellular colonies to most of the major phyla we have today (and many that no longer exist). This event is known as the Cambrian Explosion. Although this sudden emergence of so many species in the geologic column is incredible, the time-scales are so long, and the evidence so spotty, that it is likely that the Cambrian Explosion didn’t happen much faster than any other diversification event. Recent evidence shows that multi-cellular organisms date back to before this time, but they were simple, worm-like creatures. But during the Cambrian itself, multi-cellular life diversified into so many strange and interesting forms that its no wonder people have focused on this time. There are many theories for why this “explosion” occurred. Adaptive radiation events often follow extinction events, as the survivors struggle to fill the newly opened ecological niches, and there is evidence of an extinction before the Cambrian. Around this time, free oxygen in the seas and atmosphere was increasing, which may have allowed organisms to grow larger. Likewise, calcium concentrations in the seas increased, possibly allowing for the construction of skeletal tissues. Even the evolution of the eye and specific hox genes have been listed as reasons for this event. Whatever the reason, following this event, life on Earth became much more recognizable to our modern sensibilities.
Step 6: The Walking Fish
Our first ancestors that crawled up onto land were little more than fish with stiff fins, dragging themselves through the mud. Seeking to escape predation, or take advantage of new food sources on land, these first air-breathing fish gradually colonized the land, eventually evolving into reptiles, amphibians, mammals, and birds. During the Devonian period, roughly 416 million years ago, these first tetrapods inhabited shallow streams, walking along the bottoms with their fins/feet. Interestingly, the bony fish that these tetrapods evolved from were the first fish to have a swim bladder, which would evolve into a lung in tetrapods, and nearly all fish species alive today (excluding sharks and rays) are their descendants.
Step 7: Big Brains Make a Big Impact
After the evolution of tetrapods, the story of life on Earth becomes very familiar. Mammals and dinosaurs evolve at about the same time, but dinosaurs are the dominant multi-cellular life form on land until the extinction event at the end of the Cretaceous period. During this time, mammals were little more than scurrying vermin, but they were perfecting traits that would allow them to become dominant later. Since they were mainly nocturnal, mammals evolved to be warm-blooded in order to stay active through the cool nights. They gave up laying eggs in favor of live births, and mothers began to feed their young with milk glands. These traits made social life among mammals very important relative to other organisms. Warm-bloodedness, complex social structures, and finally the extinction of the dinosaurs had mammals poised to take over the world at the end of the Cretaceous period. Since the first simple brains evolved long ago, the average brain-size and intelligence of organisms has generally increased over time, and roughly 2 ½ million years ago the first members of our brainy family evolved.
Descended from tree-dwelling apes that had adapted to the swelling grasslands, these ancestors of ours lived in complex social structures like those of chimpanzees and bonobos. However, they began hunting more often than our cousins that stayed in the forests, as the game in the grasslands was larger, more common, and the most exploitable source of food available. These early hunters didn’t have deadly claws like a leopard, or crushing jaws like a hyena, but what they did have was the big brains of apes. Over times, these brains increased in size as the more intelligent hunters brought home more food, and were thus able to reproduce better. Human minds are relationship calculators, designed to keep track of the relationships between various members of our group. As group sized increased, so did the size our brains in order to keep up. Being the extremely social creatures that we are, the most important thing in our lives is impressing our fellows with our mental and physical feats, and thusly you have the origin of art, science, and religion.
The story of life on Earth is much more complex and convoluted than the simple, 7-step process I’ve outlined here. There are twists and turns unexpected by even the most imaginative biologist, and with our emerging understanding of genetics, the story is being revised all the time. Mysteries abound, from how life began to why people started farming instead of hunting and gathering. But no matter how limited our knowledge is, it’s somehow comforting to think of yourself as a component in an unbroken chain stretching 3.8 billion into the past, and who-knows how long into the future.