Big Steps in the Story of Big History – Part 4

From algae to everything.

Continuing our essay series on Brian Villmoare’s “Big History” book The Evolution of Everything: The Patterns and Causes of Big History, this essay will explore how the vast diversity of life on Earth evolved from one-celled organisms.

As Villmoare writes:

But how did the appearance of a simple, one-celled organism living in the “primordial soup” lead to all the amazing forms that evolved over the next 3.5 billion years – from sunflowers to jellyfish to grasshoppers to slime mold to humans? The answer is evolution. The definition of the word is simply this: a change in genetic variation in a population over time. The final source of change in genetic frequencies in populations is natural selection. Evolution by natural selection is a change in genetic frequencies in a population that is not simply random – it is driven by something. Some individuals reproduce, and some don’t, because some are prevented from reproducing. Often, this takes the form of some animals not surviving long enough to reproduce, and their gene combinations are therefore eliminated from the gene pool, which changes the overall population’s genetic frequencies. Sometimes, under some conditions, various elements of organisms give them an advantage in survival. A classic example is the thickness of fur coats in coyotes. If a coyote lives in the Arctic, a thick coat is an obvious advantage during a particularly cold winter. Coyotes with thinner coats may simply die (and therefore be prevented from passing their genes on to later generations). Alternately, if the coyote lives in the American Southwest, a thinner coat may provide an advantage during particularly blistering summers … [S]ometimes two populations are separated for so long that they will or can no longer interbreed. The red fox (Vulpes vulpes) and the gray fox (Urocyon cinereoargenteus) are both North American foxes, but they diverged several million years ago, and during that separation they acquired enough differences in anatomy and behavior that they will no longer interbreed. In fact, during this time there have been significant genetic changes, and these two species no longer have the same number of chromosomes, so even if they did attempt to mate there would be no fertile offspring. When enough evolutionary differences have accumulated, and the two populations no longer interbreed, evolution has generated a new species. Under stabilizing selection there is an optimum, and any organism that strays too far from the mean is selected against. One famous example of this is the human pelvis. The human pelvis does two very important jobs. The first is probably obvious – it is where the legs attach, so it must have a shape that allows for efficient locomotion. The ability to walk and run is critical in nature, and when food is sparse an animal that can forage more efficiently (burning fewer calories per mile) will be more likely to survive and reproduce. Generally, narrower hips tend to be more efficient – Olympic marathon runners, both male and female, have relatively narrow hips because humans with narrow hips tend to keep the body mass more easily centered, with less side-to-side movement. Over a marathon, the seemingly slight advantage afforded by narrow hips adds up, so you will rarely see competitive marathon runners with wide pelvises. Something similar was at work in our deep evolutionary past, except that, instead of running for fun, we were covering large distances looking for food. However, the human pelvis has another, equally (or more) important job – it provides the birth canal. Humans are born with extremely large heads, and, unlike in other animals, birth is dangerous because the human head must fit through a relatively small opening in the pelvis. Before modern medicine, death in childbirth was one of the leading causes of death in women. But one way natural selection made birth easier was to give humans wider pelvises (and in humans, females do have slightly wider pelvises than males, on average). Stabilizing selection is at work across all of nature – bird wings cannot be too small (insufficient lift) or too large (makes the bird slower, and costs many calories of food to support), squirrel fur cannot be too dark (too easy to see against snow) nor too light (too easy to spot against the tree trunk), and so forth.

The literal key to life is shaped in a structure called a double helix, which unlocks living things’ ability to replicate:

One of the strongest pieces of evidence that all life on Earth originated from a single event is that every living thing shares the same underlying biological blueprint. The most important element of this blueprint is the set of instructions built into every organism. These instructions are known as the genetic code. Although each organism has a unique set of these instructions, so that every organism is unique, the actual elements of the code are shared among all organisms. This code is a series of instructions for assembling each organism. This code is in the form of a particular type of chemical combination known as deoxyribonucleic acid, or DNA … [E]ach cell of an organism has the DNA of that organism in it … One advantage of the double helix (as opposed to a single helix) is that the nucleotide bases are chemically protected from chemical elements floating throughout the cell. Chemicals will naturally bond with other chemicals under certain circumstances. If a nucleotide made a bond with a random chemical element floating in the nucleus, the genetic code would be compromised. With a double helix, the base is effectively already bound to another base, preventing additional, potentially compromising, bonds … [T]he first life forms were simple and single-celled, and probably acquired energy for reproduction the same way plants do today – by converting energy from the Sun into the fuel needed for growth. For most of the Earth’s history, life was simply that – single-celled organisms (early cyanobacteria) that converted solar energy to carbohydrates that they could then consume, and converted carbon dioxide into carbon and free oxygen (O2) in the process.

But the DNA key was itself unlocked by the ultimate source of energy for life on Earth, namely solar power used by plants and the earliest forms of bacteria that later became incorporated into the first animal cells:

The process of converting solar energy into sugar is known as photosynthesis and it may be the single most important step that led to the evolution of all life on Earth. Photosynthesis is the conversion of the Sun’s energy into energy that a plant can use to grow. If a plant has access to water, atmospheric carbon dioxide, and sunlight, those things alone are sufficient for growth. A plant needs no other fuel to achieve this. So when we talk about photosynthesis, we really are talking about the conversion of the Sun’s energy into plant matter and the generation of oxygen as waste. You may have learned about photosynthesis in school, but it is worth repeating how this process works. Sunlight hits the surface of a cell (whether the leaf of a flower, or a mat of cyanobacteria), and a chemical in the cell known as chlorophyll absorbs the sunlight and releases a small amount of electricity. This electrical charge breaks up the bonds in the water (H2O) and carbon dioxide (CO2) and reshuffles the atoms in those two chemicals. The carbon and the hydrogen, and some of the oxygen atoms, form a new chemical called glucose (C6H12O6), and since there are some oxygen atoms left over, they form O2, the familiar form of oxygen you and I breathe. The glucose is a sugar that fuels the cell’s growth, and the oxygen is simply released into the atmosphere … The early photosynthetic bacteria formed the foundation of all later multicellular photosynthetic life, including plants. In fact, cyanobacteria are now part of every organism that undergoes photosynthesis. Inside every photosynthetic cell of a plant is a small structure known as a chloroplast. The chloroplast has the chlorophyll and is responsible for absorbing sunlight and generating the energy for the plant cell. But this small structure has its own DNA and its own cell membrane. This is because, at some time in the deep past, several billion years ago, one bacteria type absorbed a cyanobacteria and co-opted its energy-generating capabilities for its own use. That co-opting bacterium had its own DNA, and as time went on that bacterium evolved into more and more complex organisms, ultimately leading to plants. The advantage for the absorbed organisms was the protection of the outer cell wall … This happened again when energy-producing bacteria (a relative of Rickettsia) was absorbed into an early animal cell, which we now call mitochondria … Animals survive by consuming other organisms (plants or animals), so the mitochondria assist in converting the bodies of other organisms into energy that the animal cells need … [P]lants use the stored chemical energy to grow. But when animals eat those plants, they are acquiring that same stored energy, which they then convert to growth, or store as fat. If a predator eats an animal, that same energy is passed down to the predator. So, in effect, when a lion eats an impala it is eating an animal that is stored solar energy, only in chemical form. Humans access this same solar energy when they eat plants or animals, or burn logs for warmth. But what about other forms of energy? When we burn gasoline in our cars, we are using that exact same solar energy. Hundreds of millions to billions of years ago, giant mats of cyanobacteria and other organisms covered the Earth’s oceans. When they died and were buried beneath the ocean floor, over time that plant material decayed and slowly converted into what we call crude oil. A similar process occurs during coal formation, when plant matter is buried and decays over millions of years. In both cases, the product we burn is a stored accumulation of millions of years of solar energy, converted into a chemical form via photosynthesis. The only form of chemical energy on Earth that is not the result of photosynthesis is nuclear energy.

These early processors of solar energy became incorporated into more complex forms of life:

Endosymbiosis [a symbiotic, that is, mutually beneficial, relationship that occurs when one organism lives inside another] has appeared at least twice and may be critical for the evolution of more complex life. If a cell, or parts of an organism, can be relieved of the need to acquire energy by absorbing another organism, that frees the host organism to develop specialized cells. For example, the trunk of a tree does not need to have photosynthetic cells because another part of the plant, the leaves, has them and can absorb energy for the tree. Single-celled organisms don’t have this luxury because each cell is an organism that must survive on its own. Because of the seemingly infinite variation in anatomical structures available to a multicellular organism, being a eukaryote with anatomical variation among different cell types allows complex organisms to adapt to a greater variety of environments. For example, if the ground is covered by photosynthetic bacteria, there would seem to be no way for a plant to acquire enough solar energy to survive. But a plant can extend vertically, growing stalks above the ground and spreading leaves so that it can acquire solar energy. A single- celled organism like cyanobacteria has no ability to do this; all they can do is reproduce, whereas plants can extend specialized anatomy out to make sure the organism itself acquires enough sunlight to survive.

And once multi-cellular organisms developed, they opened the evolutionary floodgates that created all the forms of life we see today:

The evolution of multicellularity opened the door to an enormous range of anatomical adaptations, so that today we have organisms that can move and feed deep in the oceans beyond the reach of the Sun, as well as high in the air. What is particularly noteworthy is how rapidly this occurred. Once organisms with complex anatomy arrived on the scene, the combination of sexual reproduction, with its constant reshuffling of genes, and a complex combination of anatomies on which natural selection could work, produced a bewildering array of creatures. After more than 2 billion years of single-celled organisms as the only life on Earth, suddenly there were creatures swimming and crawling through the oceans, as well as new plant variations on which these animals could feed. The evolution of specialized anatomy is closely linked to the evolution of sexual reproduction. Without specialized anatomy, sexual organs could not exist. And once sexual reproduction takes off, more anatomical variation is possible. So these two elements act together in a feedback loop to dramatically increase the potential anatomical variation in multicelled organisms on Earth, leading to the sudden proliferation of complex life we call the Cambrian Explosion.

Amidst this abundant life are two main types of life, plant and animal:

Once these two lineages diverged, [plants and animals] adopted very different adaptive strategies – the plant lineage acquired a symbiotic relationship with cyanobacteria and became dependent on solar radiation as their energy source. Animals became dependent on the energy in other organisms: animals are the branch of life that consumes other organisms (whether plant, animal, algae, etc.) … [W]e don’t see anything that we would identify, today, as an animal until around half a billion years ago. By this time, oxygen had started to accumulate in the atmosphere in levels comparable to today, and this oxygen enabled the evolution of a wide variety of complex and larger-bodied life. The oldest animal fossil that can be identified as an animal is the sponge. Sponges are the simplest animals and act as filters, sitting on the ocean floor catching whatever passes through their porous tissues … At some point, perhaps 700 million years ago, the earliest animals evolved. These weren’t animals as we normally think of them, as they had no legs, eyes, flippers, tentacles, fins, or antennae, nor any of the other myriad physical specializations we see in most modern animals. In fact, these early animals lacked the systems we associate with animals: circulatory, nervous, and digestive systems. However, they did digest food, so the first big step in the animal lineage was abandoning the pursuit of solar energy and instead looking to other organisms as sources of energy. Paleontologists have discovered very old fossil sponges. By 580 million years ago we already have a diversified group that is well preserved in the fossil record. The next major group to evolve was jellyfish, and in this group we see many of the adaptations we associate with later animals: nervous systems, movement, specialized digestion, etc. And from there, the acquisition of specialized anatomy suddenly expanded, as evidenced by the fossil record of animals during the Cambrian Explosion. But all of these later adaptations are dependent on the initial commitment by sponges to live by eating other organisms … There are many other fossil locales that have preserved animals from the Cambrian, and during this time we also see the appearance of some familiar groups. For example, cephalopods, the family that includes nautiluses, squids, and octopuses, originated at this time. Other mollusk groups, such as bivalves (clams, oysters, mussels) and gastropods (snails and slugs on land, conch and limpets in the water), are also known from fossil record from the period around 500 million years ago (0.5 billion years ago). This is also when we see the ancestors of the vertebrates, of which humans are a member. Many of the important anatomical adaptations that we see in animals today appeared during this time, such as body shape, nerves and brains, and eyes.

In the next essay in this series, we’ll explore how neurons gave rise to complex animals, including humans.