In the beginning,
The time period of Earth’s formation is called the Hadean Eon. Occurring between 4-4.6 bya, it is named after the Greek God of the underworld, Hades, because of its fiery landscape of volcanic activity and hellish conditions. The primitive atmosphere of Earth was remarkably different from today’s, lacking oxygen and instead filled with methane, hydrogen, and carbon. However, our early planet is more accurately conceptualized as a boiling pot of prebiotic compounds than a hellish Underworld. Fueled by energy from intense solar radiation, this infernal landscape is where life first originated.
The first oceans became known by scholars as primordial soup where the amassing of organic compounds created the basis for the first forms of life on the planet. The oceans mixed together organic molecules derived from asteroids that collided with earth and components created through abiotic synthesis. Scientists have proven that the components present in early Earth, such as adenine , could produce the building blocks of life abiotically. You can think of the early evolution of life as a chef adding spices and different ingredients into a boiling pot over the stovetop of Earth’s gradually cooling mantle. The recipe for life is a unique combination that has increased in complexity over time into the world we know today and will continue to change into unknown future forms.
“You can think of the early evolution of life as a chef adding spices and different ingredients into a boiling pot over the stovetop of Earth’s gradually cooling mantle”
The first organisms that evolved out of this soupy Earth were simple, unicellular life forms that could withstand the tough conditions on Earth. One of the first fossils was of stromatolites found in the Apex chert of Western Australia as carbonaceous filaments. Stromatolites are prokaryotic organisms that are believed to be the first form of life. Since stromatolites are microscopic and share similar features with abiotic, non-living structures, the scientific community lacks a consensus on their specific time of emergence. However, scholars can agree that single-cell stromatolites are one of the first organisms and became widespread between 2.5-3.5 bya.
Around 2.4 bya, life on Earth experienced a dramatic change as oxygen became abundant in the atmosphere and organisms either adapted to the new atmospheric balance, remained in regions deficient of oxygen, or went extinct. The abundance of photosynthetic organisms like cyanobacteria and purple bacteria caused oxygen levels to dramatically rise in the Great Oxidation Event.
The Great Oxidation Event spurred multicellular life through more efficient energy production. While eukaryotic organisms may have been present beginning 2700 mya, they only grew in complexity and size with the development of mitochondria. The size of eukaryotes was limited by the amount of oxygen in the environment because anoxygenic energy production is less efficient. If an organism has to use 1/2 of its energy in order to gain more energy, then it has less to expend on growth. Mitochondrial eukaryotes received 18 times more energy and increased from 2-3 cell types into more than 50 by 1.5 mya.
Land Plants and the First Animals
Multicellular eukaryotes are believed to be the first organisms to reproduce sexually, creating a greater variety in the gene code and leading to the evolution of plants. Prokaryotic and some eukaryotic organisms reproduce through asexual reproduction, meaning the organism splits to produce two identical organisms identical to the original. In contrast, sexual reproduction is the combination of male and female gametes (or sex cells) that create a new organism with half of its DNA from each parent. The mechanisms that favored sexual reproduction are disputed, with possible explanations that evolution favored the development of sexual reproduction because it protects the replication of DNA, creates a lottery system of biodiversity, and/or raises mitochondrial efficiency. Nevertheless, sexual reproduction raises the genetic diversity of a population by recombining DNA strands from parents to create genetically unique offspring.
The development of the first land plant is believed to have occurred in stages, beginning with sexual reproduction and then zygote retention and development on the parent, as seen in the modern Coleochaete. Coleochaete has more complex features than its historic ancestor, but it demonstrates “zygote retention and subsequent development of the zygote [fertilized sex cells] on the haploid [single set of unpaired chromosome] parental body”. In simpler language, the fertilized cells remain on the parent body in a pattern that is associated with the origin of land plants. The modern plant is also promising because it grows near sea shores and can survive in aquatic environments, similar to how land plants would have begun. Sexual reproduction and expansion onto land was beneficial because it provided more nutritional value, diversity, and produced more offspring. The first plants on land quickly expanded and diversified, setting the stage for future plant life to develop.
Similar to the first plant life, the first evidence of organisms with animalistic qualities is also greatly debated amongst scholars. Models used to trace back the development of species by the average rate of mutation in genes predict that animals diverged in the early Ediacaran period, around 600 mya. Fossil evidence indicates that the Caveasphaera species is likely to have been one of the first in the animal lineage. Found in the quarries of Guizhou Province, South China, Caveasphaera are multicellular, eukaryotic organisms with a developmental process that resembles later animal embryos. In fact, Caveasphaera goes through gastrulation, an early stage of embryonic development, indicating its connection to later multicellular animals. The first animals evolved through a complex process of simple changes–possibly beginning 600 mya– and paved the path for modern creatures.
Mass Extinction and Natural Selection
By the Ordovician Period, life on earth was prospering with an abundance of marine life and land plants–until the first mass extinction. Evidence in the geological record indicates a glacial period 500 mya in the late Ordovician Period that lowered sea levels dramatically and spanned millions of years. The glacial period is unique because carbon dioxide levels (a greenhouse gas with a warming effect) in the atmosphere were incredibly high, 16 times today’s. Despite the high levels of carbon, volcanic activity that would block sunlight and carbon intake through metamorphic outgassing can explain the major glaciation event. Out of the five major extinctions in earth’s history, the Ordovician extinction is the first and second largest in terms of the percentage of species that were lost. The glaciation and resulting circulation in the ocean resulted in a 26 percent loss of marine biodiversity, measured by the amount of families.
While mass extinctions result in great losses of current species, they can result in quicker evolution through the processes of natural selection and pressure selection. As mentioned previously, DNA is known to randomly mutate and change, creating diversity within the genetic makeup of a species. The results of mutations are often negative, like a mutation that would limit the energy intake of an organism, but the mutations can occasionally be positive. A positive change is defined by enabling the organism to pass its genetic makeup to the next generation more successfully than they could have without the variation. For example, if organism A with a mutation can produce more offspring than organism B, then the mutation is more likely to spread into the next generation. It’s important to recognize that natural selection is random, since mutations cannot be chosen and most life cannot actively seek evolution. Organism A cannot choose to develop the ability to survive in colder conditions during a glacial event, but a random mutation may occur that enables a member of its species to survive and reproduce.
It’s important to recognize that natural selection is random since mutations cannot be chosen and most life cannot actively seek evolution
In the randomness of natural selection, it’s critical to expand on some of the common misconceptions about evolution. The species that survive and reproduce spread their genes to the next generation based on past performance, not future expectations. Organism A could contain a genetic advantage in extremely hot environments, but Earth went through an ice age, leading to the disappearance of the trait over time. If, after the ice age, Earth became hot again, then the trait to survive hot climates may no longer be present and could threaten the survival of the species. In evolution, past performance is the only measure available, and future performance is never guaranteed. Similarly, survival doesn’t guarantee “perfection”–just the ability to live and reproduce. If organism A contains a mutation that causes death after their prime reproduction years, the trait will continue to be passed onto its offspring despite the perceived negative mutation. Evolution has not led to perfection and organisms continue to have “flaws”.
Natural selection and evolution are always ongoing, but intense environmental events can increase the rate of diversification through pressure selection. The species that survived the Ordovician mass extinction evolved new adaptations, niches, and lifestyles. Life on earth recovered after the extinction event: new groups of brachiopods, marine organisms that live on the ocean floor, diversified. Harsh environmental conditions create pressure for species to change and be selected for the next generation, or face extinctions. For example, if oxygen became depleted, then only organisms that could survive with lower levels of oxygen would pass into the next generation. The more gradual the change in the environment, the more likely a mutation would occur that enables a species to survive. The Ordovician extinction was more critical because the cooling occurred rapidly, allowing less time for species to adapt and survive.
Life has existed for a long time by this point in Earth’s history, but evolution still has a ways to go before life happens to produce humankind. From the Carboniferous to the Permian Period 362-248 mya, the supercontinent of Pangea formed while the amount of oxygen in the atmosphere spiked and carbon dioxide dropped, compared to the atmosphere of today. Carbon decreased in the atmosphere because dead plant material turned carbon into peat and oxygen increased as great forests and ample plant life covered the earth. Meanwhile, life for animals was also diversifying with the evolution of reptilian organisms, four legged vertebrates, and flying insects. During this time period, animals began eating plants for the first time, though they remained mostly omnivorous due to insufficient protein from plants. The advances appear rapid compared to prior evolution, but the actual process spanned over a hundred million years with each generation gradually mutating and adapting.
The great rise in biodiversity was followed by the largest out of the five mass extinctions in Earth’s history, ending the Permian Period. The mass event was caused by climate change and the eruption of The Siberian Traps that were possibly provoked by an astrological impact. Oceans were deeply impacted by climate change since sea level rose, leading to anoxic areas and sudden changes in ocean currents. Then, volcanic eruptions blocked the sun and produced acid rain that killed major swaths of plants in the span of days. As a result of these and continued rapid changes in the climate, 95 percent of marine life and half of all marine families went extinct.
Here come the dinosaurs
Pangea began to break up during the Jurassic Period, 206-65 mya, and biodiversity increased as dinosaurs rose. The physical earth changed greatly throughout this period, as the supercontinent divided and sea level fell while mountains like the Himalayas formed. While Earth changed shape, so did life. Dinosaurs diverged into two groups of “bird-hipped (ornithischian) and lizard-hipped (saurischian) dinosaurs“. After the division, dinosaurs continued to diversify into a variety of species with some displaying social organization, hunting, and forming of herds. Marine life also increased in complexity as large marine lizards and modern species like turtles emerged.
The Jurassic Period too ended with an extinction known as the Cretaceous/Tertiary extinction that ended the dinosaurs and many other species. The cause of the extinction is believed to be an asteroid that hit in the Yucatan Peninsula of Mexico, which caused large tsunamis and covered the sky with dust for weeks. Similar to previous extinctions, plants died due to lack of sunlight, up to 79 percent, and the extinction also reduced the amount of mammals globally by 35 percent. Since dinosaurs and larger animals relied on plants and mammals for food, the events were especially detrimental to their survival.
The Cenozoic Period that began 65 mya lasts until today and includes major events in terms of human development, but is overall a small portion of life’s history. As the continents have continued to separate, regional ecosystems developed, like grasslands in the Americas. The greatest levels of biodiversity were seen during the beginning of the period when tree canopies covered continents. Sharks became no longer the only large animals in the sea as land animals began to enter, leading to modern whales and sharks. However, “the most successful group of large mammals today is undoubtedly the bovids—cattle, sheep, goats, and antelopes in all their diversity“, despite being some of the last organisms to evolve. Humans and apes didn’t evolve until 20 million years ago, meaning we have been alive for a small fraction of life’s evolution.
The world continues to be shaped and created by the natural processes of evolution and natural selection. The history of life has witnessed large swaths of growth in biodiversity and devastating events that have changed the course of the living world. Looking towards the future, the path of evolution is impossible to predict; it is fundamentally a random series of events, but the best predictor is the past. Life fails to perform well when faced with rapid changes because adaptation is a slow process that can require millions of years. The best hope we have as we face the current mass extinction is protecting biodiversity by preventing human caused, rapid changes to Earth’s systems.