Lecture 8 Notes: Patterns & Processes in Life History
Part 1: Conditions on early Earth
Life's intricate history on Earth began under vastly different conditions than those present today, marked by key evolutionary milestones such as the emergence of the first single-celled organisms, the development of photosynthesis, the rise of eukaryotes and multicellularity, and eventually, the colonization of land. A central question in understanding this history revolves around when each of these pivotal events occurred during various geologic periods. The early atmosphere was characterized by forming organic molecules from simpler molecules, with energy for these reactions primarily sourced from lightning and UV radiation. Early oceans were a solution rich in these organic molecules, leading to the concept of the primordial "soup" from which life is hypothesized to have arisen.
The origin of life is typically hypothesized to have occurred in four sequential steps. First, there was the abiotic synthesis of organic molecules, followed by the formation of organic polymers through the repetition of these units. The third step involved the formation of protocells, which were membrane-bound precursors to true cells. Finally, ribozymes, RNA enzymes capable of facilitating biochemical reactions, emerged. The groundbreaking Miller–Urey experiment, conducted in 1953 by Stanley Miller and Harold Urey, simulated Earth’s early atmosphere and demonstrated that amino acids, fundamental building blocks of life, could be produced abiotically. This experiment showed that abiotic processes could indeed yield the basic components of life, with electricity driving the necessary reactions.
Since life began, the world has undergone profound transformations, with past organisms differing significantly from modern ones. The fossil record serves as a crucial document of macroevolutionary changes over vast geological timescales, detailing major events such as the emergence of terrestrial vertebrates, the origin of photosynthesis, and the pervasive long-term impacts of mass extinctions. However, this record is not without limitations and biases, often favoring certain taxa, habitats, time periods, and species that were abundant. Unique fossil repositories, such as bog bodies, offer remarkably well-preserved remains due to specific conditions like low oxygen, minimal water flow, acidic soils, and cooler climates.
Dating fossils involves both relative and absolute methods. Relative age is determined by a fossil’s position within geological strata, indicating its sequence in time without providing a precise numerical age. Absolute age, conversely, is established through radiometric dating, which utilizes the predictable decay of parent isotopes into daughter isotopes. A key concept in this method is the half-life, which is the time required for half of a radioactive parent isotope in a sample to decay. Visualizations often illustrate carbon-14 () decay and its half-lives, showing how age estimates are derived from the proportion of remaining parent isotope versus accumulated daughter isotope over time. Carbon-14 dating is generally suitable for fossils up to approximately years old. Numerous geological formations, such as the Navajo Sandstone, Kayenta Formation, and others, are frequently cited and illustrated to demonstrate relative ages in dating examples.
Part 2: A timeline of life on Earth
To better comprehend the immense span of Earth's history, the Cosmic Calendar scales the universe's billion-year history into a single year. On this calendar, the Big Bang occurs on January 1st at midnight, and the present moment is midnight on December 31st. Earth itself forms around BYA, relatively early in the calendar year. Life is estimated to have arisen between and BYA, with the first prokaryotes appearing by approximately BYA. The oxygen revolution, a pivotal environmental transformation, occurred around BYA. Single-celled eukaryotes emerged around BYA, followed by multicellular eukaryotes around BYA. The first animals appeared approximately MYA, with early forms like jellyfish appearing around MYA. The Cambrian explosion, a period of rapid diversification, took place between and MYA, while the colonization of land by fungi and plants initiated around MYA, with mutualistic relationships between plants and fungi established on land by MYA, paving the way for arthropods and tetrapods to become widespread. Notably, tetrapods evolved from lobe-finned fishes around MYA.
The geologic time scale divides Earth's history into major eons: Hadean, Archaean, Proterozoic, and Phanerozoic. The Phanerozoic Eon, which encompasses the most recent period of abundant complex life, is further subdivided into the Paleozoic, Mesozoic, and Cenozoic Eras. Major transitions, such as the shift from the Proterozoic to the Phanerozoic, signify significant radiations of life forms and mass extinctions.
The Hadean Eon, spanning from Earth's formation about BYA, was a tumultuous period with no life until approximately – BYA. Following this, the Archaean Eon saw the rise of the first prokaryotes around BYA; evidence for this includes stromatolites, which are layered rock-like structures formed by ancient cyanobacteria. Prokaryotes dominated Earth for a substantial period, from to BYA. The Archaean Eon also experienced the significant oxygen revolution around BYA, a direct consequence of photosynthetic activity, which dramatically altered the atmosphere and enabled the evolution of aerobic respiration.
During the Proterozoic Eon, single-celled eukaryotes appeared around BYA, with the theory of endosymbiosis explaining the origin of organelles such as mitochondria within these cells. Multicellular eukaryotes then emerged around BYA, giving rise to ancestral forms of algae, plants, animals, and fungi later in this eon. The first animals, including sponges, appeared around MYA, with jellyfish fossils dating back to approximately MYA.
The Paleozoic Eon began with the Cambrian Explosion (between and MYA), a geological flashpoint characterized by the rapid appearance of diverse animal phyla. Prior to this event, life was generally simple, small, or soft-bodied. This era also marked the first clear signs of predation and the co-evolution of various prey defenses. A critical development in the Paleozoic was the colonization of land by plants, fungi, and later, arthropods and other lineages.
The settlement of land, commencing around MYA, involved fungi and plants establishing terrestrial habitats, often forming mutualistic relationships as indicated by fossilized roots. This transition required significant evolutionary adaptations for reproduction and prevention of dehydration. A major evolutionary step was the emergence of tetrapods, which evolved from lobe-finned fishes around MYA. In summary, key milestones include Earth's formation at BYA, life's onset at – BYA, prokaryotes at BYA, the oxygen revolution at BYA, eukaryotes at BYA, multicellularity at BYA, the first animals at MYA, the Cambrian explosion between and MYA, land colonization from – MYA, and tetrapods evolving from lobe-fins at MYA.
Part 3: The rise and fall of species
The fossil record vividly documents how life on Earth has changed over time, illustrating clear lineage changes and episodic extinctions in younger strata compared to older ones. These long-term changes are driven by large-scale processes such as continental drift, mass extinctions, and adaptive radiations. Continental drift, specifically, has significantly shaped biodiversity patterns. Land masses have converged to form supercontinents three times in Earth's history: around BYA, MYA, and MYA. The formation of the supercontinent Pangaea around MYA had profound ecological consequences, including a reduction in shallow-water habitats, leading to colder and drier inland climates. As continents shifted towards or away from the poles, global climate patterns changed, altering ocean circulation and contributing to global cooling. Conversely, the breakup of Pangaea later promoted allopatric speciation by isolating populations. The current distribution of fossils provides strong evidence of these historical continental movements.
Earth's crust, which varies in thickness from approximately to over miles, is relatively thin and underlies the dynamic process of plate tectonics. Interactions between these tectonic plates are responsible for significant geological phenomena such as the formation of mountains and islands, as well as the occurrence of earthquakes. Periodically, life on Earth has been punctuated by mass extinctions, defined as the loss of at least % of species within a geologically short interval, typically around million years. Two particularly notable mass extinctions include the End-Permian extinction (occurring MYA), which eradicated approximately % of marine species, possibly due to massive volcanism leading to global warming and oceanic anoxia. The End-Cretaceous extinction ( MYA) saw the demise of about % of species, including the dinosaurs, with compelling evidence pointing to the Chicxulub meteor impact (characterized by iridium enrichment and a large crater) as the primary cause.
Concerns are growing that Earth may be heading for a sixth mass extinction. Historically, there have been five major mass extinctions, often referred to as the "Big Five": the End Ordovician, End Devonian, End Permian, End Triassic, and End Cretaceous events. Current ecological trajectories reveal alarmingly high extinction rates, with data indicating that contemporary extinction rates significantly outpace background rates by orders of magnitude. "Our World in Data" synthesizes show that extinction rates for mammals, vertebrates, and birds are currently well above historical background levels, with estimates suggesting rates to times higher for certain groups.
Following periods of extinction, adaptive radiations often occur. These are defined as the rapid evolution of diversely adapted species from a common ancestor in response to new ecological opportunities and reduced competition. Classic examples include the rapid diversification of mammals after the extinction of the dinosaurs, and earlier radiations of photosynthetic prokaryotes, Cambrian predators, land plants, insects, and tetrapods upon colonizing land. Adaptive radiations are also prominently observed in island systems, such as the Hawaiian silverswords and related taxa, which demonstrate rapid diversification following the colonization of new island habitats, with multiple lineages diverging across several islands over millions of years.
Key Terms and Concepts
The fossil record is the chronological collection of fossils that serves as our primary means to study life's extensive history on Earth. Relative dating allows us to determine the sequence of geological events without assigning exact ages, while radiometric dating provides absolute ages using the measurable decay of radioactive isotopes. The half-life is a critical concept in radiometric dating, referring to the time required for half of a given amount of a radioactive parent isotope to decay into its daughter product. The Miller–Urey experiment is a classic demonstration that showed organic building blocks can form under the assumed prebiotic conditions of early Earth. These early oceans, rich in organic molecules, are referred to as the primordial soup, supporting the idea of abiotic synthesis, the non-biological formation of organic molecules. Protocells were membrane-bound precursors to true cells, representing an important step in abiogenesis, while ribozymes are RNA molecules that possess catalytic properties, acting as enzymes. The Cambrian Explosion refers to the rapid diversification of animal phyla that occurred approximately – MYA. The colonization of land describes the complex process by which plants, fungi, and animals adapted to terrestrial environments. Endosymbiosis is the theory explaining the origin of eukaryotic organelles, like mitochondria, through the engulfment of prokaryotic cells. Pangaea was the former supercontinent that formed around MYA, severely impacting global climates and habitats. Allopatric speciation is a form of speciation that occurs due to geographic isolation. A mass extinction signifies a drastic and widespread loss of biodiversity within a geologically short timeframe. Finally, adaptive radiation describes the rapid diversification of species into new ecological niches, often following events like mass extinctions or the colonization of new habitats. The major eons of Earth's history are the Hadean, Archaean, Proterozoic, and Phanerozoic, with the Phanerozoic further divided into the Paleozoic, Mesozoic, and Cenozoic eras.