2024 Origin of Life Lecture Notes PDF

Network of Life Lecture 4 Origin of Life / History of Life  Review Basics of Ecology ▪ The subject of ecology is the scientific study of the interactions between organisms and the environment. ▪ Ecology can be hierarchical subdivided into: or...

Network of Life Lecture 4 Origin of Life / History of Life  Review Basics of Ecology ▪ The subject of ecology is the scientific study of the interactions between organisms and the environment. ▪ Ecology can be hierarchical subdivided into: organismal-, population-, community-, ecosystem-, landscape- and global ecology. ▪ The biosphere is virtually a closed system with regard to matter, with minimal inputs and outputs. With regard to energy, it is an open system, with photosynthesis capturing solar energy. ▪ Earth’s curved shape causes latitudinal variation in the intensity of sunlight and the tilted axis of rotation causes the strong seasonality. ▪ The distribution of terrestrial biomes is controlled by climate and disturbance. ▪ Interactions between organisms and the environment limit the distribution of species. 2 WS 2024/25 – Origin of Life / History of Life hhu.de  Origin of Life / History of Life Conditions on early Earth made the origin of life possible Direct evidence of life on early Earth comes from fossils of microorganisms that lived 3.5 billion years ago. But how did the first living cells appear? Observations and experiments in chemistry, geology, and physics have led scientists to propose one scenario: They hypothesize that chemical and physical processes could have produced simple cells through a sequence of four main stages: 1. The abiotic (non-living) synthesis of small organic molecules, such as amino acids and nitrogenous bases 2. The joining of these small molecules into macromolecules, such as proteins and nucleic acids 3. The packaging of these molecules into protocells, droplets with membranes that maintained an internal chemistry different fro m that of their surroundings 4. The origin of self-replicating molecules that eventually made inheritance possible. Though speculative, this scenario leads to predictions that can be tested in the laboratory. 3 WS 2024/25 – Origin of Life / History of Life hhu.de  Origin of Life / History of Life Synthesis of Organic Compounds on Early Earth Our planet formed 4.6 billion years ago, condensing from a vast cloud of dust and rocks that surrounded the young sun. https://www.alm aobservator y.org/ Early Earth is a photograph by Lynette Cook/science Photo Library For its first few hundred million years, Earth was bombarded by huge chunks of rock and ice left over from the formation of the solar system. The collisions generated so much heat that all of the available water was vaporized, preventing the formation of seas and lakes. 4 WS 2024/25 – Origin of Life / History of Life hhu.de  Origin of Life / History of Life This massive bombardment ended 4 billion years ago, setting the stage for the origin of life. The first atmosphere had little oxygen and was likely thick with water vapor, along with compounds released by volcanic eruptions, such as nitrogen and its oxides, carbon dioxide, methane, ammonia, and hydrogen. As Earth cooled, the water vapor condensed into oceans, and much of the hydrogen escaped into space. During the 1920s, Russian chemist A. I. Oparin and British scientist J. B. S. Haldane independently hypothesized that Earth’s early atmosphere was a reducing (electron-adding) environment, in which organic compounds could have formed from simpler molecules. The energy for this synthesis could have come from lightning and UV radiation. Haldane suggested that the early oceans were a solution of organic molecules, a “primitive soup” from which life arose. In 1953, Stanley Miller, working with Harold Urey at the University of Chicago, tested the Oparin-Haldane hypothesis by creating laboratory conditions comparable to those that scientists at the time thought existed on early Earth. His apparatus yielded a variety of amino acids found in organisms today, along with other organic compounds. 5 WS 2024/25 – Origin of Life / History of Life hhu.de  Origin of Life / History of Life htt ps:/ /www.pbs.org/exploringspace/m et eorit es /murchison/page5.html Miller-Urey experiment In 1953, scientist Stanley Miller performed an experiment that may explain what occurred on primitive Earth billions of years ago. He sent an electrical charge through a flask of a chemical solution of methane, ammonia, hydrogen and water. This created organic compounds including amino acids. 6 WS 2024/25 – Origin of Life / History of Life hhu.de  Origin of Life / History of Life However, some evidence suggests that the early atmosphere was made up primarily of nitrogen and carbon dioxide and was neither reducing nor oxidizing (electron removing). In addition, small pockets of the early atmosphere, such as those near the openings of volcanoes, may have been reducing. Perhaps the first organic compounds formed near volcanoes. In a test of this hypothesis, researchers used modern equipment to reanalyse molecules that Miller had saved from one of his experiments. They found that numerous amino acids had formed under conditions that simulated a volcanic eruption. https://www.express.co.uk/pictures/galleries/ 7 WS 2024/25 – Origin of Life / History of Life hhu.de  Origin of Life / History of Life Another hypothesis is that organic compounds were first produced in deep-sea hydrothermal vents, areas on the seafloor where heated water and minerals gush from Earth’s interior into the ocean. Some of these vents, known as “black smokers,” release water so hot (300–400°C) that organic compounds formed there may have been unstable. But other deep-sea vents, called alkaline vents, release water that has a high pH (9–11) and is warm (40–90°C) rather than hot, an environment that may have been more suitable for the origin of life. 8 WS 2024/25 – Origin of Life / History of Life hhu.de  Origin of Life / History of Life 9 WS 2024/25 – Origin of Life / History of Life hhu.de  Origin of Life / History of Life sources of organic molecules 1. Volcanoes 2.Deep sea hydrothermal vents 3.Meteorites Another source of organic molecules may have been meteorites. For example, fragments of the Murchison meteorite, a 4.5- billionyear-old rock that landed in Australia in 1969, contain more than 80 amino acids, some in large amounts. Studies have shown that the Murchison meteorite also contained other key organic molecules, including lipids, simple sugars, and nitrogenous bases such as uracil. By User:Basilicofresco - Derivative work from Image:Murchison meteorite.jpg, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=4301968 10 WS 2024/25 – Origin of Life / History of Life hhu.de  Origin of Life / History of Life Abiotic Synthesis of Macromolecules The presence of small organic molecules, such as amino acids and nitrogenous bases, is not sufficient for the emergence of life as we know it. Every cell has many types of macromolecules, including enzymes and other proteins and the nucleic acids needed for self-replication. Could such macromolecules have formed on early Earth? A 2016 study demonstrated that one key step, the abiotic synthesis of RNA’s two purine bases, adenine (A) and guanine (G), can occur spontaneously from simple precursor molecules; the abiotic synthesis of the smaller cytosine (C) and uracil (U) bases was accomplished in 2009. In addition, by dripping solutions of amino acids or RNA nucleotides onto hot sand, clay, or rock, researchers have produced polymers of these molecules. The polymers formed spontaneously, without the help of enzymes or ribosomes 11 WS 2024/25 – Origin of Life / History of Life hhu.de  Origin of Life / History of Life Protocells All organisms must be able to carry out both reproduction and energy processing (metabolism). DNA molecules carry genetic information, including the instructions needed to replicate themselves accurately during reproduction. But DNA replication requires elaborate enzymatic machinery, along with an abundant supply of nucleotide building blocks provided by the cell’s metabolism. This suggests that self replicating molecules and a metabolic source of building blocks may have appeared together in early protocells. The necessary conditions may have been met in vesicles, fluid filled compartments enclosed by a membrane-like structure. Recent experiments show that abiotically produced vesicles can exhibit certain properties of life, including simple reproduction and metabolism, as well as the maintenance of an internal chemical environment different from that of their surroundings. 12 WS 2024/25 – Origin of Life / History of Life hhu.de  Origin of Life / History of Life For example, vesicles can form spontaneously when lipids or other organic molecules are added to water. When this occurs, molecules that have both a hydrophobic region and a hydrophilic region can organize into a bilayer similar to the lipid bilayer of a plasma membrane. https://www.sciencephoto.com/media/531367/view/lipid-behaviour-in-water-animation 13 WS 2024/25 – Origin of Life / History of Life hhu.de  Origin of Life / History of Life For example, vesicles can form spontaneously when lipids or other organic molecules are added to water. When this occurs, molecules that have both a hydrophobic region and a hydrophilic region can organize into a bilayer similar to the lipid bilayer of a plasma membrane. https://www.sciencephoto.com/media/531367/view/lipid-behaviour-in-water-animation Adding substances such as a soft mineral clay produced by the weathering of volcanic ash, greatly increases the rate of vesicle self-assembly. This clay, which is thought to have been common on early Earth, provides surfaces on which organic molecules become concentrated, increasing the likelihood that the molecules will react with each other and form vesicles. Abiotically produced vesicles can “reproduce” on their own, and they can increase in size (“grow”) without dilution of their contents. Finally, experiments have shown that some vesicles have a selectively permeable bilayer and can perform metabolic reactions using an external source of reagents - another important prerequisite for life. 14 WS 2024/25 – Origin of Life / History of Life hhu.de  Origin of Life / History of Life Chicken and Egg Paradox What appears first: amino acids (enzymes) or nucleic acids (genetic material) ? The discovery of catalytic activity of RNA solved the "chicken and egg" paradox of the origin of life, solving the problem of origin of peptide and nucleic acid central dogma. According to this scenario, in earliest life all enzymatic activity and genetic information encoding was done by one molecule, the RNA. 15 WS 2024/25 – Origin of Life / History of Life hhu.de  Origin of Life / History of Life Self-Replicating RNA The first genetic material was most likely RNA, not DNA. RNA plays a central role in protein synthesis, but it can also function as an enzyme-like catalyst (catalyse reactions). Such RNA catalysts are called ribozymes. RNA controls protein synthesis and also has a catalytic activity. Some ribozymes can make complementary copies of short pieces of RNA, provided that they are supplied with nucleotide building blocks. Natural selection on the molecular level has produced ribozymes capable of self-replication in the laboratory. How does this occur ? 16 WS 2024/25 – Origin of Life / History of Life hhu.de  Origin of Life / History of Life Self-Replicating RNA Unlike double-stranded DNA, which takes the form of a uniform helix, single-stranded RNA molecules assume a variety of specific three-dimensional shapes mandated by their nucleotide sequences. In a given environment, RNA molecules with certain nucleotide leadzyme hammerhead twister sequences may have shapes that enable them to replicate faster and with fewer errors than other sequences. Occasionally, a copying error will result in a molecule with a shape that is even more adept at self-replication By Lucasharr - Own work, CC BY-SA 4.0, htt ps:/ /com mons.wikimedia.org/w/index. php?curid=37125886 17 WS 2024/25 – Origin of Life / History of Life hhu.de  Origin of Life / History of Life Dr. Jack Szostak and colleagues at Harvard University succeeded in building a vesicle in which copying of a template strand of RNA could occur -> a key step toward constructing a vesicle with self-replicating RNA. On early Earth, a vesicle with such self-replicating, catalytic RNA would differ from its many neighbours that lacked such molecules. If that vesicle could grow, split, and pass its RNA molecules to its “daughters,” the daughters would be protocells. Once RNA sequences that carried genetic information appeared in protocells, many additional changes would have been possible. For example, RNA could have provided the template on which DNA nucleotides were assembled. Double stranded DNA is a more chemically stable repository for genetic information than is the more fragile RNA. DNA also can be replicated more accurately. Accurate replication was advantageous as genomes grew larger through gene duplication and other processes and as more properties of the protocells became coded in genetic information. Once DNA appeared, the stage was set for a blossoming of new forms of life - a change we see documented in the fossil record. 18 WS 2024/25 – Origin of Life / History of Life hhu.de  Origin of Life / History of Life Summary Conditions and Origin of Life Our planet formed 4.6 billion years ago, condensing from a vast cloud of dust and rocks that surrounded the young sun. Direct evidence of life on early Earth comes from fossils of microorganisms that lived 3.5 billion years ago. Haldane suggested that the early oceans were a solution of organic molecules, a “primitive soup” from which life arose, Stanley Miller confirm it in laboratory experiments. Other hypotheses suggested: Perhaps the first organic compounds formed near volcanoes. Organic compounds were first produced in deep-sea hydrothermal vents. Another source of organic molecules may have been meteorites Protocells: abiotically produced vesicles can “reproduce” on their own, and they can increase in size (“grow”) without dilution of their contents. Catalytic activity of RNA solved the "chicken and egg" paradox of the origin of life 19 WS 2024/25 – Origin of Life / History of Life hhu.de  Origin of Life / History of Life How do we know about past events? What is the library with the history of life? How do we decode the signals from the library? 20 WS 2024/25 – Origin of Life / History of Life hhu.de  Origin of Life / History of Life The fossil record documents the history of life Fossil record opens a window into the world of long ago and provides glimpses of the evolution of life over billions of years. Sedimentary rocks are the richest source of fossils. As a result, the fossil record is based primarily on the sequence in which fossils have accumulated in sedimentary rock layers, called strata. The fossil record shows that there have been great changes in the kinds of organisms on Earth at different points in time. Many past organisms were unlike organisms living today, and many organisms that once were common are now extinct. 21 WS 2024/25 – Origin of Life / History of Life hhu.de  Origin of Life / History of Life Conclusion accuracy from fossil records As substantial and significant as the fossil record is, keep in mind that it is an incomplete chronicle of evolution. Many of Earth’s organisms did not die in the right place and time to be preserved as fossils. Of the fossils that were formed, many were destroyed by later geologic processes, and only a fraction of the others have been discovered. As a result, the known fossil record is biased in favour of species that existed for a long time, were abundant and widespread in certain kinds of environments, and had hard shells, skeletons, or other parts that facilitated their fossilization. Even with its limitations, however, the fossil record is a remarkably detailed account of biological change over the vast scale of geologic time. 22 WS 2024/25 – Origin of Life / History of Life hhu.de  Origin of Life / History of Life How Rocks and Fossils Are Dated – „Decoding Process“ Fossils provide valuable data for reconstructing the history of life, but only if we can determine where they fit in that story. While the order of fossils in rock strata tells us the sequence in which the fossils were laid down -their relative ages- it does not tell us their actual ages. How can we determine the age of a fossil? 23 WS 2024/25 – Origin of Life / History of Life hhu.de  Origin of Life / History of Life Radiometric Dating One of the most common techniques is radiometric dating, which is based on the decay of radioactive isotopes. In this process, a radioactive “parent” isotope decays to a “daughter” isotope at a characteristic rate. The rate of decay is expressed by the half-life, the time required for 50% of the parent isotope to decay. Each type of radioactive isotope has a characteristic half-life, which is not affected by temperature, pressure, or other environmental variables. Examples: Carbon-14 decays relatively quickly; its half-life is 5,730 years. Uranium-238 decays slowly; its half-life is 4.5 billion years. Fossils contain isotopes of elements that accumulated in the organisms when they were alive. For example, a living organism contains the most common carbon isotope, carbon-12, as well as a radioactive isotope, carbon-14. When the organism dies, it stops accumulating carbon, and the amount of carbon-12 in its tissues does not change over time. 24 WS 2024/25 – Origin of Life / History of Life hhu.de  Origin of Life / History of Life The study of fossils has helped geologists establish a geologic record: a standard time scale that divides Earth’s history into four eons and further subdivisions. 25 WS 2024/25 – Origin of Life / History of Life hhu.de  Origin of Life / History of Life The Origin of New Groups of Organisms Some fossils document the origin of new groups of organisms. Such fossils are central to our understanding of evolution; they illustrate how new features arise and how long it takes for such changes to occur. For example the origin of mammals: Along with amphibians and reptiles, mammals belong to the group of animals called tetrapods (from the Greek tetra, four, and pod, foot), named for having four limbs. Mammals have a number of unique anatomical features that fossilize readily, allowing scientists to trace their origin. 26 WS 2024/25 – Origin of Life / History of Life hhu.de  Origin of Life / History of Life Over the course of 120 million years, mammals originated gradually from a group of tetrapods called synapsids. Shown here are a few of the many fossil organisms whose morphological features represent intermediate steps between living mammals and their early synapsid ancestors. The evolutionary context of the origin of mammals. the dagger symbol † indicates extinct lineages). 27 WS 2024/25 – Origin of Life / History of Life hhu.de  Origin of Life / History of Life Synapsids Therapsids Cynodonts 28 WS 2024/25 – Origin of Life / History of Life hhu.de  Origin of Life / History of Life Summary Fossil Records Fossil record provides glimpses of the evolution of life over billions of years. Sedimentary rocks are the richest source of fossils. Even with its limitations, however, the fossil record is a remarkably detailed account of biological change over the vast scale of geologic time. Fossil age determination by radiometric dating, which is based on the decay of radioactive isotopes. The study of fossils has helped geologists establish a geologic record: a standard time scale that divides Earth’s history into four eons and further subdivisions. 29 WS 2024/25 – Origin of Life / History of Life hhu.de  Origin of Life / History of Life Key events in life’s history: The origins of unicellular and multicellular organisms and the colonization of land 30 WS 2024/25 – Origin of Life / History of Life hhu.de  Origin of Life / History of Life Overview of the history of life over geologic time - main events 31 WS 2024/25 – Origin of Life / History of Life hhu.de  Origin of Life / History of Life The First Single-Celled Organisms Earth’s first organisms were single-celled prokaryotes that lived in the ocean. The earliest direct evidence of these organisms, dating from 3.5 billion years ago, comes from fossilized stromatolites. Stromatolites are layered rocks that form when certain prokaryotes bind thin films of sediment together. Stromatolites and other early prokaryotes were Earth’s sole inhabitants for about 1.5 billion years. As we know today, these prokaryotes transformed life on our planet. 32 WS 2024/25 – Origin of Life / History of Life hhu.de  Origin of Life / History of Life Stromatolites By P aul Harrison - Photograph taken by Paul Harrison (Reading, UK) using a Sony CyberShot DSC-H1 digital camera. , CC BY-SA 3.0, By Didier Descouens - Own work, CC BY-SA 4.0, htt ps:/ /com mons.wikimedia.org /w/index. php?curid=714512 By Benoi t Potin - Own work, CC BY-SA 4. 0, htt ps:/ /com mons.wikimedia.org/w/index. php?curid=15944367 https://comm ons.wikim edia.org/w/index.php?curi d=94327711 Modern stromatolites in Shark Bay, Paleoproterozoic oncoids from the Franceville Basin, Fossilized stromatolite in Strelley Pool chert, Western Australia Gabon, Central Africa. Oncoids are unfixed about 3.4 billion years old, from Pilbara Craton, Western Australia stromatolites ranging in size from a few millimeters to a few centimeters By V incent P oiri er - Own work, CC BY-SA 3.0, By Ruth E llis on - htt ps:/ /www.f lickr.com/photos/laruth/153584043/, CC BY 2. 0, htt ps:/ /com mons.wikimedia.org/w/index. php?curid=23972828 htt ps :/ /com mons.wikimedia.org/w/index. php?curid=1073339 By Donnie Reid, NASA Pa vilion Lake r esearch project - htt p://www.pavilionlake.com/micr obialites.php, Public Stromatolites at Highborne Cay, in Doma in , htt ps://com mons.wikimed ia.org/w/index. php?cur id=2 261455 5 Stromatolites at Lake the Exumas, The Bahamas Microbialite towers Thetis, Western Australia at Pavilion Lake, British Columbia 33 WS 2024/25 – Origin of Life / History of Life hhu.de  Origin of Life / History of Life Photosynthesis and the Oxygen Revolution Most atmospheric oxygen gas (O2) is of biological origin, produced during the water-splitting step of photosynthesis. When oxygenic photosynthesis first evolved -in photosynthetic prokaryotes similar to today’s cyanobacteria- the free O2 it produced probably dissolved in the surrounding water until it reached a high enough concentration to react with elements dissolved in water, including iron. This would have caused the iron to precipitate as iron oxide, which accumulated as sediments. These sediments were compressed into banded iron formations, red layers of rock containing iron oxide that are a source of iron ore today. Once all of the dissolved iron had precipitated, additional O2 dissolved in the water until the seas and lakes became saturated with O2. After this occurred, the O2 finally began to “gas out” of the water and enter the atmosphere - a process that began about 2.7 billion years ago. 34 WS 2024/25 – Origin of Life / History of Life hhu.de  Origin of Life / History of Life Photosynthesis and the Oxygen Revolution Red beds, like the Supai group Cyanobacteria today exposed on the sheer cliffs of Isis Temple in the Grand Canyon, are ancient soils that have a bright red color due to the abundant oxidized iron they contain. Redbeds only began to form 2 billion years ago, when the atmosphere contained enough oxygen gas to react with the iron in the sediments. When the dissolved iron encountered the oxygen produced by the photosynthesizing bacteria, the iron would have precipitated out of seawater in the form of minerals that make up the iron-rich layers of BIFs: hematite (Fe2O3) and magnetite (Fe3O4), Banded iron formations, or BIFs, consist of layers of iron-rich according to the following reactions: sediments “Cyano”bacteria fossils https://www.visionlearning.com/en/library/Earth-Science/6/The-History-of-Earths-Atmosphere-II/203/reading 35 WS 2024/25 – Origin of Life / History of Life hhu.de  Origin of Life / History of Life The rise of atmospheric oxygen. Chemical analyses of ancient rocks have enabled this reconstruction of atmospheric oxygen levels during Earth’s history. ❖ The amount of atmospheric O2 increased gradually from about 2.7 to 2.4 billion years ago, but then shot up relatively rapidly to between 1% and 10% of its present level. ❖ This “oxygen revolution” had an enormous impact on life. In some of its chemical forms, oxygen attacks chemical bonds and can inhibit enzymes and damage cells. ❖ Diverse adaptations to the changing atmosphere evolved, including cellular respiration, which uses O2 in the process of harvesting the energy stored in organic molecules. 36 WS 2024/25 – Origin of Life / History of Life hhu.de  Origin of Life / History of Life The rise in atmospheric O2 levels left a huge imprint on the history of life. A few hundred million years later, another fundamental change occurred: The origin of the eukaryotic cell. 37 WS 2024/25 – Origin of Life / History of Life hhu.de  Origin of Life / History of Life The First Eukaryotes The oldest widely accepted fossils of eukaryotes are of single-celled organisms that lived 1.8 billion years ago. How did the eukaryotes evolve from their prokaryotic ancestors? Current evidence indicates that the eukaryotes originated by endosymbiosis when a prokaryotic cell engulfed a small cell that would evolve into an organelle found in all eukaryotes, the mitochondrion. The small, engulfed cell is an example of an endosymbiont, a cell that lives within another cell, called the host cell. 38 WS 2024/25 – Origin of Life / History of Life hhu.de  Origin of Life / History of Life A hypothesis for the origin of mitochondria and plastids through serial endosymbiosis. Evidence that supports the endosymbiotic origin of mitochondria and plastids: The inner membranes of both organelles have enzymes and transport systems that are homologous to those found in the membranes of living bacteria. Mitochondria and plastids replicate by a splitting process that is similar to that of certain bacteria. Mitochondria and plastids also have the cellular machinery (including ribosomes) needed to transcribe and translate their DNA into proteins. Finally, in terms of size, RNA sequences, and sensitivity to certain antibiotics, the ribosomes of mitochondria and plastids are more similar to bacterial ribosomes than they are to the cytoplasmic ribosomes of eukaryotic cells. 39 WS 2024/25 – Origin of Life / History of Life hhu.de  Origin of Life / History of Life The Origin of Multicellularity 40 WS 2024/25 – Origin of Life / History of Life hhu.de  Origin of Life / History of Life The Origin of Multicellularity After the first eukaryotes appeared, a great range of unicellular forms evolved, giving rise to the diversity of single-celled eukaryotes that continue to flourish today. Another wave of diversification also occurred: Some single celled eukaryotes gave rise to multicellular forms, whose descendants include a variety of algae, plants, fungi, and animals. Fossil evidence and DNA sequence data suggest that multicellular eukaryotes emerged about 1.3 billion years ago. The oldest known fossils of multicellular eukaryotes that can be resolved taxonomically are of relatively small red algae that lived 1.2 billion years ago. Larger and more diverse multicellular eukaryotes do not appear in the fossil record until about 600 million years ago. 41 WS 2024/25 – Origin of Life / History of Life hhu.de  Origin of Life / History of Life The rise of large eukaryotes in the Ediacaran period represents an enormous change in the history of life. Before that time, Earth was a microbial world: Its only inhabitants were single-celled prokaryotes and eukaryotes, along with an assortment of microscopic, multicellular eukaryotes. As the diversification of the Ediacaran biota came to a close about 541 million years ago, the stage was set for another, even more spectacular burst of evolutionary change: The “Cambrian explosion.” 42 WS 2024/25 – Origin of Life / History of Life hhu.de  Origin of Life / History of Life Appearance of selected animal groups. The white bars indicate the earliest fossil record of these animal groups. The Cambrian Explosion Many present-day animal phyla appear suddenly in fossils formed 535–525 million years ago, early in the Cambrian period. This phenomenon is referred to as the Cambrian explosion. 43 WS 2024/25 – Origin of Life / History of Life hhu.de  Origin of Life / History of Life Prior to the Cambrian explosion, all large animals were soft bodied. The fossils of large pre-Cambrian animals reveal little evidence of predation. Instead, these animals appear to have been grazers (feeding on algae), filter feeders, or scavengers, not hunters. The Cambrian explosion changed all of that. In a relatively short period of time (10 million years), predators over 1 m in length emerged that had claws and other features for capturing prey; simultaneously, new defensive adaptations, such as sharp spines and heavy body armor, appeared in their prey. Although the Cambrian explosion had an enormous impact on life on Earth, it appears that many animal phyla originated long before that time. Overall, fossils and DNA analyses suggest that animals originated about 700 million years ago and then remained small for over 100 million years - until they diversified explosively during the Cambrian and beyond. 44 WS 2024/25 – Origin of Life / History of Life hhu.de  Origin of Life / History of Life The Colonization of Land The colonization of land was another milestone in the history of life. There is fossil evidence that some prokaryotes lived on terrestrial surfaces as early as 3.2 billion years ago. However, larger forms of life, such as fungi, plants, and animals, did not begin to colonize land until about 500 million years ago. This gradual evolutionary venture out of aquatic environments was associated with adaptations that made it possible to reproduce on land and that helped prevent dehydration. Plants appear to have colonized land in the company of fungi. Even today, the roots of most plants are associated with fungi that aid in the absorption of water and minerals from the soil These root fungi (or mycorrhizae), in turn, obtain their organic nutrients from the plants. Such mutually beneficial associations of plants and fungi are evident in some of the oldest fossilized plants, dating this relationship back to the early spread of life onto land. 45 WS 2024/25 – Origin of Life / History of Life hhu.de  Origin of Life / History of Life Tetrapods include humans, although we are late arrivals on the scene. The human lineage diverged from other primates around 6-7 million years ago, and our species originated only about 195,000 years ago. If the clock of Earth’s history were rescaled to represent an hour, humans appeared less than 0.2 second ago. 46 WS 2024/25 – Origin of Life / History of Life hhu.de  Origin of Life / History of Life Summary Key Events in Life’s History Six main key events in life’s history: appearance of prokaryotes, atmospheric oxygen, single-cell eukaryotes, multi-cellular eukaryotes, animals, land colonization Earth’s first organisms were single-celled prokaryotes that lived in the ocean ~ 3.5 billion years ago. Stromatolites are layered rocks that form when certain prokaryotes bind thin films of sediment together. Most atmospheric oxygen gas (O2) is of biological origin, produced during the water-splitting step of photosynthesis. O2 finally began to “gas out” about 2.7 billion years ago. The oldest widely accepted fossils of eukaryotes are of single-celled organisms that lived 1.8 billion years ago. Current evidence indicates that the eukaryotes originated by endosymbiosis. Some single celled eukaryotes gave rise to multicellular forms, whose descendants include a variety of algae, plants, fungi, and animals. Many present-day animal phyla appear suddenly in fossils formed 535–525 million years ago, early in the Cambrian period. This phenomenon is referred to as the Cambrian explosion. Larger forms of life, such as fungi, plants, and animals, began to colonize land about 500 million years ago. 47 WS 2024/25 – Origin of Life / History of Life hhu.de