| Evolutionary Timeline: This timeline is broken into the three following parts: Part A) The formation of the earth and the changes in the earth’s physical characteristics that made it hospitable to life. Part B) The development of cellular life. Part C) The development of multicellular life. Here we consider the development of our ancestors: worms, fish, amphibians, reptiles, mammals and apes. We will also consider the development of some of our non- ancestral cousins including: plants, insects, dinosaurs and birds. Part A) This section recounts the noteworthy occurrences that took place during the formation of the earth. More specifically it outlines the changes in the earth’s physical characteristics that made it hospitable to life. 4,600,000,000 years ago: The Solar System Forms Out of the Remnants of a Supernova Our earth, our sun and the other 9 planets in our solar system are material remnants of a tremendous interstellar explosion – a type II supernova. Over 4.6 billion years ago this huge explosion created a shock wave that helped to compress some of its own left over matter into tiny clumps. Eventually these clumps of matter and gas coalesced to form the sun, the planets, the moons, the asteroids and the comets of today’s solar system. The earth formed into a homogeneous (uniformly mixed) solid by the process of planetessimal accretion (it used its gravity to attract and retain loose debris floating around in the solar system). For a long time the earth was a red hot ball of matter. The surface eventually cooled, but as we know, even today the center is still extremely hot and the molten, partially liquid core affects the earth’s surface in many ways (the effects are evident in volcanoes, hydrothermal vents, and plate tectonics). The heat from the center of the earth (which is fueled by radioactive decay as well as residual heat from the formation of the earth) is one of the two sources of energy (the other being the light from the sun) that allowed life on earth to form and florish. Life necessitates energy and these two forms of energy, chemical energy and light energy, were always available on earth. Despite this other physical characteristics of the earth had to change in order to allow for the formation of life. 4,300,000,000 years ago: As the Earth Cools Outgassing Allows for an Atmosphere and Oceans Melting of the earth’s core due to radioactive and gravitational heating leads to its differentiated interior composition. The heating also leads to the process of outgassing. This refers to the process where solids under the earth’s crust would heat up and boil into gasses. These gasses then escaped out of the earth’s crust to form the early atmosphere. This atmosphere was composed of certain atoms and simple molecules such as water vapor, methane, ammonia, hydrogen, nitrogen, and carbon dioxide. These molecules are relatively heavy (heavier than hydrogen and helium which, by the way float away into space because the earth’s gravity is too weak to retain them) and so they are kept close to the surface of the earth (again in the form of an atmosphere), because of the earth’s gravity. Before the earth went through its outgassing period it had no atmosphere, and thus could not contain life. Life most probably first formed underwater, but without an atmosphere it is impossible for water to condense on the surface of the earth. This is because a liquid at room temperature will boil away at a vaccum. The pressure created by a true atmosphere is the only way for bodies of water to remain on the surface of the earth for an extended time. If our atmosphere today was suddenly removed, our oceans would boil off very quickly. The creation of the ozone layer was important for living organisms as well. Water vapor molecules that made up part of the early atmosphere underwent photodissociation after being bombarded with ultraviolet light from the sun. The process of photodissociation causes the H20 molecules to separate into oxygen atoms and hydrogen molecules. The oxygen atoms, because of their inherent lightness, formed the ozone layer which later on protected early life forms from the high energy ultra violet light from the sun. This is the same ozone layer that protects us from ultraviolet rays today. 4,000,000,000 years ago: The Earth is a Less Dangerous Place The bombardment of the Earth by planetessimals (small planets careening around our early solar system that added their mass to the earth’s mass during the earth’s early stages) ceases. A more peaceful earth soon becomes home to some of the simplest precursors to life, complex organic molecules. For an understanding of how such molecules got a start deep within the earth’s oceans please see the The Stanley Miller Experiment. At this time the atmosphere does not contain any free oxygen. Oxygen is a reactive, destructive molecule and if it had been present it might have impeded the formation of life. 3,800,000,000 years ago The Earth’s crust becomes solid. The oldest rocks found and carbon dated by man appear around this time. 3,800,000,000 years ago Water vapor found in the atmosphere, that originated from the outgassing of the earth condenses into water to form the earth’s early oceans. Frozen water from the icy comets that bombarded the earth, melt into liquid to supplement these oceans. The precursors to cells- self-replicating RNA molecules appear around this time. To read more about self-replicators click here. Part B) This section describes how cellular life developed from interactions between carbon, hydrogen and oxygen molecules. 3,500,000,000 - 2,800,000,000 years ago: We Have Cells, Thus We Have Life The first cells appear around this time and a major distinction is drawn at this point. According to convention nothing is referred to as living matter unless it is a cell or composed of cells. Many combinations of complex molecules that led up to the formation of cells might have seemed life like but were not alive until they formed the first cell. The complex macromolecules, the amino acid chains and the protein strands, even the self replicating RNA molecules that led up to the formation of these first cells are not considered living matter. Therefore this time period saw the formation and evolution of the first life forms on the planet. Our oldest fossils that are evidence for the existence of life date back to this time period. The processes that led up to the formation of cells must have started well before this period. A great deal of scientific research is still being conducted to understand the events that led up to the first cells. The early cells resembled prokaryotic cells (as opposed to the more advanced eukaryotic cells which we are composed of) and were very similar in appearance and functionality to what we know as bacteria. These prokaryotic cells are distinguished from eukaryotic cells (which you and I are made from), because they lack membrane bound nuclei and organelles (the functioning structures that float freely inside of the cell bodies). Also prokaryotic DNA is not organized into chromosomes and it is generally more simplistic in structure and function. These simple prokaryotic cells actually merged to form the first eukaryotic cells by a process called endosymbiosis. The First Cellular Organisms: These first organisms were chemoautotrophs, this means that they supply themselves with energy from chemicals in their environment. Unlike heterotrophs (like you and I, all animals and fungi and some types of bacteria) they did not need to feed off of other forms of life to stay alive. Also unlike photoautotrophs (like green plants and some bacteria) they did not need sunlight for energy either. These organisms probably developed on the beds of the ocean floor near underwater hotspots, also known as hydrothermal vents. Here the chemoautotrophs took energy from the chemicals in the hot waters and used them to create and maintain their bodies. They used carbon dioxide as a source of carbon (the most important element in living organisms) and they metabolized inorganic materials to supply themselves with energy. These early unicellular organisms and the multicellular organisms that evolved to be able to consume them were probably our very first cellular ancestors. Many forms of chemoautotrophic bacteria and the organisms that feed off of them (shrimp, worms and fish) still exist today in hydrothermal vents. The Atmosphere Gains Oxygen Very simplistic aquatic plants evolved from the earliest unicellular organisms. Blue-green algae began to acquire energy in a new and different way. It began to gather energy from sunlight by using a process called photosynthesis. This algae began to release oxygen molecules into the oceans and these molecules quickly found their way to the atmosphere above. The accumulation of oxygen in the atmosphere increased the ozone layer and changed the atmosphere from a chemically reducing atmosphere to a chemically oxidizing one. This new atmosphere featured a positive feedback mechanism which quickly increased the supply of oxygen in the atmosphere. The large amount of oxygen was toxic to some early organisms (anoxic organisms), but it also allowed other organisms that burned energy more efficiently (by oxidation) to come about. These new organisms used oxygen in their form of cellular respiration and it is obvious that they are our direct progenitors because our bodies are made up of cells that undergo the same type of cellular respiration. 1,500,000,000 years ago The first Eukaryotic organisms develop. These are single cell organisms that have membrane enclosed nuclei and organelles. Eukaryotic organism had more evolutionary potential and because of some of their fundamental properties allowed for a great variety of different creatures. Part C) As multicellular organisms evolved from their unicellular ancestors they moved out of the oceans, onto the land and even into the air 1,500,000,000 – 600,000,000 years ago The rise of multicellular organisms. Unicellular or single celled life is no longer the only type of life on earth. Most animals though could not survive without unicellular organisms and so even today there is still a type of symbiosis between multicellualr and unicellular organisms. 545,000,000 years ago The cambrian period featured an explosion of hard bodied organisms. These organisms were mostly marine invertebrates, such as trilobites. 500,000,000 - 450,000,000 years ago The rise of fish was seen by this time period. Fish are actually the first vertebrates and at this time they were the most complex, most intelligent animal type on the earth. At this time fish were much simpler than many modern day fish. For example, none of the fish at this time had yet evolved jaws. It took several million years for fish to slowly evolve the jaw that we see them with now. Interestingly, some fish, and other fish like animals (including lancelets, hagfish and lampreys) on the earth today do not have jaws. We ultimately inherited our own jaws, our eye structure and our vertebrae, not to mention countless other homological structures, from these fish. They were truly the evolutionary pioneers of our planet. 430,000,000 years ago Waxy coated algae begin to live on the dry land. This form of algae was definitely one of the first terrestrial forms of life on the earth. Plant life on the surface evolved from some of these first dry algae plants. 420,000,000 years ago Millipedes are the first land animals. Millipedes along with all other insects share common marine ancestors with humans (like sponges) but not terrestrial ancestors. Humans evolved from amphibians that moved on to the land. Insects on the other hand evolved from arthropods that moved onto the land. Arthropods are marine animals like crab, lobster and shrimp. The similarities between arthropods and insects are obvious, the antennae, the segmented body, the paired and jointed limbs and the exoskeletons are unmistakably similar. 375,000,000 years ago The Appalachians are formed due to a plate collision involving modern day North America, Africa and Europe 375,000,000 years ago Primitive sharks appear. Sharks along other marine animals with cartilage instead of bone (rays and skates) actually came before most fish. 350,000,000 – 300,000,000 years ago Fish begin to form legs from their fins and lungs from their gills. One fish in particular, a lobe finned fish in the Devonian period called the Eusthenopteron evolved the limb bones in its fins that were later necessary for the transition to land. Early amphibians such as Ichthyostega and Acanthostega were probably closely related to this lobe finned fish. The first of these amphibians moved onto the land very slowly and they represent the transition from aquatic life to terrestrial life. Later, reptiles which could no longer breathe under the water (and which are born terrestrially) evolved from these early amphibians. 350,000,000 years ago A large variety of primitive insects have evolved on the earth’s surface. 350,000,000 years ago Primitive ferns evolve and become the first plant to use roots. 300,000,000 – 200,000,000 years ago Rise of reptiles. Reptiles evolve from amphibians and develop into many shapes and sizes. 300,000,000 years ago Winged insects evolve. At this point winged insects represent the only form of life that can fly. Birds will develop wings much later in the evolutionary history. Bats will develop wings much later still, after the rise of mammals. 280,000,000 years ago Beetles and weevils evolve 250,000,000 years ago The Permian period mass extinction is the largest mass extinction ever recorded. It kills many types of animals including the majority of marine invertebrates. 250,000,000 – 210,000,000 years ago The dinosaurs appear in the late Triassic and dominate from the Triassic, through the Jurassic and die out in the Cretaceous. Archosaurs were a group of large reptiles that came to prominence in the Triassic. They included a wide range of carnivores and herbivores and were the ancestors to dinosaurs, birds, crocodiles, and pterosaurs. 230,000,000 years ago Roaches and termites evolve 225,000,000 years ago Modern ferns and modern bees have evolved 200,000,000 years ago Pangaea, the huge super-continent, begins to break up into the continents that we know today. First this super- continent breaks into Gondwanaland in the south and Laurasia in the north. Laurasia went on to break up into North America, Europe and Asia. Gondwanaland, on the other hand would break up into India, Australia, Africa, Antarctica and South America. 200,000,000 years ago Primitive crocodiles have evolved 200,000,000 years ago Very small very simplistic mammals evolve from reptiles. These mammals have traits that are vastly different from reptiles. Please keep in mind though that it took many thousand of years for these traits to evolve. It is thought that one of the first true intermediaries between reptiles and mammals was the cynodont, a small carnivorous reptile. Mammals developed live birth (viviparous birth), teat milk (lactation) and warm blood (endothermy). The dinosaurs were also warm blooded. It seems that being warm blooded allowed animals to be more active and energetic. 145,000,000 years ago A dinosaur- the Archaeopteryx- which may have been the first flying vertebrate appears. This animal was considered a reptile, more specifically a dinosaur, but its descendants are birds. It may or may not be obvious to you by their movements, habits and vocal calls that birds evolved directly from dinosaurs. 136,000,000 years ago Primitive kangaroos have evolved 120,000,000 years ago The earliest known placental mammal, the mouse-sized Eomaia appears. Before placental mammals came around though, two other groups of mammals were created as we diverged from reptiles- the marsupials and the monotremes. 100,000,000 years ago Primitive cranes have evolved 90,000,000 years ago Modern sharks have evolved 65,000,000 years ago The K-T boundary- the extinction of the Dinosaurs has taken place and mammals begin to proliferate. Mammals were kept very small before this time as the dinosaurs truly dominated the earth before the mass extinction that killed them out. There have actually been many mass extinctions (as observed in the fossil record) this one was not the biggest but it killed off almost all of the dinosaurs (except for birds). Finally, mammals had their chance to evolve from very small shrew like animals to some of the bigger mammals that we see today. One interesting thing to note is that many of the early mammals began to look slightly different very early on. Very early camels, horses, rhinos, sheep, pigs, dogs, and cats all looked somewhat similar at one point in time. They were able to grow larger and they became vastly different looking as they began to fill different ecological niches. They began to take up different spots on the food chain and began to move to vastly different climates, this is why we see so much variety in modern mammals. 60,000,000 years ago Rats, mice and squirrels have evolved 55,000,000 years ago Rabbits, hares and the first primates evolve 50,000,000 years ago Primitive monkeys evolve. The most primitive of monkeys (prosimians), which are still extant (the opposite of extinct) today, were very similar in appearance to the other early mammals. 28,000,000 years ago Koalas have evolved 20,000,000 years ago Parrots and pigeons have evolved 20,000,000 -12,000,000 years ago Chimpanzees, and hominids (our bipedal ancestors) evolve. 4,000,000 years ago Hominids (the family of primates in which homo sapiens is the only extant species) evolve from apes after bipedalism (the ability to walk on two feet) develops. This ability to walk on two feet was thought to be a response to some of the factors that jeopardized the lives of early hominids. Most apes, as opposed to humans, knuckle walk. They use all four “legs,” (their hands and feet) to move around in their environment. Most monkeys would not benefit from bipedalism because they find most of their safety and food in trees, and they are well adapted to tree living. Early hominids were forced to hunt for food on the plains, away from trees. The ability to walk on two feet allowed them to see from a higher vantage point, over tall grasses and gave them an advantage in hunting and avoiding predators. Bipedalism also frees the hands allowing hominids to carry heavy objects, tools or weapons long distances. Finally bipedalism is a much more energy efficient means of locomotion. Without bipedalism, early hominids could never have gathered enough food on the plains to support their energy consuming knuckle walking. 4,000,000 – 1,000,000 years ago Australopithecus appears 3,500,000 years ago The Australopithecus Lucy walks the Earth. 2,000,000 years ago Widespread use of stone tools 2,000,000 – 10,000 years ago The most recent ice age occurs 1,600,000 – 200,000 years ago Homo erectus exists 1,000,000 - 500,000 years ago Homo erectus uses fire as a tool 300,000 years ago Geminga supernova occurs and creates an explosion nearly as bright as the moon from 60 parsecs away. 200,000 – 30,000 years ago Homo sapiens neanderthalensis exist. 200,000-50,000 years ago Modern humans, Homo sapiens, have developed a brain that is extremely massive in comparison to their body weight. Their brain size and cognitive capacity reflects their reliance on social interaction and teamwork and also on complex visuo-spatial, conceptual and verbal problem solving. At this point nature had developed an animal that was able to modify and develop its own behavior. It was probably bipedalism, society and an opposable thumb that allowed human brains to enlarge to such a relatively huge size. 100,000 years ago It is estimated that modern (or very close to modern) humans migrated out of Africa at this time. Before this mass migration all Homo sapiens were located in Africa and the Middle East; in Asia there was Homo erectus and in Europe there was Homo Neanderthalensis. It is not known weather Homo sapiens killed the Homo erectus and Homo neanderthalensis after moving out of Africa, or if they interbred. Either way, homo sapiens were the only ones that made it to modern times. 50,000 – 0 years ago Homo sapiens sapiens appear. 40,000 – 12,000 years ago Homo sapiens sapiens enter Australia from southeastern Asia. They also enter North America from northeastern Asia by crossing the Bering Strait land bridge. 25,000 – 10,000 years ago The most recent glaciation creates an ice sheet that covers most of the northern United States. 20,000 years ago Homo sapiens sapiens paint the Altamira cave 12,000 years ago Homo sapiens sapiens have domesticated dogs in Kirkuk, Iraq 10,000 years ago The first Homo sapiens sapiens settlements pop up. 10,000 years ago Homo sapiens sapiens learn to cast copper and harden pottery 6,000 years ago Writing developed in Sumeria |
| Organization for the Advancement of Interdisciplinary Learning |