History of Life on Earth

• Through the years, Earth has become witness to countless life-forms that have lived in its every conceivable corner
• Life emerged about 3.8 billion years ago with prokaryotic organisms, while multicellular organisms evolved only in the last 540 million years
• Forest (385 million years ago) and land plants (475 million years ago) have followed, populating the planet
• Mammals have only evolved until 200 million years ago, whereas, Homo Sapiens first appearance 200,00 years ago, which makes us only 0.004 percent old in Earth's life history
• Eons


  • Hadean
  • Archean
  • Proterozoic
  • Phanerozoic
• Hadean


  The oldest eon, officially recognized only in 2012, named after the word Hades (God of the underworld in Greek mythology), referring to the hellish conditions and violent cosmic collisions that characterized Earth four billion years ago
• Archean


  Still not much clear among geologists due to the few fossil or mineral evidence that could support it
• Proterozoic


  Characterized by the presence of oxygen in the atmosphere due to the emergence of cyanobacteria, allowing other organisms, such as eukaryotes and other multicellular organisms to live
• Phanerozoic eon


  Spans within the past 541 million years up to the present, linked with the Cambrian explosion, a rapid evolutionary event when complex organisms believed to have first evolved on Earth appeared
• Precambrian


  Covering about 88% of earth's life history, the earliest forms of organisms that first emerged in this Precambrian Earth were probably similar to present-day bacteria
• Eras

  • Paleozoic
  • Mesozoic
  • Cenozoic

  
• Paleozoic


  Lasted 300 million years, many of the organisms that have emerged during this period were invertebrates or animals without backbones
• Mesozoic


  Lasted 180 million years and is known for being the age of the dinosaurs, although mammals have already evolved in this particular era

  
• Cenozoic


  Began 65 million years ago until present, also known as the Age of Mammals because they are most common and dominant species of this era
• Mesozoic era is divided into three periods: Triassic, Jurassic, and Cretaceous
• Periods have been further subdivided by geologists into epochs, because the Cenozoic era is almost complete in terms of the fossil records, it would be easier for geologists to further examine it using epochs
• Bacteria and insects can evolve quickly due to their ability to reproduce quickly, generate mutations at a rapid rate, and transfer adaptive traits among the different members of their population
• Greeks believed that the stripes of zebras were created at the beginning of time and remained exactly the same ever since life on earth has begun
• Aristotle recognized that organisms on earth are related to one another in a hierarchy from simple to complex forms, this thought will dominate the scientific community for nearly 2000 years
• In the 1600s, with the idea of creationism predominating the common thinking at that time, other scientific exploration began to unfold new facts that contradicted the present-day thinking that all organisms were products of a single creation with unchanging species

  
• George-Louis Leclerc, Comte de Buffon (1707-1788) suggested that closely related species may have arisen from a common ancestor, and the change may have evolved due to changes in the environment or even by chance
• Erasmus Darwin (Grandfather of Charles Darwin) believed that evolution could occur in living organisms as well as in humans, but it was unclear to him how evolution might occur in organisms

  
• Jean Baptiste Lamarck proposed the idea that species, including humans, descended from other species, and attributed the change to natural laws, not to miracles, and proposed a mechanism of how it can all happen

  
• Paleontologists discovered fossils in the late 1700s to the early 1800s, which convinced many scientists that plants and animals had changed or evolved from the simple to complex organisms
• Geologists made concrete discoveries and analyzed Earth's erosion, volcanic eruption, and mountain building, proving that Earth could be millions of years older and that it must be undergoing a major but slow continuous change over time
• Charles Darwin and Alfred Russel Wallace both devised a theory by natural selection to support what they saw from their voyages, with Darwin attracting more attention when he published his monumental and well-documented manuscript, titled On the Origin of Species by Means of Natural Selection

  
• Darwin and Wallace's observation from their stay in the remote islands of South America and Galapagos provided them evidence that evolution had occurred

  
• Individual members within a population of species are varied and some of these variations could be inherited by future generations
• Members of the population have the capacity to reproduce more offspring than the environment could provide, such as food, shelter, or space, putting pressure within members and making them compete with each other for these limited resources
• Some individuals, whose hereditary characteristics favored them to cope better more efficiently in their local environment, are more likely to survive and produce offspring than individuals who do not possess those traits
• Due to natural selection, certain inherited traits flourished, becoming more common in a population across many generations
• Inherited traits or adaptations can be physical, physiological, or behavioral characteristics that allowed organisms to be better fitted in their current environment
• Over time, natural selection paved the way for favorable traits to be modified or maintained to improve and increase the organism's chance of survival and reproduction
• Biologists accept Darwin's principle of natural selection as the mechanism that explains how evolution occurs in nature, but natural selection is not responsible for variations among species, it simply facilitates the individuals with better traits to withstand current environmental challenges while producing offspring in the next generation
• Paleontologists provided concrete evidence of what extinct organisms looked like in their previous time in history, and discovered transitional forms of organisms in between rock layers

  
• During Darwin's visit to Galapagos islands, he noticed the uniqueness of animals and plants that lived in the isolated islands compared to the Cape Verde Islands off the coasts of Africa, and hypothesized that Galapagos Islands' residents harbored organisms that probably have swam, flown, or accidentally rafted to the nearest continental shore to the Galapagos volcanic islands

  
• Alfred Russel Wallace noticed a sudden distinct change in the fauna distribution in the Malay Archipelago, and drew the Wallace Line which separates animals from Asia and Australia, allowing the proliferation of unique species on each side of the line