5 LIFE HOW ARE WE STILL EVOLVING?

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1 5 LIFE HOW ARE WE STILL EVOLVING?

2 UNIT 5 LIFE CONTENTS UNIT 5 BASICS 3 Unit 5 Overview 4 Unit 5 Learning Outcomes 5 Unit 5 Lessons 6 Unit 5 Key Concepts LOOKING BACK 8 What Happened in Unit 4? KEY CONTENT 10 A Big History of Everything 11 Threshold 5: Life on Earth 13 Life & Purpose 14 How Did Life Begin and Change? 15 Mini-Thresholds of Life 16 Life in All Its Forms 17 What Is the Biosphere? 18 How Do Earth and Life Interact? 19 How We Proved an Asteroid Wiped Out the Dinosaurs 20 Darwin, Evolution, and Faith 21 Watson, Crick, and Franklin 22 Codes LOOKING AHEAD 24 What s Next in Unit 6? 2

3 UNIT 5 OVERVIEW Key Discipline: Biology Timespan: The first life forms appeared about 3.8 billion years ago Driving Question: How are we still evolving? Threshold for this Unit: Threshold 5: Life 3

4 UNIT 5 LEARNING OUTCOMES By the end of Unit 5, students should be able to: 1. Describe the conditions that made it possible for life to emerge on Earth. 2. Explain the differences between life and nonlife. 3. Describe the major events in the development of life on Earth and explain what is meant by the term biosphere. 4. Use evidence to explain adaptation and evolution, including Darwin s theory of natural selection and DNA. 4

5 UNIT 5 LESSONS 5.0 What Is life? Among living things, there are strong similarities and remarkable differences all explained by the amount of their shared DNA. But what does that really mean? What is life? 5.1 How Did Life Begin and Change? The appearance of life marked a profound beginning. Simple life forms dominated most of the history of the Earth, but over time simple creatures were transformed, evolving into complex organisms. 5.2 How Do Earth and Life Interact? The biosphere is the home to millions of living things, but it is subject to change as a result of astronomical, geological, and biological forces. Changes in the biosphere can create challenges for the survival of living things. Mass extinction events, like the asteroid impact killed off the dinosaurs, are the most extreme examples of these challenges. 5.3 Ways of Knowing: Life The work of biologists has transformed the way humans view life in the last 150 years, first with the theory of evolution and then with the discovery of the structure of DNA. Continued next slide 5

6 UNIT 5 KEY CONCEPTS adaptation adaptive radiation bacteria biodiversity biology biosphere brain Cambrian explosion dinosaurs DNA eukaryotes evolution extinction event fossils fossil fuel gene homeostasis iridium K-T boundary last universal common ancestor life mammals marsupials metabolism multicelled (multicellular) organism natural selection niche organism photosynthesis prokaryotes proteins reproduction RNA species 6

7 LOOKING BACK

8 WHAT HAPPENED IN UNIT 4? Unit 4 focused on the formation of our Solar System and our planet, Earth. We learned: How Earth and the rest of our Solar System formed over a very long period of time. About the Earth s violent and unstable beginning. How plate tectonics keeps the Earth s surface in constant motion. How we learn about the Earth s changes over time through the science of geology. 8

9 KEY CONTENT

10 A BIG HISTORY OF EVERYTHING Video Life on Earth probably first appeared about 3.8 billion years ago, and consisted of single-celled bacteria. Many scientists now favor a deep-sea origin for life. DNA, which is the blueprint for every living thing, emerged early on. Photosynthesis was also an early development. Oxygen, a byproduct of photosynthesis, is the most important waste product in the history of the world. Some bacteria learned to live on oxygen; for others it was toxic and led to death. In the end, oxygen proved to be a more efficient way to produce energy, and allowed bigger, more complex life forms to appear. The Cambrian explosion, about 550 million years ago, was biology s Big Bang, and resulted in a huge proliferation of plants and animals. Amphibians moved from the sea to the land. They developed a new type of egg, one with a hard shell that kept the contents from drying out, which allowed the eggs to be laid on land. Dinosaurs evolved about 250 million years ago, and dominated the Earth for over 150 million years. Our ancestors, the early mammals, were very small no bigger than cats as long as the dinosaurs were around. The dinosaur extinction made way for bigger mammals like us. Primates are mammals that evolved after the extinction of the dinosaurs. They possessed forward-facing eyes and five-fingered, grasping hands. Our ancestors eventually left the trees and began walking on two legs, which allowed them to carry things (including babies). 10

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13 LIFE & PURPOSE Article / Ursula Goodenough Biologists have found it difficult to agree on a definition of life. They have found more common ground when it comes to naming the characteristics that all living things possess. The biologist Ursula Goodenough argues that self-generation, self-maintenance, purpose, and the ability to evolve are the features that distinguish living from nonliving things. Goodenough defines self-generation as the process of making a self. She defines selfmaintenance as the process of getting the energy and materials needed to survive. By purpose she means the ability of the genome to take actions that allow for self-maintenance and selfgeneration. Finally, she argues that living things have the ability to evolve. 13

14 HOW DID LIFE BEGIN AND CHANGE? Video Talk / David Christian Life is incredibly diverse: scientists estimate that 5 30 million species exist today, and believe this is just a small fraction of the living things that have existed during all of Earth s history. Three Goldilocks Conditions needed to be satisfied for life to emerge: there needed to be a variety and abundance of chemical elements, energy (not too much and not too little), and a water environment. Many scientists favor the theory that life originated close to the surface of the water. Others believe that life originated deep in the ocean near volcanic vents. Scientists continue to search for evidence that shows where life emerged. Life on Earth has evolved over a 3-billion year period. 14

15 MINI-THRESHOLDS OF LIFE Video Talk / David Christian Life as we know it results from a long and complex process. This story, like the larger story of Big History, can be told by focusing on critical turning points, or mini-thresholds, in that story. The appearance of photosynthesizing plants, representing a new way of generating energy, is the first mini-threshold. The appearance of eukaryotes, organisms more complex than prokaryotes because their DNA is enclosed in a nucleus and they have organelles, is the second. The appearance of multi-celled organisms, organisms whose cells perform specialized functions but share the same DNA, is the third. The appearance of brains allowed for the coordination of the activities of the many types of cells in a multi-celled organism, and represents the fourth mini-threshold. The move of animals from the water to the land created new pressures for living things to adapt and evolve. This represents the fifth mini-threshold. The appearance of mammals represents the sixth mini-threshold. The fact that they are warm blooded, have fur or hair, and have placental birth, makes them different from other land species. 15

16 LIFE IN ALL ITS FORMS Video Most of the history of life has been characterized by a lack of diversity. For most of the history of life, the only living things were simple, single-celled prokaryotes. The recent history of life has been very different. The biosphere is home to a huge number of species, and scientists have found it challenging to catalog all of the Earth s species. Determining the number of species is a hands-on activity and usually focuses on a limited area. Scientists cordon off a space, cover it with nets, and begin counting. They generally expect to find thousands of species in a few square yards of area. While the methods for counting species have not changed dramatically over time, our understanding about the number of species has. Carl Linnaeus tried to count and organize the number of living species on Earth, and he came up with the number 4,000. In 1900, scientists estimated that half a million species existed. Today, many scientists believe that number is closer to 8.7 million. 16

17 WHAT IS THE BIOSPHERE? Article The term biosphere was coined by the geologist Eduard Suess in a four-volume book published between 1885 and 1908, and was used to identify the portion of the Earth that supports life. In the Big History course, the biosphere is defined as the network of all life on Earth. Specific, smaller regions within the biosphere are referred to as ecosystems or habitats. 17

18 HOW DO EARTH AND LIFE INTERACT? Video Talk / David Christian The biosphere is the network of all living things on Earth. Changes in our climate can have a massive impact on living things, and scientists have identified five mass extinctions in the history of the Earth. Our biosphere is influenced by factors such as: astronomical events geological processes biological processes 18

19 HOW WE PROVED AN ASTEROID WIPED OUT THE DINOSAURS Video Talk / Walter Avarez Geologist Walter Alvarez gets to the bottom of a real scientific mystery by investigating the rock record. Evidence of a asteroid impact in the K-T boundary, a layer of rock 65 million years old, indicates that a mass extinction event happened, killing off the dinosaurs. Alvarez worked with different types of scientists to examine evidence and develop an hypothesis that is later accepted as fact. 19

20 DARWIN, EVOLUTION, AND FAITH Article / John Haught Since Darwin published his ideas about evolution in 1859, they have been controversial. Many people denounced his ideas as an assault on religion. Do evolution and religion have to be in conflict? Are they trying to answer the same questions? Is there common ground between the two perspectives? Many Christians, because they believe that the Bible is the literal word of God, have rejected Darwin s ideas because the evolutionary story he tells is so different from what is found in the Bible. For others who see the Bible as more of a guide for faith and morality, Darwin s work does not undermine the role of God or the importance of humans in the world. Professor John Haught describes interpretations of the relationship between evolution and religion. He calls them conflict, contrast, and convergence: First, for some the views are in total conflict; second, some say that the evolution and religion answer totally different questions, so they can t be in conflict; and third, some see truth in both views, so they believe the two must ultimately be reconcilable. 20

21 WATSON, CRICK, AND FRANKLIN Article / Cynthia Stokes Brown Darwin showed how natural selection allowed individuals to pass on traits that made them successful, but he was unable to explain the biological process that allowed these traits to be passed on. By the 1950s, it was clear to many scientists that DNA was key to this puzzle, but both its structure and the mechanism it used to enable the inheritance of traits were unclear. Watson and Crick identified the double-helix structure of DNA while working at Cambridge University. The x-ray crystallography images of DNA that had been taken by Rosalind Franklin of King s College provided an important foundation for their work. Watson and Crick proposed that the DNA copied itself by splitting the two strands, allowing each strand to acquire free floating DNA to reassemble complete strands, resulting in full copies of the originals. 21

22 CODES Video The instructions for building a living organism can be boiled down to a code. That code is called a DNA sequence. So many things around us, both living and nonliving, are code based: computer programs, chemical formulas, and DNA. Each provides detailed instructions for important tasks. Codes are as important to life as they are to technology. Computer viruses and biological viruses are both made of codes (binary code for computer viruses and DNA for biological viruses), and both can cause destruction. Codes have changed warfare because computer viruses can be used to attack nuclear weapons systems without having to put soldiers lives at risk. Computers are similar to living organisms in that both have programs that make them run. In the case of a cell, this is the DNA. 22

23 LOOKING AHEAD

24 WHAT S NEXT? In Unit 6, we will focus on early humans and the appearance of our species, Homo sapiens. We will learn: How the first humans lived. What physical adaptations have made us different from our primate cousins. What role human language plays in collective learning. How collective learning allows us to pass knowledge from one generation to the next. 24