The History of Life Chapter 17

Fossils and Ancient Life The fossil record provides evidence about the history of life on earth. It also shows how organisms have changed over time.

Fossil Formation Fossils range in size from tiny fragments of bone or plant to entirely preserved animals. Fossils usually occur in sedimentary rock (made up of small layered particles). As layers of sediment build up around the dead organisms, pressure and chemical activity eventually turn the organisms into rock. Dead organisms are buried by layers of sediment, which forms new rock. The preserved remains may later be discovered and studied. Water carries small rock particles to lakes and seas.

Interpreting the Fossil Record Scientists look for similarities between living organisms and fossils. Scientists try to date the fossilized organisms by using two different dating techniques Relative Dating Radiocarbon Dating

Relative Dating The age of the fossil is determined by comparing its placement with that of fossils in other layers of rock (strata) Exact age cannot be determined; you can only say that something appears earlier or later than something else.

Radiocarbon Dating Scientists determine the age of a sample based on the amount of remaining radioactive isotopes it contains. Calculations are based on half-life – the time it takes for ½ of the radioactive sample to decay. Different elements have different half-lives The half life for C-14 to decay into N-14 is 5,730 years 100% C-14  0% N-14 = 0 years 50% C-14  50% N-14 = 5,730 years 25% C-14  75% N-14 = 11,460 years In 60,000 years, the sample is all N-14 K-40  Ar-40 has a half life of 1.26 billion years

Geologic Time Scale Represents evolutionary time Based on rock layers and fossils Divided into eras and periods to make the time more manageable, but the divisions are not equal in time.

The Formation of Earth Earth was formed 4.6 billion years ago. It is speculated that early Earth’s atmosphere lacked oxygen and was rich in nitrogen and hydrogen- containing gases (NH3, H20, H2, CH4). A few deep breaths of early Earth’s atmosphere would have killed you.

The First Organic Molecules In the 1950’s Stanley Miller and Harold Urey tried to simulate early Earth’s conditions in an apparatus they designed and built Water = Ocean = Atmosphere Sparks = Lightening Condensation = rain Samples could be removed at the base. A variety of organic molecules were formed – 6 of the 20 amino acids. Similar experiments by others have produced a variety of biologically important molecules such as RNA, DNA, lipids, Carbohydrates, and ATP.

How Did Life Begin? RNA is suspected as being the first genetic molecule. Small chains of RNA will form and RNA molecules can act as a catalyst in forming other molecules This theory is often referred to as primordial soup Origin of the first cells Lipids are thought to have formed the first cell membrane. DNA did not evolve until after simple cells were abundant.

How Did Life Begin? First Life in the Sea The first fossil has been dated to 3.5 billion years old (Bacteria) As a result of bacteria populating the Earth the atmosphere began to change. Due to photosynthesis, oxygen was released into the air- this caused the evolution of oxygen-breathing organisms, and the extinction of many organisms who could not survive in an oxygen-filled environment. After billions of years the atmosphere consisted of 21% oxygen, very similar to today’s atmosphere. Eukaryotic cells evolved about 1.5 billion years ago. The endosymbiotic theory proposes that eukaryotic cells arose from living communities formed by prokaryotic organisms.

Evolution of Cells

Patterns of Evolution These patterns of evolution are all driven by natural selection Extinction Adaptive Radiation Convergent Evolution Coevolution Punctuated Equilibrium

Extinction Many now extinct organisms lived during a period known as the Cambrian period, 500 – 550 million years ago. There have been at least five periods in the history of Earth where large percentages of existing species die. These events are called mass extinctions – the causes are questionable.

Adaptive Radiation This is when a number of different forms evolve from a single species. Example: Darwin’s finches

Convergent Evolution This is when two unrelated organisms independently evolve similarities when adapting to a new environment; these organisms tend to look very similar to each other Examples: sharks, dolphins, penguins and seals all have body shapes (streamlined) and fins and flippers that are very similar in form and function.

Coevolution This is when two species evolve in response to changes in each other. Example: Central American swollen-thorn acacias and symbiotic ants Acacia provides a home, and protects acacia from herbivores and prunes away competitive plants.

Gradualism vs. Punctuated Equilibrium Darwin believed that evolution was a gradual, or slow and steady, process with increasing change. In punctuated equilibrium, species remain stable for long periods of time and evolution occurs suddenly, with new species branching from existing ones. Today most scientists use the term punctuated equilibrium to describe the process of evolution The evolution of a new species is usually driven by environment or ecological separation, mutation, or a combination