14.2: The Origin of Life

Origins: Early Ideas Spontaneous Generation Life arises from nonlife. Example) People once believed that mice were created by placing damp hay and corn in a dark corner. How was Spontaneous Generation Disproved? In 1668, Francesco Redi tested the idea that flies arose spontaneously from rotting meat. He hypothesized that flies, not meat, produced other flies.

Redi’s Experiment Redi placed rotting meat in several flasks that were open and several flasks that were covered. Maggots formed on the meat in the uncovered flask and open to flies. The flasks that were covered had no maggots. Despite Redi’s successful experiment, people still believed in spontaneous generation. Microscopes were just beginning to be used at this time and people knew that organisms too small to be seen were everywhere. Some people thought these microbes must arise spontaneously even if flies don’t (Vital Force in the air).

Redi’s Experiment

Pasteur’s Experiments Disproved Vital Force Hypothesis. In the mid-1800s, Pasteur designed an experiment that would disproved spontaneous generation of microorganisms and prove the theory of biogenesis. The idea that living organisms come from living organisms.

Pasteur’s Experiment The steps of Pasteur's experiment are outlined below: First, Pasteur prepared a nutrient broth similar to the broth one would use in soup. Next, he placed equal amounts of the broth into two long-necked flasks. He left one flask with a straight neck. The other he bent to form an "S" shape. Then he boiled the broth in each flask to kill any living matter in the liquid. The sterile broths were then left to sit, at room temperature and exposed to the air, in their open-mouthed flasks.

Pateur’s Experiment Continued After several weeks, Pasteur observed that the broth in the straight-neck flask was discolored and cloudy, while the broth in the curved-neck flask had not changed. He concluded that germs in the air were able to fall unobstructed down the straight-necked flask and contaminate the broth. The other flask, however, trapped germs in its curved neck,­ preventing them from reaching the broth, which never changed color or became cloudy.

Pateur’s Experiment Continued If spontaneous generation had been a real phenomenon, Pasteur argued, the broth in the curved-neck flask would have eventually become re-infected because the germs would have spontaneously generated. But the curved-neck flask never became infected, indicating that the germs could only come from other germs.

Pasteur’s Experiment Continued http://phschool.com/atschool/phsciexp/active_art/redi_pasteur_experiment/index.html

Origins: Modern Ideas Most biologist agree that life originated through a series of chemical events. During these events, complex organic molecules were made from simpler ones. Eventually, simple metabolic pathways developed. These pathways enabled molecules to be broken down. These pathways might have led to the origin of life.

How did early organic molecules form? In the 1920s, Alexander Oparin suggested the primordial soup hypothesis. He thought that if early Earth’s atmosphere had a mix of certain gases, organic molecules could have been made from simple reactions involving those gases in the early oceans. UV light from the Sun and lightning might have provided the energy for the reactions. These organic molecules would eventually lead to life.

How was Oparin’s Hypothsis Tested? In 1953, Stanley Miller and Harold Urey conducted an experiment showing that simple organic molecules could be made from inorganic compounds. They built a glass apparatus to simulate early Earth conditions. They filled the apparatus with water and gases that they thought had made up the early atmosphere and boiled the mixture, shined UV light on it to simulate sun, and charged it with electricity to simulate lightning. The result: The mixture contained amino acids, the building blocks of protein.

Miller and Urey’s Experiment http://glencoe.mcgraw-hill.com/sites/0078802849/student_view0/unit4/chapter14/concepts_in_motion.html# Later, other scientists found that hydrogen cyanide could be formed from simpler molecules, which can react with itself to eventually form adenine. Many other experiments have been carried out under a variety of conditions and the final products have been amino acids, sugars, and nucleotides. These experiments showed that reactions for the origin of life were possible on early Earth.

Other Hypotheses Some scientists hypothesize that Earth’s surface, with its high UV levels and meteorite strikes, was too destructive to have provided a safe place for life. They suggest that the organic reactions occurred in hydrothermal volcanic vents of the deep sea, where sulfur is the base of a unique food chain. Others think meteorites might have brought the first organic molecules to Earth.

How were the first proteins made? Amino acids can bond to one another, but they can separate just as easily. Complex organic molecules might have originated in pools of water where small molecules had concentrated and been warmed. Proteins might have formed when an amino acid stuck to a particle of clay. Clay would have been a common sediment in early oceans. Clay could have provided a framework for protein assembly.

What’s RNA got to do with it? A genetic code is a requirement for life and many biologists think RNA was life’s first coding system. RNA systems are capable of evolution by natural selection. Some RNAs can behave like enzymes. These molecules could have carried out some early life processes. Some scientists think clay particles could have been a template for RNA replication and that the resulting molecules developed a replication mechanism.

How did the first cells arise? The formation of membranes. Scientists have tested ways to enclose molecules in membranes that allow metabolic and reproductive pathways to develop. However, we might never know the steps that led to cell formation.

Cellular Evolution Scientists don’t know what the earliest cells were like, chemicals found in rocks suggest life was present 3.8 billion years ago even though no fossils remain. In 2004, scientists discovered what appeared to be fossilized microbes in volcanic rock that is 3.5 billion years old. What does this mean? Cellular activity had become established and that early life might have been linked to volcanic environments.

Cellular Evolution Continued Scientists think the first cells were prokaryotes, which lack a nucleus and most other organelles. A prokaryote called archaea are the closest relatives of Earth’s first cells. These microbes live in extreme environments like hot springs and volcanic vents in the deep sea; which are similar to early Earth.

When did photosynthetic organisms appear? Archaea are autotrophs that get their energy from inorganic compounds like sulfur. They also don’t make oxygen. Scientists think oxygen wasn’t present on Earth until 1.8 billion years ago and any oxygen that was here bonded with iron ions. This allowed for oxygen to accumulate in the environment.

When did photosynthetic organisms appear? Cyanobacteria, could perform photosynthesis evolved about 3.5 billion years ago and they released oxygen into the atmosphere; eventually producing enough oxygen to support an ozone layer. The ozone layer provided a shield from the Sun’s damaging UV radiation and made conditions right for eukaryotes.

When did eukaryotic cells evolve? About 1.8 billion years ago. Characteristics: Larger than prokaryotes Complex internal membranes, which enclose many organelles including the nucleus.

What is the endosymbiont theory? Proposed by Lynn Margulis States that ancestors of eukaryotic cells lived together in association with prokaryotic cells. In some cases, prokaryotes might even have lived inside eukaryotes. HOW? They might have entered as undigested prey or they might have been internal parasites. Eventually, the relationship benefitted both cells & the prokaryotes became organelles inside the eukaryotic cells.

Endosymbiont Theory Continued Evidence suggests that mitochondria and chloroplasts formed by endosymbiosis. HOW? Both contain their own DNA arranged in circular patterns like the DNA of prokaryotes. Both have ribosomes that are more similar to the ribosomes in prokaryotes that to those in eukaryotes. Both reproduce by fission independent from the rest of the cell.

Endosymbiont Theory Animation http://www.sumanasinc.com/webcontent/animations/content/organelles.html

Conclusions Scientists don’t know the early steps that lead to life or to its evolution. However, they do continue to test theories and evaluate new evidence as they seek answers to understand what led to life on Earth. QUESTIONS??