Copyright 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Transcription

1

2 Chemical and physical processes on early Earth produced very simple cells through a sequence of stages. Could this be true???? How could it happen?? Let s follow it step by step. Here are the objectives

3 Describe a scientific hypothesis about the origin of life on Earth. [LO 1.27, Evaluate scientific questions based on hypotheses about the origin of life on Earth. [LO 1.28, SP 3.3] Describe the reasons for revisions of scientific hypotheses about the origin of life on Earth. [LO 1.29, SP 6.3] Evaluate scientific hypotheses about the origin of life on Earth. [LO 1.30, SP 6.5] Evaluate the accuracy and legitimacy of data to answer scientific questions about the origin of life on Earth. [LO 1.31, SP 4.4] Justify the selection of geological, physical, and chemical data that reveal early Earth conditions. [LO 1.32, SP 4.1]

4 Step One: Make Simple Organic Monomers Earth formed about 4.6 billion years ago Along with the rest of the solar system Earth s early atmosphere and oceans contained water vapor and many chemicals released by volcanic eruptions such as and, but no. But how can these simple molecules become complex molecules like proteins and DNA???

5 Miller and Urey showed us one way. CONCLSION. CH 4 Electrode Water vapor H 2 O Cooled water containing organic molecules Figure 26.2

6 Time for 6 minutes of Carl Sagan to show us how? Check it out on the Cosmos DVD: cosmic Fugue, Ch To emphasize a reducing atmosphere (no molecular oxygen) was a key. What would oxygen do to these chemicals that were combining?

7 Let s try a practice question 1. By discharging electric sparks into a laboratory chamber atmosphere that consisted of water vapor, hydrogen gas, methane, and ammonia, Stanley Miller obtained data that showed that a number of organic molecules, including many amino acids, could be synthesized. Miller was attempting to model early Earth conditions as understood in the 1950s. The results of Miller s experiments best support which of the following hypotheses? (A) The molecules essential to life today did not exist at the time Earth was first formed. (B) The molecules essential to life today could not have been carried to the primordial Earth by a comet or meteorite. (C) The molecules essential to life today could have formed under early Earth conditions. (D) The molecules essential to life today were initially self-replicating proteins that were synthesized approximately four billion years ago.

8 Instead of forming in the atmosphere, it could have been deep sea vents, or maybe deep in ice. Alkaline vents, not black smokers, seem to be the current favorite (proton gradient handout) Figure 26.3

9 Watch here. 4 min. This is pretty good. 9 min. It starts with a decent review of natural selection, and then starts to compare chemical evolution to biological around the 3 minute mark. This is GOOD. Show this!

10 Latest update Sutherland s team reports that it created nucleic acid precursors starting with just hydrogen cyanide (HCN), hydrogen sulfide (H 2 S), and ultraviolet (UV) light. What is more, Sutherland says, the conditions that produce nucleic acid precursors also create the starting materials needed to make natural amino acids and lipids. That suggests a single set of reactions could have given rise to most of life s building blocks simultaneously.

11 Science best explanation. Proof of how? Sutherland s team argues that early Earth was a favorable setting for those reactions. HCN is abundant in comets, which rained down steadily for nearly the first several hundred million years of Earth s history. The impacts would also have produced enough energy to synthesize HCN from hydrogen, carbon, and nitrogen. Likewise, Sutherland says, H 2 S was thought to have been common on early Earth, as was the UV radiation that could drive the reactions and metal-containing minerals that could have catalyzed them.

12 Step Two: Make Polymers Now that you have small organic molecules, they might polymerize when they are concentrated on hot sand, clay, or rock. Or maybe even in ice??? Check out this 1:21 to see how fatty acids can get made and concentrated, and then this :30 for nucleic acids. OR here s a link to the whole series.

13 Step Three: Form Protobionts Now you are getting closer to something that is almost a cell, a Proto what??? Check out this :25, and this :33 to see how micelles and liposomes form naturally.

14 Inside these membrane bound things, we get this. 20 m Glucose-phosphate Glucose-phosphate Phosphorylase Phosphate Starch Maltose Amylase Figure 26.4a, b (a) Simple reproduction. This liposome is giving birth to smaller liposomes (LM). Maltose (b) Simple metabolism. If enzymes in this case, phosphorylase and amylase are included in the solution from which the droplets self-assemble, some liposomes can carry out simple metabolic reactions and export the products.

15 The RNA World and the Dawn of Natural Selection The first genetic material Was probably RNA, not DNA RNA can fold into a specific shape :25, and catalyze like an enzyme. Where have we seen RNA doing this?

16 Watch what ribozymes can do Self-splicing watch :25 Making complementary copies of short stretches of their own sequence or other short pieces of RNA :40 3 Ribozyme (RNA molecule) Template Nucleotides Figure 26.5 Complementary RNA copy 5 5

17 How can these RNA s get inside a liposome? If you are more efficient, natural selection will take care of you watch :35 Watch :35 to see how simple protocell division can be. Watch for a good summary. No, if there is time, which there never is, take a look at a neat new finding on the evolution of multicellularity.

18 Extraterrestrial Sources of Organic Compounds How s this for a crazy idea? Panspermia Let s look at the Murchison meteorite.

19 But wait!!! Another new idea!!!! TNA world? It differs from RNA and DNA in its sugar backbone: TNA uses threose where RNA uses ribose and DNA deoxyribose. That gives TNA a key advantage, says John Chaput of Arizona State University in Tempe: it is a smaller molecule than ribose or deoxyribose, possibly making TNA easier to form. Chaput and his colleagues have now created a TNA molecule that folds into a three-dimensional shape and clamps onto a specific protein. These are key steps towards creating a TNA enzyme that can control a chemical reaction, just like RNA.

20 So let s break down the steps If you want life to start, the first step is: Then